The authors from the University of Wyoming gratefully acknowledge support as part of the Center for Mechanistic Control of Water-Hydrocarbon-Rock Interactions in Unconventional and Tight Oil Formations (CMC-UF), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under DOE (BES) Award DE-SC0019165. The authors from the University of Kansas would like to acknowledge the National Science Foundation EPSCoR Research Infrastructure Improvement Program: Track -2 Focused EPSCoR Collaboration award (OIA- 1632892) for funding of this project. Authors also extend their appreciation to Jindi Sun from the Chemical Engineering Department, University of Wyoming for her generous help in instrument training. SAA thanks Kyle Winkelman from the University of Wyoming for his help with constructing the imaging and UV stands. Last but not the least, the authors gratefully acknowledge John Wasserbauer from microGlass, LLC for useful discussions regarding the SLE technique.

CAUTION: This protocol involves handling a high-pressure setup, a high-temperature furnace, hazardous chemicals, and UV light. Please read all relevant material safety data sheets carefully and follow chemical safety guidelines. Review pressure testing (hydrostatic and pneumatic) safety guidelines including required training, safe operation of all equipment, associated hazards, emergency contacts, etc. before starting the injection process.
1. Design geometrical patterns
<ol>
	<li>Design a photomask comprising geometrical features and flow pathways of interest (Figure 1, Supplementary File 1: Figure S1).</li>
	<li>Define the bounding box (surface area of the device) to identify the area of the substrate and confine the design to the dimensions of the desired medium.</li>
	<li>Design inlet/outlet ports. Choose port dimensions (e.g., 4 mm in diameter in this case) to achieve a relatively uniform distribution of foam prior to entering the medium (Figure 1).</li>
	<li>Prepare a photomask of the designed geometrical pattern by printing the design onto a sheet of transparent film or a glass substrate.
	<ol>
		<li>Extrude the two-dimensional design to the third dimension and incorporate inlet and outlet ports (for use in SLE). 
		NOTE: The SLE technique requires a three-dimensional drawing (Figure 2).</li>
	</ol>
	</li>
</ol>
2. Transfer the geometric patterns to the glass substrate using photolithography
NOTE: Etchants and piranha solutions must be handled with extreme care. Use of personal protective equipment including facepiece reusable respirator, goggles, gloves and use of acid/corrosion resistant tweezers (Table of Materials) is recommended.
<ol>
	<li>Prepare the solutions needed in the wet-etching process by following these steps (also see the electronic supporting information provided as Supplementary File 1).
	<ol>
		<li>Pour an adequate amount of chrome etchant solution in a beaker such that the substrate can be submerged in the etchant. Heat up the fluid to approximately 40 &#176;C.</li>
		<li>Prepare a solution of developer (Table of Materials) in deionized water (DI water) with a volumetric ratio of 1:8 such that the substrate is able to be fully submerge in the mixture.</li>
	</ol>
	</li>
	<li>Imprint the geometrical pattern on a borosilicate substrate coated with a layer of chromium and a layer of photoresist using UV irradiation.
	<ol>
		<li>Using gloved hands, place the mask (glass substrate or the transparent film bearing the geometrical pattern) directly on the side of the borosilicate substrate that is covered with chrome and photoresist.</li>
		<li>Place the photomask and substrate combination under the UV light with the photomask facing the source. 
		NOTE: This work uses UV light with a wavelength of 365 nm (to match the peak sensitivity of the photoresist) and at an average intensity of 4.95 mW/cm2.</li>
		<li>Transfer the geometrical pattern into the layer of photoresist by exposing the stack of the substrate and the mask to UV light. 
		NOTE: Optimum exposure time is a function of the thickness of the photoresist layer and the strength of UV radiation. Photoresist is sensitive to light and the entire process of imprinting the pattern must be performed in a dark room equipped with yellow lighting.</li>
	</ol>
	</li>
	<li>Develop the photoresist.
	<ol>
		<li>Remove the photomask and substrate stack from the UV stage using gloved hands.</li>
		<li>Remove the photomask and submerge the substrate in the developer solution for approximately 40 s, thereby transferring the pattern to the photoresist.</li>
		<li>Cascade-rinse the substrate by flowing DI water from the top of the substrate and over all its surfaces a minimum of three times and allow the substrate to dry.</li>
	</ol>
	</li>
	<li>Etch the pattern in the chrome layer.
	<ol>
		<li>Submerge the substrate in a chrome etchant heated to about 40 &#176;C for approximately 40 s, thereby transferring the pattern from the photoresist to the chrome layer.</li>
		<li>Remove the substrate from the solution, cascade-rinse the substrate using DI water and allow it to dry.</li>
	</ol>
	</li>
	<li>Etch the pattern in the borosilicate substrate. 
	NOTE: A buffered etchant (Table of Materials) is used to transfer the geometrical pattern to the glass substrate. Prior to the use of the buffered etchant, the backside of the substrate is coated with a layer of photoresist to shield it from the etchant. The thickness of this protective layer is immaterial to the overall fabrication process.
	<ol>
		<li>Using a brush, apply several layers of hexamethyldisilazane (HMDS) on the uncovered face of the substrate and allow it to dry. 
		NOTE: HMDS helps promote adhesion of photoresist to the surface of the borosilicate substrate.</li>
		<li>Apply one layer of photoresist on top of the primer. Place the substrate in an oven at 60&#8210;90 &#176;C for 30&#8211;40 min.</li>
		<li>Pour an adequate amount of the etchant into a plastic container and fully submerge the substrate in the etchant. 
		NOTE: The etching rate is influenced by the concentration, temperature and duration of exposure. The buffered etchant used in this work etches an average of 1&#8210;10 nm/min.</li>
		<li>Leave the patterned substrate in the etchant solution for a predetermined amount of time based on the desired channel depths. 
		NOTE: Etching time may be reduced by intermittent bath sonication of the solution.</li>
		<li>Remove the substrate from the etchant using a solvent-resistant pair of tweezers and cascade-rinse the substrate using DI water.</li>
		<li>Characterize the etched features on the substrate to ensure desired depths have been achieved. 
		NOTE: This characterization may be done using a laser scanning confocal microscope (Figure 3). In this work, a 10x magnification is used for data acquisition. Once channel depths are satisfactory, move to the cleaning and bonding stage.</li>
	</ol>
	</li>
</ol>
3. Clean and bond
<ol>
	<li>Remove photoresist and chrome layers.
	<ol>
		<li>Remove the photoresist from the substrate by exposing the substrate to an organic solvent, such as N-Methyl-2-pyrrolidone (NMP) solution heated using a hot plate under a hood to approximately 65 &#176;C for approximately 30 min.</li>
		<li>Cascade-rinse the substrate with acetone (ACS grade), followed by ethanol (ACS grade) and DI water.</li>
		<li>Place the cleaned substrate in chrome etchant heated using a hot plate under a hood to approximately 40 &#176;C for about 1 min, thus removing the chrome layer from the substrate.</li>
		<li>Once the substrate is free from chrome and photoresist, characterize the channel depths using laser scanning confocal microscopy. 
		NOTE: This work uses a 10x magnification for data acquisition (Figure 4).</li>
	</ol>
	</li>
	<li>Prepare the cover plate and the etched substrate for bonding.
	<ol>
		<li>Mark the positions of the inlet/out holes on a blank borosilicate substrate (cover plate) by aligning the cover plate against the etched substrate.</li>
		<li>Blast through-holes in the marked locations using a micro abrasive sandblaster and 50 &#181;m aluminum-oxide micro sandblasting media. 
		NOTE: Alternatively, the ports may be created using a mechanical drill.</li>
		<li>Cascade rinse both the etched substrate and the cover plate with DI water.</li>
		<li>Perform an RCA wafer cleaning procedure to remove contaminants prior to bonding using standard technique. Perform the wafer cleaning steps under a hood due to the volatility of the solutions involved in the process.</li>
		<li>Bring a 1:4 by volume H2O2:H2SO4 piranha solution to a boil and submerge the substrate and the cover plate in the solution for 10 min under a hood.</li>
		<li>Cascade rinse the substrate and the cover plate with DI water.</li>
		<li>Submerge the substrate and the cover plate in the buffered etchant for 30&#8211;40 s.</li>
		<li>Cascade rinse the substrate and the cover plate with DI water.</li>
		<li>Submerge the substrate and the cover plate for 10 min in a 6:1:1 by volume DI water:H2O2:HCl solution that is heated to approximately 75 &#176;C. 
		NOTE: Etching and bonding are preferably performed in a cleanroom. If a cleanroom is not available, performing the following steps in a dust-free environment is recommended. In this work, steps 3.2.9&#8211;3.2.12 are performed in a glovebox to minimize the possibility of contamination of the substrates.</li>
		<li>Press the substrate and the cover plate tightly against each other while submerged.</li>
		<li>Remove the substrate and the cover plate from DI water:H2O2:HCl solution. Cascade rinse with DI water and submerge in DI water.</li>
		<li>Make sure the substrate and the cover plate are firmly attached together and carefully remove the two while pressed against each other from DI water.</li>
	</ol>
	</li>
	<li>Bond the substrates thermally.
	<ol>
		<li>Place the stacked substrates (the etched substrate and the cover plate) between two smooth, 1.52 cm-thick, glass-ceramic plates for bonding.</li>
		<li>Place the glass-ceramic plates between two metallic plates made of Alloy X (Table of Materials), which is able to withstand the required temperatures without significant distortion.</li>
		<li>Center the glass wafers in the ceramic-metallic holder. 
		NOTE: This work uses glass-ceramic plates that are 10 cm x 10 cm x 1.52 cm in thickness. The stacked setup is secured using 1/4&#8221; bolts and nuts (Figure 5).</li>
		<li>Hand-tighten the nuts and place the holder in a vacuum chamber for 60 min at approximately 100 &#176;C.</li>
		<li>Remove the holder from the chamber and carefully tighten the nuts using approximately 10 lb-in of torque.</li>
		<li>Place the holder inside a furnace and execute the following heating program. Raise the temperature at 1 &#176;C/min up to 660 &#176;C; keep the temperature constant at 660 &#176;C for 6 h followed by a cooling step at approximately 1 &#176;C/min back down to room temperature.</li>
		<li>Remove the thermally bonded microfluidic device, rinse it with DI water, place it in HCl (12.1 M) and bath-sonicate (40 kHz at 100 W of power) the solution for one hour (Figure 6).</li>
	</ol>
	</li>
</ol>
4. Fabricate laser-etched glass microfluidic devices
NOTE: Device fabrication was performed by a third-party glass 3D printing service (Table of Materials) via an SLE process and using a fused silica substrate as the precursor.
<ol>
	<li>Write the desired pattern in a fused silica substrate using a linearly polarized laser beam oriented perpendicular to the stage generated via a femtosecond laser source with a pulse duration of 0.5 ns, a repetition rate of 50 kHz, pulse energy of 400 nJ, and a wavelength of 1.06 &#956;m.</li>
	<li>Remove the glass from the written pattern inside the fused silica substrate using a KOH solution (32 wt%) at 85 &#176;C with ultrasound sonication (Figure 7).</li>
</ol>
5. Perform high-pressure testing
<ol>
	<li>Saturate the microfluidic device with the resident fluid (e.g., DI water, surfactant solution, oil, etc. depending on the type of experiment) using a syringe pump.</li>
	<li>Prepare foam-generating fluids and related instruments.
	<ol>
		<li>Prepare the brine solution (resident fluid) with the desired salinity and dissolve the surfactant (such as lauramidopropyl betaine and alpha-olefin-sulfonate) with the desired concentration (according to surfactant&#8217;s critical micelle concentration) in the brine.</li>
		<li>Fill the tanks of the CO2 and water pumps with adequate amounts of fluids per the experiment at room temperature.</li>
		<li>Fill the brine accumulator and flow lines with the surfactant solution using a syringe. This work uses an accumulator with a capacity of 40 mL.</li>
		<li>Rinse the brine line with the brine solution.</li>
		<li>Rinse the line connecting the accumulator to the device and the outlet lines with the resident fluid (the brine solution in this case).</li>
		<li>Place the saturated microfluidic device in a pressure-resistant holder and connect the inlet/outlet ports to the appropriate lines using 0.010&#8221; inner diameter tubing (Figure 8, Supplementary File 1: Figure S5).</li>
		<li>Increase the temperature of the circulating bath, which controls the temperature of the brine and CO2 lines, to the desired temperature (e.g., 40 &#176;C here (Figure 9)).</li>
		<li>Check all the lines to ensure the integrity of the setup prior to injection.</li>
	</ol>
	</li>
	<li>Generate the foam.
	<ol>
		<li>Begin injecting the brine at a rate of 0.5 mL/min and check the flow of surfactant solution into the device and the backpressure line.</li>
		<li>Increase the backpressure and brine-pump pressure simultaneously in gradual steps (~ 0.006 MPa/s) while maintaining continuous flow from the outlet of the backpressure regulator (BPR). Increase the pressure up to ~7.38 MPa (minimum required scCO2 pressure) and stop the pumps.</li>
		<li>Increase the CO2 line pressure up to a pressure above 7.38 MPa (minimum scCO2 pressure).</li>
		<li>Open the CO2 valve and allow the scCO2 mixed with the high-pressure surfactant solution to flow through an inline mixer to generate foam.</li>
		<li>Wait until flow is fully developed inside the device and the channels are saturated. Monitor the outlet for the onset of foam generation. 
		NOTE: Auxiliary ports may be used to help pre-saturate the medium fully with the resident fluid (Figure 1). Inconsistencies in the rate of pressure build-up and sudden increases in BPR may lead to breakage (Figure 10). Fluid pressures and backpressure must be raised gradually to minimize the risk of damage to the device.</li>
	</ol>
	</li>
	<li>Perform real-time imaging and data analysis.
	<ol>
		<li>Turn on the camera to capture detailed images of flow inside the channels. This work uses a camera featuring a 60 megapixel, monochromatic, full-frame sensor.</li>
		<li>Launch the dedicated shutter control software (Table of Materials). Select a shutter speed of 1/60, a focal ratio (f-number) of f/8.0, and select the appropriate lens.</li>
		<li>Launch the dedicated camera software (Table of Materials). Select the camera, the desired format (e.g., IIQL) and an ISO setting of 200 in the pulldown menu under the &#8220;CAMERA&#8221; setting of the software.</li>
		<li>Adjust the working distance of the camera to the medium as needed to focus on the medium. Capture images at prescribed time-intervals by pressing the capture button in the software.</li>
	</ol>
	</li>
	<li>Depressurize the system back to ambient conditions.
	<ol>
		<li>Stop injection (gas and liquid pumps), close the CO2 and brine pump inlets, open the rest of the line valves and turn off the heaters.</li>
		<li>Decrease the backpressure gradually (e.g., at a rate of 0.007 MPa/s) until the system reaches ambient pressure conditions. Decrease the brine and CO2 pump pressures separately. 
		NOTE: Decreasing the scCO2 pressure may result in inconsistent or turbulent BPR outflow, therefore the pressure drawdown must be executed with requisite care.</li>
	</ol>
	</li>
	<li>Clean the microfluidic device thoroughly after each experiment as needed by flowing the following sequence of solutions through the medium: isopropanol/ethanol/water (1:1:1), 2 M HCl solution, DI water, a basic solution (DI water/NH4OH/H2O2 at 5:5:1) and DI water.</li>
	<li>Post-process collected images.
	<ol>
		<li>Isolate the pore scape by excluding the background from the images.</li>
		<li>Correct minor misalignments by performing perspective transformation and implementing a local thresholding strategy as needed to account for non-uniform illumination28.</li>
		<li>Calculate geometrical and statistical parameters relevant to the experiment such as average bubble size, bubble size distribution and bubble shape for each foam microstructural images in the channel.</li>
	</ol>
	</li>
</ol>

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<li>Jiménez-martínez, J., Porter, M. L., Hyman, J. D., Carey, J. W., Viswanathan, H. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mixing+in+a+three-phase+system%3A+Enhanced+production+of+oil-wet+reservoirs+by+CO2+injection%5BTitle%5D)+AND+%22Geophysical+Research+Letters%22%5BJournal%5D)">Mixing in a three-phase system: Enhanced production of oil-wet reservoirs by CO2 injection.</a> Geophysical Research Letters. 43, 196-205 (2016).</li>
<li>Rognmo, A. U., Fredriksen, S. B., Alcorn, Z. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Pore-to-Core+EOR+Upscaling+for+CO2+Foam+for+CCUS%5BTitle%5D)+AND+%22SPE+Journal%22%5BJournal%5D)">Pore-to-Core EOR Upscaling for CO2 Foam for CCUS.</a> SPE Journal. 24, 1-11 (2019).</li>
<li>Erickstad, M., Gutierrez, E., Groisman, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+low-cost+low-maintenance+ultraviolet+lithography+light+source+based+on+light-emitting+diodes%5BTitle%5D)+AND+%22Lab+on+a+Chip%22%5BJournal%5D)">A low-cost low-maintenance ultraviolet lithography light source based on light-emitting diodes.</a> Lab on a Chip. 15, 57-61 (2015).</li>
<li>Guo, F., Aryana, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((An+Experimental+Investigation+of+Flow+Regimes+in+Imbibition+and+Drainage+Using+a+Microfluidic+Platform%5BTitle%5D)+AND+%22Energies%22%5BJournal%5D)">An Experimental Investigation of Flow Regimes in Imbibition and Drainage Using a Microfluidic Platform.</a> Energies. 12 (7), 1-13 (2019).</li>
<li>Burshtein, N., Chan, S. T., Toda-peters, K., Shen, A. Q., Haward, S. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((3D-printed+glass+microfluidics+for+fluid+dynamics+and+rheology%5BTitle%5D)+AND+%22Current+Opinion+in+Colloid+%26+Interface+Science%22%5BJournal%5D)">3D-printed glass microfluidics for fluid dynamics and rheology.</a> Current Opinion in Colloid & Interface Science. 43, 1-14 (2019).</li>
<li>Wang, Y., Aryana, S. A. Creation of Saturation Maps from Two-Phase Flow Experiments in Microfluidic Devices. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aWang%2C+Y+%22Advances+in+Petroleum+Engineering+and+Petroleum+Geochemistry.+Advances+in+Science%2C+Technology+%26+Innovation%22">Advances in Petroleum Engineering and Petroleum Geochemistry. Advances in Science, Technology & Innovation</a>. Banerjee, S., Barati, R., Patil, S. , Springer. 77-80 (2019).</li>
<li>Hermans, M., Gottmann, J., Riedel, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Selective%2C+Laser-Induced+Etching+of+Fused+Silica+at+High+Scan-Speeds+Using+KOH%5BTitle%5D)+AND+%22Journal+of+Laser+Micro%2FNanoengineering%22%5BJournal%5D)">Selective, Laser-Induced Etching of Fused Silica at High Scan-Speeds Using KOH.</a> Journal of Laser Micro/Nanoengineering. 9, 126-131 (2014).</li>
<li>Iliescu, C., Jing, J., Tay, F. E. H., Miao, J., Sun, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Characterization+of+masking+layers+for+deep+wet+etching+of+glass+in+an+improved+HF%2FHCl+solution%5BTitle%5D)+AND+%22Surface+%26+Coatings+Technology%22%5BJournal%5D)">Characterization of masking layers for deep wet etching of glass in an improved HF/HCl solution.</a> Surface & Coatings Technology. 198, 314-318 (2005).</li>
</ol>

The authors declare no conflicts of interest and disclosure.

This work presents a protocol related to a fabrication platform to create robust, high-pressure glass microfluidic devices. The protocol presented in this work alleviates the need for a cleanroom by performing several of the final fabrication steps inside a glovebox. The use of a cleanroom, if available, is recommended to minimize the potential for contamination. Additionally, the choice of the etchant should be based on the desired surface roughness. The use of a mixture of HF and HCl as the etchant tends to reduce surf...

Hydraulic fracturing has been used for quite some time as a means to stimulate flow especially in tight formations1. Large amounts of water needed in hydraulic fracturing are compounded with environmental factors, water-availability issues2, formation damage3, cost4 and seismic effects5. As a result, interest in alternate fracturing methods such as waterless fracturing and the use of foams is on the rise. Alternative methods may provide important benefits such as reduction in water use6, compatibility with water sensitive formations7, minimal to no plugging of the formation8, high apparent viscosity of the fracturing fluids9, recyclability10, ease of clean-up and proppant carrying capability6. CO2 foam is a potential waterless fracturing fluid that contributes to more efficient production of petroleum fluids and improved CO2 storage capacities in the subsurface with a potentially smaller environmental footprint compared to conventional fracturing techniques6,7,11.
Under optimal conditions, supercritical CO2 foam (scCO2 foam) at pressures beyond the minimum miscibility pressure (MMP) of a given reservoir provides a multi-contact miscible system that is able to direct flow into less permeable parts of the formation, thereby improving sweep efficiency and recovery of the resources12,13. scCO2 delivers gas like diffusivity and liquid like density14 and is well suited for subsurface applications, such as oil recovery and carbon capture, utilization and storage (CCUS)13. The presence of the constituents of foam in the subsurface helps reduce the risk of leakage in long-term storage of CO215. Moreover, coupled-compressibility-thermal shock effects of scCO2 foam systems may serve as effective fracturing systems11. Properties of CO2 foam systems for subsurface applications have been studied extensively at various scales, such as characterization of its stability and viscosity in sand-pack systems and its effectiveness in displacement processes3,6,12,15,16,17. Fracture level foam dynamics and its interactions with porous media are less studied aspects that are directly relevant to the use of foam in tight and fractured formations.
Microfluidic platforms enable direct visualization and quantification of the relevant microscale processes. These platforms provide real-time control of the hydrodynamics and chemical reactions to study pore-scale phenomena alongside recovery considerations1. Foam generation, propagation, transport and dynamics may be visualized in microfluidic devices emulating fractured systems and fracture-microcrack-matrix conductive pathways relevant to oil recovery from tight formations. Fluid exchange between fracture and matrix is directly expressed in accordance with the geometry18, thereby highlighting the importance of simplistic and realistic representations. A number of relevant microfluidic platforms have been developed over the years to study various processes. For example, Tigglaar and coworkers discuss fabrication and high-pressure testing of glass microreactor devices through in-plane connection of fibers to test flow through glass capillaries connected to the microreactors19. They present their findings related to bond inspection, pressure tests and in-situ reaction monitoring by 1H NMR spectroscopy. As such, their platform may not be optimal for relatively large injection rates, pre-generation of multiphase fluid systems for in situ visualization of complex fluids in permeable media. Marre and coworkers discuss the use of a glass microreactor to investigate high-pressure chemistry and supercritical fluid processes20. They include results as a finite-element simulation of stress distribution to explore the mechanical behavior of modular devices under the load. They use nonpermanent modular connections for interchangeable microreactor fabrication, and the silicon/Pyrex microfluidic devices are not transparent; these devices are suited for kinematic study, synthesis and production in chemical reaction engineering where visualization is not a primary concern. The lack of transparency makes this platform unsuitable for direct, in situ visualization of complex fluids in surrogate media. Paydar and coworkers present a novel way to prototype modular microfluidics using 3D printing21. This approach does not seem well-suited for high-pressure applications since it uses a photocurable polymer and the devices are able to withstand only up to 0.4 MPa. Most microfluidic experimental studies related to transport in fractured systems reported in literature focus on ambient temperature and relatively low-pressure conditions1. There have been several studies with a focus on direct observation of microfluidic systems that mimic subsurface conditions. For example, Jimenez-Martinez and co-workers introduce two studies on critical pore-scale flow and transport mechanisms in a complex network of fractures and matrix22,23. The authors study three-phase systems using microfluidics under reservoir conditions (8.3 MPa and 45 &#176;C) for production efficiency; they assess scCO2 usage for re-stimulation where the leftover brine from a prior fracturing is immiscible with CO2 and the residual hydrocarbon23. Oil-wet silicon microfluidic devices have relevance to mixing of oil-brine-scCO2 in Enhanced Oil Recovery (EOR) applications; however, this work does not directly address pore-scale dynamics in fractures. Another example is work by Rognmo et al. who study an upscaling approach for high-pressure, in situ CO2 foam generation24. Most of the reports in literature that leverage microfabrication are concerned with CO2-EOR and they often do not include important fabrication details. To the best of the authors&#8217; knowledge, a systematic protocol for fabrication of high-pressure capable devices for fractured formations is currently missing from the literature.
This work presents a microfluidic platform that enables the study of scCO2 foam structures, bubble shapes, sizes and distribution, lamella stability in the presence of oil for EOR and hydraulic fracturing and aquifer remediation applications. The design and fabrication of microfluidic devices using optical lithography and Selective Laser-induced Etching29 (SLE) are discussed. Additionally, this work describes fracture patterns that are intended to simulate the transport of fluids in fractured tight formations. Simulated pathways may range from simplified patterns to complex microcracks based on tomography data or other methods that provide information regarding realistic fracture geometries. The protocol describes step-by-step fabrication instructions for glass microfluidic devices using photolithography, wet-etching and thermal bonding. An in-house developed collimated Ultra-Violet (UV) light source is used to transfer the desired geometric patterns onto a thin layer of photoresist, which is ultimately transferred to the glass substrate using a wet-etching process. As part of quality assurance, the etched patterns are characterized using confocal microscopy. As an alternative to photolithography/wet-etching, an SLE technique is employed to create a microfluidic device and a comparative analysis of the platforms is presented. The setup for flow experiments comprise gas cylinders and pumps, pressure controllers and transducers, fluid mixers and accumulators, microfluidic devices, high-pressure capable stainless-steel holders along with a high-resolution camera and an illumination system. Finally, representative samples of observations from flow experiments are presented.

<table><tbody><tr><td>1/4&amp;rdquo; bolts and nuts</td><td></td><td></td><td>For fabrication of the metallic plates to sandwich the glass chip between them for thermal bonding</td></tr><tr><td>3.45 x 3.45 mm UV LED</td><td>Kingbright</td><td></td><td>To emitt LED light</td></tr><tr><td>3D measuring Laser microscope</td><td>OLYMPUS</td><td>LEXT OLS4000</td><td>To measure channel depths</td></tr><tr><td>40 mm x 40 mm x 10 mm 12V DC Cooling Fan</td><td>Uxcell</td><td></td><td>To cool the UV LED lights</td></tr><tr><td>120 mm x 38 mm 24V DC Cooling Fan</td><td>Uxcell</td><td></td><td>To cool the UV LED lights</td></tr><tr><td>5 ml (6 ml) NORM-JECT Syringe</td><td>HENKE SASS WOLF</td><td>Lot #16M14CB</td><td>To rinse the chip before each experiment</td></tr><tr><td>Acetone (Certified ACS)</td><td>Fisher Chemical</td><td>Lot #177121</td><td>For cleaning</td></tr><tr><td>Acid/ corossion resistive tweezer</td><td>TED PELLA</td><td></td><td>To handle the glass piece in corosive solutions</td></tr><tr><td>Acid/solvent resistance tweezers</td><td>TED PELLA, INC</td><td>#53009 and #53010</td><td>To handle the glass in corrosive solutions</td></tr><tr><td>Alloy X</td><td>AMERICAN SPECIAL METALS</td><td>Heat Number: ZZ7571XG11</td><td>Thermal bonding</td></tr><tr><td>Ammonium hydroxide (ACS reagent)</td><td>Sigma Aldrich</td><td>Lot #SHBG9007V</td><td>To clean the chip at the end of process</td></tr><tr><td>AutoCAD</td><td>Autodesk, San Rafael, CA</td><td></td><td>To design 2D patterns and 3D chips</td></tr><tr><td>BD Etchant for PSG-SiO&lt;sub&gt;2&lt;/sub&gt; systems</td><td>TRANSENE</td><td>Lot #028934</td><td>An improved buffered etch formulation for delineation of phosphosilica glass &amp;ndash; SiO&lt;sub&gt;2&lt;/sub&gt; (PSG), and borosilica glass &amp;ndash; SiO&lt;sub&gt;2&lt;/sub&gt; (BSG) systems</td></tr><tr><td>Blank Borofloat substrate</td><td>TELIC</td><td>CG-HF</td><td>Upper substrate for UV etching</td></tr><tr><td>Borofloat substrate with metalizations</td><td>TELIC</td><td>PG-HF-LRC-Az1500</td><td>Lower substrate for UV etching</td></tr><tr><td>Capture One photo editing software</td><td>Phase One</td><td></td><td>To Capture/Edit/Convert the pictures taken by Phase One Camera</td></tr><tr><td>Capture station</td><td>DT Scientific</td><td>DT Versa</td><td>To place of the chip in the field of view of the camera</td></tr><tr><td>Carbon dioxide gas (Grade E)</td><td>PRAXAIR</td><td>UN 1013, CAS Number 124-38-9</td><td>non-aqeous portion of foam</td></tr><tr><td>Chromium etchant 1020</td><td>TRANSENE</td><td>Lot #025433</td><td>High-purity ceric ammonium nitrate systems for precise, clean etching of chromium and chromium oxide films.</td></tr><tr><td>Circulating baths with digital temperature controller</td><td>PolyScience</td><td></td><td>To control the brine and CO&lt;sub&gt;2&lt;/sub&gt; temperatures</td></tr><tr><td>Computer</td><td></td><td>NVIDIA Tesla K20 Graphic Card - 706 MHz Core - 5 GB GDDR5 SDRAM - PCI Express 2.0 x16</td><td>To process and visualize the images obtained via the Phase One camera</td></tr><tr><td>Custom made high pressure glass chip holder</td><td></td><td></td><td>To tightly hold the chip and its connections for high pressure testing</td></tr><tr><td>Cutrain (Custom)</td><td></td><td></td><td>To protect against UV/IR Radiations</td></tr><tr><td>Deionized water (DI)</td><td></td><td></td><td>For cleaning</td></tr><tr><td>Digital camera with monochromatic 60 MP sensor</td><td>Phase One</td><td>IQ260</td><td>Visualization system</td></tr><tr><td>Ethanol, Anhydrous, USP Specs</td><td>DECON LABORATORIES, INC.</td><td>Lot #A12291505J, CAS# 64-17-5</td><td>For cleaning</td></tr><tr><td>Facepiece reusable respirator</td><td>3M</td><td>6502QL, Gases, Vapors, Dust, Medium</td><td>To protect against volatile solution inhalation</td></tr><tr><td>Fused Silica (UV Grade) wafer</td><td>SIEGERT WAFER</td><td>UV grade</td><td>Glass precursor for SLE printing</td></tr><tr><td>GIMP</td><td>Open-source image processing software</td><td></td><td>To characterize image texture and properties</td></tr><tr><td>Glovebox (vinyl anaerobic chamber)</td><td>Coy</td><td></td><td>To provide a clean, dust-free environment</td></tr><tr><td>Heated ultrasonic cleaning bath</td><td>Fisher Scientific</td><td></td><td>To accelerate the etching process</td></tr><tr><td>Hexamethyldisilazane (HMDS) Cleanroom&amp;reg; MB</td><td>KMG</td><td>62115</td><td>Primer for photoresist coating</td></tr><tr><td>Hose (PEEK tubing)</td><td>IDEX HEALTH &amp;amp; SCIENCE</td><td>Natural 1/16&quot; OD x .010&quot; ID x 5ft, Part # 1531</td><td>Flow connections</td></tr><tr><td>Hydrochloric acid, certified ACS plus</td><td>Fisher Chemical</td><td>Lot # 187244</td><td>Solvent in RCA semiconductor cleaning protocol</td></tr><tr><td>Hydrogen Peroxide</td><td>Fisher Chemical</td><td>H325-500</td><td>Solvent in RCA semiconductor cleaning protocol</td></tr><tr><td>ImageJ</td><td>NIH</td><td></td><td>To characterize image texture and properties</td></tr><tr><td>ISCO syringe pump</td><td>TELEDYNE ISCO</td><td>D-SERIES (100DM, 500D)</td><td>To pump the fluids</td></tr><tr><td>Kaiser LED light box</td><td>Kaiser</td><td></td><td>To illuminate the chip</td></tr><tr><td>Laser printing machine</td><td>LightFab GmbH, Germany.</td><td>FILL</td><td>Glass-SLE chip fabrication</td></tr><tr><td>Laser safety glasses</td><td>FreeMascot</td><td>B07PPZHNX4</td><td>To protect against UV/IR Radiations</td></tr><tr><td>LED Engin 5W UV Lens</td><td>LEDiL</td><td></td><td>To emitt LED light</td></tr><tr><td>Light Fab 3D Printer (femtosecond laser)</td><td>Light Fab</td><td></td><td>To selectively laser Etch of fused silica</td></tr><tr><td>LightFab 3D printer</td><td>LightFab GmbH, Germany</td><td></td><td>To SLE print the fused silica chips</td></tr><tr><td>MATLAB</td><td>MathWorks, Inc., Natick, MA</td><td></td><td>Image processing</td></tr><tr><td>Metallic plates</td><td></td><td></td><td></td></tr><tr><td>Micro abrasive sand blasters (Problast 2)</td><td>VANIMAN</td><td>Problast 2 &amp;ndash; 80007</td><td>To craete holes in cover plates</td></tr><tr><td>MICROPOSIT 351 developer</td><td>Dow</td><td>10016652</td><td>Photoresist developer solution</td></tr><tr><td>Muffle furnace</td><td>Thermo Scientific</td><td>Thermolyne Type 1500</td><td>Thermal bonding</td></tr><tr><td>N&lt;sub&gt;2&lt;/sub&gt; pure research grade</td><td>Airgas</td><td>Research Plus - NI RP300</td><td>For drying the chips in each step</td></tr><tr><td>NMP semiconductor grade - 0.1&amp;mu;m Filtered</td><td>Ultra Pure Solutions, Inc</td><td>Lot #02191502T</td><td>Organic solvent</td></tr><tr><td>Oven</td><td>Gravity Convection Oven</td><td>18EG</td><td></td></tr><tr><td>Phase One IQ260 with an achromatic sensor</td><td>Phase One</td><td>IQ260</td><td>To visulize transport in microfluidic devices using an ISO 200 setting and an aperture at f/8.</td></tr><tr><td>Photomask</td><td>Fine Line Imaging</td><td>20,320 DPI FILM</td><td>Pattern of channels</td></tr><tr><td>Photoresist (SU-8)</td><td>MICRO CHEM</td><td>Product item: Y0201004000L1PE, Lot Number: 18110975</td><td>Photoresist</td></tr><tr><td>Polarized light microscope</td><td>OLYMPUS</td><td>BX51</td><td>Visual examination of micro channels</td></tr><tr><td>Ports (NanoPort Assembly)</td><td>IDEX HEALTH &amp;amp; SCIENCE</td><td>NanoPort Assembly Headless, 10-32 Coned, for 1/16&quot; OD, Part # N-333</td><td>Connections to the chip</td></tr><tr><td>Python</td><td>Python Software Foundation</td><td></td><td>Image processing</td></tr><tr><td>Safety face shield</td><td>Sellstrom</td><td>S32251</td><td>To protect against UV/IR Radiations</td></tr><tr><td>Sealing film (Parafilm)</td><td>Bemis Company, Inc</td><td></td><td>Isolation of containers</td></tr><tr><td>Shutter Control Software</td><td>Schneider-Kreuznach</td><td></td><td>To adjust shutter settings</td></tr><tr><td>Smooth ceramic plates</td><td></td><td></td><td>Thermal bonding</td></tr><tr><td>Stirring hot plate</td><td>Corning&amp;reg;</td><td>PC-620D</td><td>To heat the solutions</td></tr><tr><td>Sulfuric acid, ACS reagent 95.0-98.0%</td><td>Sigma Aldrich</td><td>Lot # SHBK0108</td><td>Solvent in RCA semiconductor cleaning protocol</td></tr><tr><td>Syringe pump (Standard Infuse/Withdraw PHD ULTRA)</td><td>Harvard Apparatus</td><td>70-3006</td><td>To saturate the chip before each experiment</td></tr><tr><td>Torque wrench</td><td>Snap-on</td><td>TE25A-34190</td><td>To tighten the screws</td></tr><tr><td>UV power meter</td><td>Optical Associates, Incorporated</td><td>Model 308</td><td>To measure the intesity of UV light</td></tr><tr><td>UV power meter</td><td>Optical Associates, Incorporated</td><td>Model 308</td><td>To quantify the strength of UV light</td></tr><tr><td>UV radiation stand (LED lights)</td><td></td><td></td><td>To transfer the pattern to glass (photoresist layer)</td></tr><tr><td>Vaccum pump</td><td>WELCH VACCUM TECHNOLOGY, INC</td><td>1380</td><td>To dry the chip</td></tr><tr><td>Variable DC power supplies</td><td>Eventek</td><td>KPS305D</td><td>To power the UV LED lights</td></tr></tbody></table>

microfluidic fabrication techniques for high-pressure testing of microscale supercritical co2 foam transport in fractured unconventional reservoirs

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a result, these platforms have not been fully exploited for direct observation of high-pressure transport in fractures. This work introduces microfluidic platforms that enable direct observation of multiphase flow in devices featuring surrogate permeable media and fractured systems. Such platforms provide a pathway to address important and timely questions such as those related to CO2 capture, utilization and storage. This work provides a detailed description of the fabrication techniques and an experimental setup that may serve to analyze the behavior of supercritical CO2 (scCO2) foam, its structure and stability. Such studies provide important insights regarding enhanced oil recovery processes and the role of hydraulic fractures in resource recovery from unconventional reservoirs. This work presents a comparative study of microfluidic devices developed using two different techniques: photolithography/wet-etching/thermal-bonding versus Selective Laser-induced Etching. Both techniques result in devices that are chemically and physically resistant and tolerant of high pressure and temperature conditions that correspond to subsurface systems of interest. Both techniques provide pathways to high-precision etched microchannels and capable lab-on-chip devices. Photolithography/wet-etching, however, enables fabrication of complex channel networks with complex geometries, which would be a challenging task for laser etching techniques. This work summarizes a step-by-step photolithography, wet-etching and glass thermal-bonding protocol and, presents representative observations of foam transport with relevance to oil recovery from unconventional tight and shale formations. Finally, this work describes the use of a high-resolution monochromatic sensor to observe scCO2 foam behavior where the entirety of the permeable medium is observed simultaneously while preserving the resolution needed to resolve features as small as 10 &#181;m.

This section presents examples of physical observations from scCO2 foam flow through a main fracture connected to array of micro-cracks. A glass microfluidic device made via photolithography or SLE is placed inside a holder and in the field of view of a camera featuring a 60 megapixel, monochromatic, full-frame sensor. Figure 11 illustrates the process of fabrication microfluidic devices and their placement in the experimental setup. Figure 12 is illu...

Watch this Scientific Journal Video about Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs at JoVE.co

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

This paper describes a protocol along with a comparative study of two microfluidic fabrication techniques, namely photolithography/wet-etching/thermal-bonding and Selective Laser-induced Etching (SLE), that are suitable for high-pressure conditions. These techniques constitute enabling platforms for direct observation of fluid flow in surrogate permeable media and fractured systems under reservoir conditions.

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a ...

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6.7K Views.  University of Kansas. Dit protocol is een multidisciplinaire samenwerking tussen de Universiteit van Kansas en de Universiteit van Wyoming. We hadden behoefte aan het testen van onze gepatenteerde, polyelektrolyt complexe nanodeeltjes met behulp van hoge druk microfluidics. En het begon een samenwerking tussen onze onderzoeksgroep en dr. Aryana's onderzoeksgroep om te profiteren van zijn state-of-the-art faciliteiten.We willen NSF bedanken voor de financiering van een deel van dit project, en danken het ter...

Video: Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Reservoir Condition Pore-scale Imaging of Multiple Fluid Phases Using X-ray Microtomography

X-ray microtomography was used to image, at a resolution of 6.6 &#181;m, the pore-scale arrangement of residual carbon dioxide ganglia in the pore-space of a carbonate rock at pressures and temperatures representative of typical formations used for CO2 storage. Chemical equilibrium between the CO2, brine and rock phases was maintained using a high pressure high temperature reactor, replicating conditions far away from the injection site. Fluid flow was controlled using high pressure high temperature syringe pumps. To maintain representative in-situ conditions within the micro-CT scanner a carbon fiber high pressure micro-CT coreholder was used. Diffusive CO2 exchange across the confining sleeve from the pore-space of the rock to the confining fluid was prevented by surrounding the core with a triple wrap of aluminum foil. Reconstructed brine contrast was modeled using a polychromatic x-ray source, and brine composition was chosen to maximize the three phase contrast between the two fluids and the rock. Flexible flow lines were used to reduce forces on the sample during image acquisition, potentially causing unwanted sample motion, a major shortcoming in previous techniques. An internal thermocouple, placed directly adjacent to the rock core, coupled with an external flexible heating wrap and a PID controller was used to maintain a constant temperature within the flow cell. Substantial amounts of CO2 were trapped, with a residual saturation of 0.203 &#177; 0.013, and the sizes of larger volume ganglia obey power law distributions, consistent with percolation theory.

We present a methodology for the imaging of multiple fluid phases at reservoir conditions by the use of x-ray microtomography. We show some representative results of capillary trapping in a carbonate rock sample.

X-ray microtomography was used to image, at a resolution of 6.6 &#181;m, the pore-scale arrangement of residual carbon dioxide ganglia in the pore-space ...

Microfluidic Devices for Characterizing Pore-scale Event Processes in Porous Media for Oil Recovery Applications

Microfluidic devices are versatile tools for studying transport processes at a microscopic scale. A demand exists for microfluidic devices that are resistant to low molecular-weight oil components, unlike traditional polydimethylsiloxane (PDMS) devices. Here, we demonstrate a facile method for making a device with this property, and we use the product of this protocol for examining the pore-scale mechanisms by which foam recovers crude oil. A pattern is first designed using computer-aided design (CAD) software and printed on a transparency with a high-resolution printer. This pattern is then transferred to a photoresist via a lithography procedure. PDMS is cast on the pattern, cured in an oven, and removed to obtain a mold. A thiol-ene crosslinking polymer, commonly used as an optical adhesive (OA), is then poured onto the mold and cured under UV light. The PDMS mold is peeled away from the optical adhesive cast. A glass substrate is then prepared, and the two halves of the device are bonded together. Optical adhesive-based devices are more robust than traditional PDMS microfluidic devices. The epoxy structure is resistant to swelling by many organic solvents, which opens new possibilities for experiments involving light organic liquids. Additionally, the surface wettability behavior of these devices is more stable than that of PDMS. The construction of optical adhesive microfluidic devices is simple, yet requires incrementally more effort than the making of PDMS-based devices. Also, though optical adhesive devices are stable in organic liquids, they may exhibit reduced bond-strength after a long time. Optical adhesive microfluidic devices can be made in geometries that act as 2-D micromodels for porous media. These devices are applied in the study of oil displacement to improve our understanding of the pore-scale mechanisms involved in enhanced oil recovery and aquifer remediation.

The goal of this procedure is to easily and rapidly produce a microfluidic device with customizable geometry and resistance to swelling by organic fluids for oil recovery studies. A polydimethylsiloxane mold is first generated, and then used to cast the epoxy-based device. A representative displacement study is reported.

Microfluidic devices are versatile tools for studying transport processes at a microscopic scale. A demand exists for microfluidic devices that are ...

Environment

Microfluidic Pneumatic Cages: A Novel Approach for In-chip Crystal Trapping, Manipulation and Controlled Chemical Treatment

The precise localization and controlled chemical treatment of structures on a surface are significant challenges for common laboratory technologies. Herein, we introduce a microfluidic-based technology, employing a double-layer microfluidic device, which can trap and localize in situ and ex situ synthesized structures on microfluidic channel surfaces. Crucially, we show how such a device can be used to conduct controlled chemical reactions onto on-chip trapped structures and we demonstrate how the synthetic pathway of a crystalline molecular material and its positioning inside a microfluidic channel can be precisely modified with this technology. This approach provides new opportunities for the controlled assembly of structures on surface and for their subsequent treatment.

Herein, we describe the fabrication and operation of a double-layer microfluidic system made of polydimethylsiloxane (PDMS). We demonstrate the potential of this device for trapping, directing the coordination pathway of a crystalline molecular material and controlling chemical reactions onto on-chip trapped structures.

The precise localization and controlled chemical treatment of structures on a surface are significant challenges for common laboratory technologies. ...

Chemistry

High Throughput Microfluidic Rapid and Low Cost Prototyping Packaging Methods

In this work, 3 different packaging and assembly techniques are presented. They can be classified into two categories: one-time use and reusable packaging techniques.
The one-time use packaging technique employs UV-based and temperature curing epoxies to connect microtubes to access holes, wire-bonding for integrated circuit connections, and silver epoxy for electrical connections. This method is based on a robust assembly technique that can support relatively high pressure close to 1 psi and does not need any support to strengthen the microfluidic architecture.
Reusable packaging techniques consist of PDMS-based microtube interconnectors and anisotropic adhesive films for electrical connections. These devices are more sensitive and fragile. Consequently, Plexiglas support is added to the microfluidic structure to improve the electrical contact when anisotropic adhesive films are used, and also to strengthen the microfluidic architecture. In addition, a micromanipulator is needed to maintain tubes while using a thin PDMS layer to connect them to the access holes. Different PDMS layer thicknesses, ranging from 0.45-3 mm, are tested to compare the best adherence versus injection rates. Applied injection rates are varied from 50-300 &#956;l/hr for 0.45-3 mm PDMS layers, respectively. These techniques are mainly applicable for low-pressure applications. However, they can be extended for high-pressure ones through plasma-oxygen process to permanently seal the PDMS to glass substrates. The main advantage of this technique, besides the fact that it is reusable, consists of keeping the device observable when the microchannel length is very short (in the range of 3 mm or lower).

In this article we describe different techniques for microfluidic rapid prototyping platforms. The proposed techniques are based on ultraviolet (UV) sensitive and temperature curing epoxies, polydimethylsiloxane (PDMS) based tubing, wire-bonding, and anisotropic adhesive films. The assembling procedures presented are developed for both one-time use devices as well as reusable microfluidic systems.

In this work, 3 different packaging and assembly techniques are presented. They can be classified into two categories: one-time use and reusable packaging ...

Bioengineering

A Microfluidic Technique to Probe Cell Deformability

Here we detail the design, fabrication, and use of a microfluidic device to evaluate the deformability of a large number of individual cells in an efficient manner. Typically, data for ~102 cells can be acquired within a 1 hr experiment. An automated image analysis program enables efficient post-experiment analysis of image data, enabling processing to be complete within a few hours. Our device geometry is unique in that cells must deform through a series of micron-scale constrictions, thereby enabling the initial deformation and time-dependent relaxation of individual cells to be assayed. The applicability of this method to human promyelocytic leukemia (HL-60) cells is demonstrated. Driving cells to deform through micron-scale constrictions using pressure-driven flow, we observe that human promyelocytic (HL-60) cells momentarily occlude the first constriction for a median time of 9.3 msec before passaging more quickly through the subsequent constrictions with a median transit time of 4.0 msec per constriction. By contrast, all-trans retinoic acid-treated (neutrophil-type) HL-60 cells occlude the first constriction for only 4.3 msec before passaging through the subsequent constrictions with a median transit time of 3.3 msec. This method can provide insight into the viscoelastic nature of cells, and ultimately reveal the molecular origins of this behavior.

We demonstrate a microfluidics-based assay to measure the timescale for cells to transit through a sequence of micron-scale constrictions.

Here we detail the design, fabrication, and use of a microfluidic device to evaluate the deformability of a large number of individual cells in an ...

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion co-flow device to generate uniform high-viscosity droplets in a low-viscosity fluid. The microfluidic capillary device has a common co-flow structure with its exit connecting to a wider tube. Elongated droplets of the low-viscosity fluid are first encapsulated by the high-viscosity fluid in the co-flow structure. As the elongated low-viscosity droplets flow through the exit, which is treated to be wetted by the low-viscosity fluid, phase inversion is then induced by the adhesion of the low viscosity droplets to the tip of the exit, which results in the subsequent inverse encapsulation of the high-viscosity fluid. The size of the resultant high-viscosity droplets can be adjusted by changing the flow rate ratio of the low-viscosity fluid to the high-viscosity fluid. We demonstrate several typical examples of the generation of high-viscosity droplets with a viscosity up to 11.9 Pas, such as glycerol, honey, starch, and polymer solution. The method provides a simple and straightforward approach to generate monodisperse high-viscosity droplets, which may be used in a variety of droplet-based applications, such as materials synthesis, drug delivery, cell assay, bioengineering, and food engineering.

A phase-inversion co-flow device is demonstrated to generate monodisperse high-viscosity droplets above 1 Pas, which is difficult to realize in droplet microfluidics.

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion ...

Combining Microfluidics and Microrheology to Determine Rheological Properties of Soft Matter during Repeated Phase Transitions

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement during previous phase changes and applied shear. To determine the extent of these changes, we have developed a microfluidic device that enables repeated phase transitions induced by exchange of the surrounding fluid and microrheological characterization while limiting shear on the sample. This technique is &#181;2rheology, the combination of microfluidics and microrheology. The microfluidic device is a two-layer design with symmetric inlet streams entering a sample chamber that traps the gel sample in place during fluid exchange. Suction can be applied far away from the sample chamber to pull fluids into the sample chamber. Material rheological properties are characterized using multiple particle tracking microrheology (MPT). In MPT, fluorescent probe particles are embedded into the material and the Brownian motion of the probes is recorded using video microscopy. The movement of the particles is tracked and the mean-squared displacement (MSD) is calculated. The MSD is related to macroscopic rheological properties, using the Generalized Stokes-Einstein Relation. The phase of the material is identified by comparison to the critical relaxation exponent, determined using time-cure superposition. Measurements of a fibrous colloidal gel illustrate the utility of the technique. This gel has a delicate structure that can be irreversibly changed when shear is applied. &#181;2rheology data shows that the material repeatedly equilibrates to the same rheological properties after each phase transition, indicating that phase transitions do not play a role in microstructural changes. To determine the role of shear, samples can be sheared prior to injection into our microfluidic device. &#181;2rheology is a widely applicable technique for the characterization of soft matter enabling the determination of rheological properties of delicate microstructures in a single sample during phase transitions in response to repeated changes in the surrounding environmental conditions.

We demonstrate the fabrication and use of a microfluidic device that enables multiple particle tracking microrheology measurements to study the rheological effects of repeated phase transitions on soft matter.

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement ...

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO2 Source

Herein is presented a general strategy to perform reactions under mild to moderate CO2 pressures with dry ice. This technique obviates the need for specialized equipment to achieve modest pressures, and can even be used to achieve higher pressures in more specialized equipment and sturdier reaction vessels. At the end of the reaction, the vials can be easily depressurized by opening at room temperature. In the present example CO2 serves as both a putative directing group as well as a way to passivate amine substrates, thereby preventing oxidation during the organometallic reaction. In addition to being easily added, the directing group is also removed under vacuum, obviating the need for extensive purification to remove the directing group. This strategy allows the facile &#947;-C(sp3)-H arylation of aliphatic amines and has the potential to be applied to a variety of other amine-based reactions.

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO2 Source

Here we present a protocol for performing reactions in simple reaction vessels under low-to-moderate pressures of CO2. The reactions can be performed in a variety of vessels simply by administering the carbon dioxide in the form of dry ice, without the need for costly or elaborate equipment or set-ups.

Herein is presented a general strategy to perform reactions under mild to moderate CO2 pressures with dry ice. This technique obviates the need for ...

Multi-step Variable Height Photolithography for Valved Multilayer Microfluidic Devices

Microfluidic systems have enabled powerful new approaches to high-throughput biochemical and biological analysis. However, there remains a barrier to entry for non-specialists who would benefit greatly from the ability to develop their own microfluidic devices to address research questions. Particularly lacking has been the open dissemination of protocols related to photolithography, a key step in the development of a replica mold for the manufacture of polydimethylsiloxane (PDMS) devices. While the fabrication of single height silicon masters has been explored extensively in literature, fabrication steps for more complicated photolithography features necessary for many interesting device functionalities (such as feature rounding to make valve structures, multi-height single-mold patterning, or high aspect ratio definition) are often not explicitly outlined. 
Here, we provide a complete protocol for making multilayer microfluidic devices with valves and complex multi-height geometries, tunable for any application. These fabrication procedures are presented in the context of a microfluidic hydrogel bead synthesizer and demonstrate the production of droplets containing polyethylene glycol (PEG diacrylate) and a photoinitiator that can be polymerized into solid beads. This protocol and accompanying discussion provide a foundation of design principles and fabrication methods that enables development of a wide variety of microfluidic devices. The details included here should allow non-specialists to design and fabricate novel devices, thereby bringing a host of recently developed technologies to their most exciting applications in biological laboratories.

Multilayer microfluidic devices often involve the fabrication of master molds with complex geometries for functionality. This article presents a complete protocol for multi-step photolithography with valves and variable height features tunable to any application. As a demonstration, we fabricate a microfluidic droplet generator capable of producing hydrogel beads.

Microfluidic systems have enabled powerful new approaches to high-throughput biochemical and biological analysis. However, there remains a barrier to ...

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO₂ Foam Transport in Fractured Unconventional Reservoirs

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