We acknowledge the financial support from the Rice University Consortium for Processes in Porous Media (Houston, TX, USA).

Caution: This protocol involves handling a high temperature oven, toxic chemicals, and UV light. Please read all the material safety data sheets carefully and follow your institution&#39;s chemical safety guidelines.
1. Device Design
<ol>
	<li>Design a photomask in a CAD software application.
	<ol>
		<li>Draw a rectangular channel that is 3 cm long and 0.5 cm wide (Figure 1b-top right).</li>
		<li>Create an array of enclosed shapes representing the grains of the porous media. 
		NOTE: These shapes are referred to as posts because they will become three-dimensional structures during the soft lithography process. The shape and size of the posts should be on the order of tens of microns, and have a spacing of ten to one hundred microns. Multiple post sizes may be employed to create heterogeneity, and a section can be left bare of posts to simulate a fracture in the media.</li>
		<li>Draw inlet and outlet channels that are approximately one-third as wide as the porous media section. Draw a channel stemming from the inlet port to act as a drain.</li>
		<li>Draw a bounding box around the entire design with a minimum of 1.0 cm of clearance from the design. 
		NOTE: The area between the bounding box and the borders of the design, as well as the posts, are to be made transparent on the photomask.</li>
	</ol>
	</li>
	<li>Submit the CAD file to a company for high-resolution CAD printing 
	NOTE: Optional: For a foam displacement experiment, design a microfluidic foam generator (Figure 1a). Repeat step 1, omitting the design heterogeneity and bounding box. A flow-focusing geometry is recommended at the inlet before the porous media design. The flow spaces should be transparent on the photomask.</li>
</ol>
2. PDMS Mold Fabrication
<ol>
	<li>Create a photoresist-patterned silicon wafer master mold in a clean room
	<ol>
		<li>Spin-coat a 20 &#181;m layer of photoresist onto a new silicon wafer at 2,000 rpm for 30 s.</li>
		<li>Soft bake the wafer on a hot plate in two increments: 65 &#176;C for 1 min followed by 95 &#176;C for 3 min.</li>
		<li>Use a mask aligner to pattern the photoresist layer with the CAD design using a constant dosage of 150 mJ/cm2.</li>
		<li>Perform a post-exposure bake on a hot plate in two increments: 65 &#176;C for 1 min followed by 95 &#176;C for 3 min. Allow the wafer to cool for 5 min.</li>
		<li>Immerse the wafer in 100 mL of propylene-glycol-methyl-ether-acetate in a glass crystallization dish. Gently agitate by hand for 10 min to develop the photoresist pattern. Rinse it with isopropanol and dry the wafer under a stream of dry air.</li>
		<li>Hard bake the wafer on a hot plate in two increments: 120 &#176;C for 5 min followed by 150 &#176;C for 10 min. Allow the wafer to cool for 15 min.</li>
	</ol>
	</li>
	<li>Cast PDMS onto the silicon wafer master mold
	<ol>
		<li>Mix a total of 30 g of the PDMS elastomer and curing agent in a 5:1 ratio inside a dust-free disposable container.</li>
		<li>Degas the PDMS in a vacuum desiccator for 30 min.</li>
		<li>Pour the PDMS onto the photoresist-patterned silicon wafer master mold in a 150 mm glass Petri dish.</li>
		<li>Place the Petri dish containing the wafer and PDMS in an 80 &#176;C oven for 1 h.</li>
		<li>Remove the Petri dish from the oven and allow the contents to reach room temperature. 
		NOTE: The procedure may be paused at this point.</li>
	</ol>
	</li>
	<li>Prepare the PDMS mold for pattern transfer to optical adhesive
	<ol>
		<li>Carefully cut the PDMS mold out using a scalpel, and peel the mold away from the wafer.</li>
		<li>Clean and protect the PDMS mold using clear adhesive tape. 
		NOTE: The procedure may be paused at this point.</li>
		<li>Place the PDMS mold, pattern-side up, in the bottom of a dust-free 60 mm plastic Petri dish. Allow 10 s for the PDMS to stick to the plastic.</li>
		<li>Protect the surface of the PDMS with clear plastic tape until step 3.1.1. 
		NOTE: Optional: To make the foam generator, repeat steps 2.1. through 2.3.2. for the foam-generator design.</li>
	</ol>
	</li>
</ol>
3. Optical Adhesive Device Fabrication
<ol>
	<li>Cast optical adhesive onto the PDMS mold
	<ol>
		<li>Remove the tape from the patterned surface of the PDMS, and pour optical adhesive into the 150 mm Petri dish to a depth of approximately 0.9 cm above the top surface of the PDMS mold. Gently remove any bubbles with any type of cotton swab.</li>
	</ol>
	</li>
	<li>Cure the optical adhesive under UV light for a total of 40 min as outlined in steps 3.2.1 - 3.2.5 in a PSD-UV system. 
	Caution: Wear appropriate protection when working with UV light.
	<ol>
		<li>Expose the Petri dish to UV light (254 nm) for 5 min.</li>
		<li>Invert the Petri dish such that the bottom is now facing the UV source, and expose the under-side to UV light for 5 min.</li>
		<li>Invert the Petri dish, return it to the upright position, and re-expose the top side to UV light for 5 min.</li>
		<li>Invert the Petri dish upside down again, and re-expose the bottom side to UV light for 10 min.</li>
		<li>Invert the Petri dish back to the upright position, and re-expose the top side to UV light for 15 min. 
		NOTE: The curing procedure in steps 3.2.1 through 3.2.5 is only applicable when the specified PSD-UV apparatus is used (Table of Materials). Cure times will vary depending on the specific lamp that is used and on the exact thickness of the optical adhesive layer.</li>
	</ol>
	</li>
	<li>Remove the cured optical adhesive from the PDMS mold
	<ol>
		<li>Use a box cutter to carefully break the optical adhesive out of the Petri dish mold. 
		Caution: Box cutter blades are very sharp and can easily cut flesh. Be careful when working around the sharp edges of broken Petri dishes.</li>
		<li>Use a sturdy pair of scissors to remove excess optical adhesive from the edge of the design.</li>
		<li>Slowly peel the PDMS mold away from the optical adhesive puck. Protect the patterned portions of the optical adhesive surface and the PDMS surface with clear tape.</li>
		<li>Use a 1.0 mm biopsy punch to create inlet, outlet, and drain holes. Protect the patterned optical adhesive with clear tape.</li>
	</ol>
	</li>
	<li>Prepare the substrate
	<ol>
		<li>Dispense 1 mL of optical adhesive onto a new glass slide, and spin-coat the slide in two steps: 500 rpm for 5 s then 4,000 rpm for 20 s.</li>
		<li>Quickly transfer the substrate to the UV light treatment, and partially cure the thin optical adhesive layer under UV light for 30 s.</li>
	</ol>
	</li>
	<li>Bond the optical adhesive cast to the substrate
	<ol>
		<li>Place the optical adhesive cast, patterned side up, and the substrate, coated-side up, in an O2 plasma cleaner. Plasma clean the surface for 20 s at 540 mTorr.</li>
		<li>Firmly press the two treated surfaces together until all undesired air pockets have been minimized or removed.</li>
		<li>Fully cure the device under UV light for 20 min. 
		Caution: For UV light, wear appropriate protection such as protective glasses, lab coat, gloves, etc.</li>
		<li>Place the device on a hot plate at 50 &#176;C for 18 h.</li>
	</ol>
	</li>
	<li>Insert a 6-inch long segment of 0.58 mm ID low density polyethylene tubing (PE/3) into each of the ports on the device.</li>
	<li>Use a 5 min quick-set epoxy to secure the tubing in place. 
	NOTE: Optional: To complete the foam generator, repeat steps 3.5.1, 3.5.2, 3.6, and 3.7. using the foam-generator PDMS cast and a new glass slide, instead of the optical adhesive cast and prepared substrates, respectively.</li>
</ol>
4. Oil Displacement Experiment
<ol>
	<li>Prepare the microfluidic device to be imaged on an inverted microscope equipped with a high-speed camera. Fix the device to the microscope stage using tape. Using a 4X objective, focus on the area of interest (AOI).</li>
	<li>Prepare the injection fluids 
	NOTE: For three-phase systems, a dye should be added to clear displacing fluids to provide color contrast for image analysis.
	<ol>
		<li>Load 3 mL of crude oil or model oil sample into a 10 mL glass syringe equipped with a 23 gauge industrial dispensing tip. Secure the syringe in the syringe pump holder and set the appropriate diameter value on the syringe pump settings.</li>
		<li>Load 1 mL of the displacing fluid into a 3 mL plastic syringe equipped with a 23 gauge industrial dispensing tip. Secure the syringe in the syringe pump holder and set the appropriate diameter value on the syringe pump settings. 
		NOTE: Optional: For foam generation experiments, connect a 10 m long 25 &#181;m diameter glass capillary tube to an N2 gas tank and set the gas pressure to the desired value for the required gas flow rate as obtained from a calibration curve. Allow 10 min for the gas flow to equilibrate.</li>
	</ol>
	</li>
	<li>Saturate the OPTICAL ADHESIVE model porous media device with oil
	<ol>
		<li>Connect the displacing fluid to the inlet of the device by inserting the needle tip into the PE/3 tubing. 
		NOTE: Optional: When foam is used as the displacing phase, connect the displacing fluid syringe to the inlet of the foam generator. Connect the gas capillary to the second inlet port on the foam generator by inserting the capillary tube into a 23-gauge industrial dispensing tip and sealing the annulus with quick-set epoxy. The outlet of the foam generator is then connected to the inlet of the optical adhesive device using a 23-gauge connector.</li>
		<li>Connect the oil-filled syringe to the inlet of the device by inserting the needle tip into the PE/3 tubing.</li>
		<li>Begin flowing the oil into the outlet port of the OPTICAL ADHESIVE device at 2 mL/h while simultaneously flowing the displacing fluid into the inlet port at 0.8 mL/h such that the two fluids both flow out the drain port. The displacing fluid should not enter the porous media. Collect the effluent in a 20 mL glass vial.</li>
	</ol>
	</li>
	<li>Begin filming the AOI on the porous media device at a frame rate fast enough to capture the desired phenomena. A typical frame rate is 50 fps. Capture a still image of the 100% oil-saturated area.</li>
	<li>Swiftly and simultaneously cut the PE/3 tubing that is flowing in the oil using scissors while clamping the drain tube with a 5 cm binder clamp.</li>
	<li>Allow the displacing fluid to invade the device until either the oil displacement reaches steady-state or the camera runs out of memory.</li>
</ol>
5. Image and Data Analysis
<ol>
	<li>Use a free image analysis software such as Image J or use the image analysis toolbox in MATLAB to analyze the footage from the experiment.
	<ol>
		<li>Using the still image of the 100% oil-saturated channel, calculate the porosity in units of percent for the porous media AOI.</li>
	</ol>
	</li>
	<li>Calculate the pore volume using the following equation: 
	<img alt="Equation 1" src="/files/ftp_upload/56592/56592eq1.jpg" /></li>
	<li>Use image analysis software to determine the oil saturation, as a fraction of the total flow space, in each frame of the video footage from the experiment. For two phase displacement experiments, the displacing phase saturation in each frame may be calculated as: 
	<img alt="Equation 2" src="/files/ftp_upload/56592/56592eq2.jpg" /></li>
	<li>Prepare a plot of oil saturation in percent vs. pore volumes of injected fluid 
	NOTE: Optional: For three phase systems such as those of foam displacement experiments, use the MATLAB image analysis toolbox to categorize each displacing phase by color using the characteristic RGB range for each phase. Prepare a plot showing the saturations of all three phases with injected pore volumes.</li>
</ol>

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The authors have nothing to disclose.

This protocol for studying oil recovery processes in optical adhesive micromodels strikes a balance between the robustness of non-polymeric micromodels &#8211; such as glass or silicon &#8211; and the facile fabrication of PDMS microfluidic devices. Unlike micromodels made of glass or optical adhesive, PDMS devices lack resistance to light organic species. PDMS micromodels are also not ideal for many experiments because the surfaces of these devices have unstable wetting properties, and the polymer matrix is permeable to...

The purpose of this method is to visualize and analyze multi-phase, multi-component fluid interactions and complex pore-scale dynamics in porous media. Fluid flow and transport in porous media have been of interest for many years because these systems are applicable to several subsurface processes such as oil recovery, aquifer remediation, and hydraulic fracturing1,2,3,4,5. Using micromodels to mimic these complex pore-structures, unique insights are gained by visualizing pore-level dynamic events between the different fluid phases and the media6,7,8,9,10,11.
The fabrication of traditional silica-based micromodels is expensive, time consuming, and challenging, yet constructing micromodels from optical adhesive offers a relatively inexpensive, fast, and easy alternative12,13,14,15. Compared with other polymer-based micromodels, optical adhesive exhibits more stable surface wetting properties. For example, polydimethylsiloxane (PDMS) micromodel surfaces will quickly become hydrophobic during the course of a typical displacement experiment16. Furthermore, the Young&#39;s modulus of PDMS is 2.5 MPa whereas that of optical adhesive is 325 MPa13,17,18. Thus, optical adhesive is less prone to pressure induced deformation and channel failure. Importantly, cured optical adhesive is much more resistant to swelling by low molecular weight organic components, which allows experiments involving crude oil and light solvents to be conducted18. Overall, optical adhesive is a superior alternative to PDMS for displacement studies involving crude oil when silica-based micromodels are prohibitively complex or expensive and high temperature and pressure studies are not required.
The protocol described in this publication provides the step-by-step fabrication instructions for optical adhesive micromodels and reports the subtle tricks that ensure success in the manipulation of small quantities of fluids. The design and fabrication of optical adhesive based micromodels with soft lithography is first described. Then, the fluid displacement strategy is given for ultra-low flow rates that are commonly unattainable with mass flow controllers. Next, a representative experimental result is given as an example. This experiment reveals foam destabilization and propagation behavior in the presence of crude oil and heterogeneous porous media. Lastly, typical image processing and data analysis is reported.
The method provided here is appropriate for visualization applications involving multi-phase flow and interactions in confined microchannel spaces. Specifically, this method is optimized for characteristic micro-feature resolutions greater than 5 and less than 700 &#181;m. Typical flow rates are on the order of 0.1 to 1 mL/h. In studies of crude oil or light solvent displacement by aqueous or gaseous fluids on the order of these optimized parameters at ambient conditions, this protocol should be appropriate.

<table><tbody><tr><td>3 mL Leur-Lok Syringe</td><td>Fischer Scientific</td><td>14-823-435</td><td></td></tr><tr><td>10 mL Glass Syringe</td><td>Fischer Scientific</td><td>1482698G</td><td></td></tr><tr><td>Photomask</td><td>CAD/Art Services</td><td></td><td></td></tr><tr><td>Silicon Wafer</td><td>University Wafer</td><td>452</td><td></td></tr><tr><td>Propylene-Glycol-Methyl-Ether-Acetate&amp;nbsp;</td><td>Sigma Aldrich</td><td>484431-4L</td><td></td></tr><tr><td>150 mm Glass Petri Dish</td><td>Carolina Biological Supply</td><td>#721134</td><td></td></tr><tr><td>60 mm Plastic Petri Dish</td><td>Carolina Biological Supply</td><td>#741246</td><td></td></tr><tr><td>Mask Aligner</td><td>EV Group</td><td>EVG 620</td><td></td></tr><tr><td>1 mm Biopsy Punch</td><td>Miltex, Plainsboro, NJ</td><td>69031-01</td><td></td></tr><tr><td>Industrial Dispensing Tip</td><td>CML Supply</td><td>Gauge 23</td><td></td></tr><tr><td>Inverted Microscope</td><td>Olympus</td><td>IX-71</td><td></td></tr><tr><td>Plasma System</td><td>Harrick Plasma</td><td>PDC-32G</td><td>Plasma cleaner</td></tr><tr><td>Polydimehtylsiloxane (PDMS)</td><td>Dow Corning, Midland, MI</td><td>SYLGARD 184</td><td></td></tr><tr><td>Norland Optical Adhesive 81 (NOA81) or (OA)</td><td>Norland Products Inc.</td><td>8116</td><td>Optical adhesive</td></tr><tr><td>Quick-Set Epoxy</td><td>Fisher Scientific</td><td>4001</td><td></td></tr><tr><td>Glass Slides</td><td>Globe Scientic Inc.</td><td>1321</td><td></td></tr><tr><td>SU-8 2015 Photoresist</td><td>MicroChem</td><td>SU-8 2015</td><td>Photo resist</td></tr><tr><td>Syringe Pump</td><td>Harvard Apparatus</td><td>Fusion 400</td><td></td></tr><tr><td>Glass Capillary Tubing</td><td>SGE Analytical Science</td><td>1154710C</td><td></td></tr><tr><td>High-Speed Camera</td><td>Vision Research</td><td>V 4.3</td><td></td></tr><tr><td>Polyethylene Tubing</td><td>Scientific Commodities Inc.</td><td>#BB31695-PE/3</td><td></td></tr></tbody></table>

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.

In this example experiment, aqueous foam is used to displace Middle East crude oil (with a viscosity of 5.4 cP and API gravity of 40&#176;) in a heterogeneous porous media with layered permeability contrast. A PDMS foam generator is connected to an optical adhesive micromodel which was previously completely saturated with crude oil. Figure 1a shows the CAD design of the photomask for the PDMS foam generator, the photoresist-patterned silicon wafer, and the co...

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

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 ...

microfluidic-devices-for-characterizing-pore-scale-event-processes

Research

JoVE Journal

Environment

10.4K Views.  Rice University. Het doel van deze procedure is te gemakkelijk en snel produceren een microfluidic-apparaat met aanpasbare geometrie en weerstand tegen de zwelling door organische vloeistoffen voor olie herstel studies. Een Polydimethylsiloxaan schimmel is eerst gegenereerd, en vervolgens gebruikt om het apparaat op basis van epoxy gegoten. Een representatieve verplaatsing studie wordt gerapporteerd.

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

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 ...

Engineering

Capillary-based Centrifugal Microfluidic Device for Size-controllable Formation of Monodisperse Microdroplets

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal microfluidic device. W/O microdroplets have recently been used in powerful methods that enable miniaturized chemical experiments. Therefore, developing a versatile method to yield monodisperse W/O microdroplets is needed. We have developed a method for generating monodisperse W/O microdroplets based on a capillary-based centrifugal axisymmetric co-flowing microfluidic device. We succeeded in controlling the size of microdroplets by adjusting the capillary orifice. Our method requires equipment that is easier-to-use than with other microfluidic techniques, requires only a small volume (0.1-1 &#181;l) of sample solution for encapsulation, and enables the production of hundreds of thousands number of W/O microdroplets per second. We expect this method will assist biological studies that require precious biological samples by conserving the volume of the samples for rapid quantitative analysis biochemical and biological studies.

Here, we demonstrate a simple production method for size-controllable, monodisperse, water-in-oil (W/O) microdroplets using a capillary-based centrifugal microfluidic device. This method requires only a small sample volume and enables high-yield production. We expect this method will be useful for rapid biochemical and cellular analyses.

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal ...

Combining Fluidic Devices with Microscopy and Flow Cytometry to Study Microbial Transport in Porous Media Across Spatial Scales

Understanding the transport, dispersion and deposition of microorganisms in porous media is a complex scientific task comprising topics as diverse as hydrodynamics, ecology and environmental engineering. Modeling bacterial transport in porous environments at different spatial scales is critical to better predict the consequences of bacterial transport, yet current models often fail to up-scale from laboratory to field conditions. Here, we introduce experimental tools to study bacterial transport in porous media at two spatial scales. The aim of these tools is to obtain macroscopic observables (such as breakthrough curves or deposition profiles) of bacteria injected into transparent porous matrices. At the small scale (10-1000 &#181;m), microfluidic devices are combined with optical video-microscopy and image processing to obtain breakthrough curves and, at the same time, to track individual bacterial cells at the pore scale. At larger scale, flow cytometry is combined with a self-made robotic dispenser to obtain breakthrough curves. We illustrate the utility of these tools to better understand how bacteria are transported in complex porous media such as the hyporheic zone of streams. As these tools provide simultaneous measurements across scales, they pave the way for mechanism-based models, critically important for upscaling. Application of these tools may not only contribute to the development of novel bioremediation applications but also shed new light on the ecological strategies of microorganisms colonizing porous substrates.

Breakthrough curves (BTCs) are efficient tools to study the transport of bacteria in porous media. Here we introduce tools based on fluidic devices in combination with microscopy and flow cytometric counting to obtain BTCs.

Understanding the transport, dispersion and deposition of microorganisms in porous media is a complex scientific task comprising topics as diverse as ...

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 ...

Pore-scale Imaging and Characterization of Hydrocarbon Reservoir Rock Wettability at Subsurface Conditions Using X-ray Microtomography

In situ wettability measurements in hydrocarbon reservoir rocks have only been possible recently. The purpose of this work is to present a protocol to characterize the complex wetting conditions of hydrocarbon reservoir rock using pore-scale three-dimensional X-ray imaging at subsurface conditions. In this work, heterogeneous carbonate reservoir rocks, extracted from a very large producing oil field, have been used to demonstrate the protocol. The rocks are saturated with brine and oil and aged over three weeks at subsurface conditions to replicate the wettability conditions that typically exist in hydrocarbon reservoirs (known as mixed-wettability). After the brine injection, high-resolution three-dimensional images (2 &#181;m/voxel) are acquired and then processed and segmented. To calculate the distribution of the contact angle, which defines the wettability, the following steps are performed. First, fluid-fluid and fluid-rock surfaces are meshed. The surfaces are smoothed to remove voxel artefacts, and in situ contact angles are measured at the three-phase contact line throughout the whole image. The main advantage of this method is its ability to characterize in situ wettability accounting for pore-scale rock properties, such as rock surface roughness, rock chemical composition, and pore size. The in situ wettability is determined rapidly at hundreds of thousands of points. 
The method is limited by the segmentation accuracy and X-ray image resolution. This protocol could be used to characterize the wettability of other complex rocks saturated with different fluids and at different conditions for a variety of applications. For example, it could help in determining the optimal wettability that could yield an extra oil recovery (i.e., designing brine salinity accordingly to obtain higher oil recovery) and to find the most efficient wetting conditions to trap more CO2 in subsurface formations.

This protocol is presented to characterize the complex wetting conditions of an opaque porous medium (hydrocarbon reservoir rock) using three-dimensional images obtained by X-ray microtomography at subsurface conditions.

In situ wettability measurements in hydrocarbon reservoir rocks have only been possible recently. The purpose of this work is to present a protocol to ...

A Microfluidic Platform to Study Bioclogging in Porous Media

Bacterial biofilms are found in several environmental and industrial porous media, including soils and filtration membranes. Biofilms grow under certain flow conditions and can clog pores, thereby redirecting the local fluid flow. The ability of biofilms to clog pores, the so-called bioclogging, can have a tremendous effect on the local permeability of the porous medium, creating a pressure buildup in the system, and impacting the mass flow through it. To understand the interplay between biofilm growth and fluid flow under different physical conditions (e.g., at different flow velocities and pore sizes), in the present study, a microfluidic platform is developed to visualize biofilm development using a microscope under externally-imposed, controlled physical conditions. The biofilm-induced pressure buildup in the porous medium can be measured simultaneously using pressure sensors and, later, correlated with the surface coverage of the biofilm. The presented platform provides a baseline for a systematic approach to investigate bioclogging caused by biofilms in porous media under flow conditions and can be adapted to studying environmental isolates or multispecies biofilms.

The present protocol describes a microfluidic platform to study biofilm development in quasi-2D porous media by combining high-resolution microscopy imaging with simultaneous pressure difference measurements. The platform quantifies the influence of pore size and fluid flow rates in porous media on bioclogging.

Bacterial biofilms are found in several environmental and industrial porous media, including soils and filtration membranes. Biofilms grow under certain ...

Dissolved Solute Sampling Across an Oxic-Anoxic Soil-Water Interface Using Microdialysis Profilers

Biogeochemical processes shift rapidly in both spatial (millimeter scale) and temporal (hour scale to day scale) dimensions at the oxic-anoxic interface in response to disturbances. Deciphering the rapid biogeochemical changes requires in situ, minimally invasive tools with high spatial and temporal sampling resolution. However, the available passive sampling devices are not very useful in many cases either due to their disposable nature or the complexity and extensive workload for sample preparation.
To address this problem, a microdialysis profiler with 33 individual polyethersulfone nanomembrane tubes (semipermeable, &#60;20 nm pore size) integrated into the one-dimensional skeleton (60 mm) was established to iteratively sample the dissolved compounds in porewater across the soil-water interface at a high resolution of 1.8 mm (outer diameter plus one spacing, i.e., 0.1 mm between probes). The sampling mechanism is based on the principle of concentration gradient diffusion. The automatic loading of degassed water allows minimal disturbance to the chemical species across the oxic-anoxic interface.
This paper describes the procedures of device setup and continuous porewater sampling across the soil-water interface on a daily basis. Concentration-depth profiles were selectively measured before (on Day 6) and after (on Day 7) disturbances induced by irrigation. The results showed that concentration-depth profiles were undergoing rapid changes, especially for redox-sensitive elements (i.e., iron and arsenic). These protocols can help investigate the biogeochemical responses across the soil-water interface under various disturbances caused by physical, chemical, and biological factors. The paper thoroughly discusses the advantages and disadvantages of this method for potential use in the environmental sciences.

A microdialysis profiler is described to sample dissolved porewater solutes across an oxic-anoxic soil-water interface in situ with minimal disturbance. This device is designed to capture rapid changes in concentration-depth profiles induced by disturbances at the soil-water interface and beyond.

Biogeochemical processes shift rapidly in both spatial (millimeter scale) and temporal (hour scale to day scale) dimensions at the oxic-anoxic interface ...

A Microfluidic Platform for Precision Small-volume Sample Processing and Its Use to Size Separate Biological Particles with an Acoustic Microdevice

A major advantage of microfluidic devices is the ability to manipulate small sample volumes, thus reducing reagent waste and preserving precious sample. However, to achieve robust sample manipulation it is necessary to address device integration with the macroscale environment. To realize repeatable, sensitive particle separation with microfluidic devices, this protocol presents a complete automated and integrated microfluidic platform that enables precise processing of 0.15&#8211;1.5 ml samples using microfluidic devices. Important aspects of this system include modular device layout and robust fixtures resulting in reliable and flexible world to chip connections, and fully-automated fluid handling which accomplishes closed-loop sample collection, system cleaning and priming steps to ensure repeatable operation. Different microfluidic devices can be used interchangeably with this architecture. Here we incorporate an acoustofluidic device, detail its characterization, performance optimization, and demonstrate its use for size-separation of biological samples. By using real-time feedback during separation experiments, sample collection is optimized to conserve and concentrate sample. Although requiring the integration of multiple pieces of equipment, advantages of this architecture include the ability to process unknown samples with no additional system optimization, ease of device replacement, and precise, robust sample processing.

This protocol describes a system architecture for performing automated small volume (0.15&#8211;1.5 ml) particle separations using a microfluidic device, and discusses methods to optimize acoustofluidic device performance and operation.

A major advantage of microfluidic devices is the ability to manipulate small sample volumes, thus reducing reagent waste and preserving precious sample. ...

Bioengineering

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.

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.

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

Window on a Microworld: Simple Microfluidic Systems for Studying Microbial Transport in Porous Media

Microbial growth and transport in porous media have important implications for the quality of groundwater and surface water, the recycling of nutrients in the environment, as well as directly for the transmission of pathogens to drinking water supplies. Natural porous media is composed of an intricate physical topology, varied surface chemistries, dynamic gradients of nutrients and electron acceptors, and a patchy distribution of microbes. These features vary substantially over a length scale of microns, making the results of macro-scale investigations of microbial transport difficult to interpret, and the validation of mechanistic models challenging. Here we demonstrate how simple microfluidic devices can be used to visualize microbial interactions with micro-structured habitats, to identify key processes influencing the observed phenomena, and to systematically validate predictive models. Simple, easy-to-use flow cells were constructed out of the transparent, biocompatible and oxygen-permeable material poly(dimethyl siloxane). Standard methods of photolithography were used to make micro-structured masters, and replica molding was used to cast micro-structured flow cells from the masters. The physical design of the flow cell chamber is adaptable to the experimental requirements: microchannels can vary from simple linear connections to complex topologies with feature sizes as small as 2 &mu;m. Our modular EcoChip flow cell array features dozens of identical chambers and flow control by a gravity-driven flow module. We demonstrate that through use of EcoChip devices, physical structures and pressure heads can be held constant or varied systematically while the influence of surface chemistry, fluid properties, or the characteristics of the microbial population is investigated. Through transport experiments using a non-pathogenic, green fluorescent protein-expressing Vibrio bacterial strain, we illustrate the importance of habitat structure, flow conditions, and inoculums size on fundamental transport phenomena, and with real-time particle-scale observations, demonstrate that microfluidics offer a compelling view of a hidden world.

Microfluidic devices can be used to visualize complex natural processes in real time and at the appropriate physical scales. We have developed a simple microfluidic device that mimics key features of natural porous media for studying growth and transport of bacteria in the subsurface.

Microbial growth and transport in porous media have important implications for the quality of groundwater and surface water, the recycling of nutrients in ...

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

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