Name: combining microfluidics and microrheology to determine rheological properties of soft matter during repeated phase transitions
Uploaded: 2018-04-19T14:00:00
Description: Watch this Scientific Journal Video about Combining Microfluidics and Microrheology to Determine Rheological Properties of Soft Matter during Repeated Phase Transitions at JoVE.com

Funding for this work was provided by the Procter &amp; Gamble Co. and the American Chemical Society Petroleum Research Fund (54462-DNI7). Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. The authors would like to acknowledge Dr. Marco Caggioni for helpful discussions.

1. Fabrication of the Microfluidic Device
<ol>
	<li>Microfluidic stamp fabrication. 
	Note: This step requires the use of volatile materials and should be done in a chemical fume hood.
	<ol>
		<li>Use a negative printed design with the same dimensions as the glass slide (75&#215;50 mm), the channels colored white, and the background colored black (See Figure 1). Print this design on a clear acetate sheet (transparency) with a resolution of 1200 dpi.</li>
		<li>If the ...

<ol>
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The two-layer microfluidic device (Figure 1) can be easily made by following well-documented microfluidic fabrication techniques29. Glass supports are added to the bottom of the device to decrease vibrational effects on probe movement. The glass slide is very thin (0.10 mm) in order to accommodate the working distance of the microscope objective. This makes the device susceptible to small vibrations in the building and sample environment that are then measured with th...

Phase transitions in soft matter can change the scaffold structure, which has implications in the processing and final stability of the material1,2,3. The characterization of soft materials during dynamic phase transitions provides essential information about the relationship between structural evolution and equilibrium structure and rheological properties. For example, many home care products require a phase change during consumer use. Also, during manufacturing, processing steps, including dilution and mixing, can impart shear affecting the rheological properties and final microstructure of the product. Understanding the rheological properties throughout a phase change ensures that the product performs as designed. Additionally, if forces alter the starting rheology of the material during manufacturing, phase transitions can yield unexpected and undesired results, changing the intended function and effectiveness.At the critical gelation point, defined as the point where the material transitions from a solution of associated colloids or polymers to a sample-spanning gel network, material properties change drastically with slight changes to association. Any modification to the structure at the critical gel point can impact the end product4. During these dynamic transitions, soft materials have weak mechanical properties and measurements that use classical experimental techniques can be within the measurement noise limit5,6,7. To account for this, techniques such as microrheology, which is sensitive in the low moduli range (10-3 - 4 Pa), are used to characterize the weak incipient gel during dynamic evolution. Some materials are susceptible to changes in microstructure due to external forces, which presents a challenge during characterization, as any transfer of material or fluid can affect the structure and, ultimately, the final material properties. To avoid altering the material microstructure, we have developed a microfluidic device that can exchange the environmental fluid around a sample while minimizing shear. By exchanging the fluid environment, changes in rheological properties and microstructure are measured during phase transitions with minimal contributions from shear. The device is combined with multiple particle tracking microrheology (MPT) in a technique called &#181;2rheology. This technique is used to quantify material properties during consecutive phase changes of a gel in response to an external driving force. The technique will be illustrated using a fibrous colloidal gel, hydrogenated castor oil (HCO)9,10,11.
Gel scaffolds can undergo changes in association and dissociation due to their sample environment12,13,14,15. The driving force for gelation and degradation are material specific and must be tailored for each material of interest. &#181;2rheology can be used to characterize gel systems that respond to external stimuli, including colloidal and polymeric networks. Altering pH, osmotic pressure or salt concentration are examples of driving forces that can induce changes in the material microstructure. For example, HCO undergoes controlled phase transitions by creating an osmotic pressure gradient. When a concentrated HCO gel sample (4 wt% HCO) is submerged in water, the attractive forces between colloidal particles weaken, causing degradation. Alternatively, when a dilute solution of HCO (0.125 wt% HCO) is contacted with a hydrophilic material (referred to as the gelling agent and composed of mostly glycerin and surfactant), the attractive forces return, causing gelation. This gel system will be used to show the operation of the device as a tool for measuring consecutive phase transitions on a single sample9,10. To characterize these gel scaffolds during dynamic transitions and the delicate incipient gel structure at the critical phase transition, we use MPT to characterize these materials with high spatio-temporal resolution.
Microrheology is used to determine gel properties and structure, especially at the critical transition, of an array of soft materials, including colloidal and polymeric gels5,6,9,16. MPT is a passive microrheological technique that uses video microscopy to record the Brownian motion of fluorescent probe particles embedded within a sample. The particle positions throughout the videos are precisely determined to within 1/10th of a pixel using classical tracking algorithms17,18. The ensemble averaged mean-squared displacement (MSD, (&#916;r2(t))) is calculated from these particle trajectories. The MSD is related to material properties, such as the creep compliance, using the Generalized Stokes-Einstein Relation17,19,20,21,22,23. The state of the material is determined by calculating the logarithmic slope of the MSD curve as a function of lag time, &#945;,
<img alt="Equation 1" src="/files/ftp_upload/57429/57429eq1.jpg" />
where t is the lag time, and comparing it to the critical relaxation exponent, n. n is determined using time-cure superposition, a well-documented technique that was modified to analyze MPT data by Larsen and Furst6. By comparison of n to &#945; the state of the material is quantitatively determined. When &#945; &#62; n the material is a sol, and when &#945; &#60; n the material is a gel. Previous work has characterized the HCO system using microrheology to determine the critical relaxation exponent9. Using this information, we precisely determine when the material transitions from a gel to a sol during an experiment. Additionally, the non-Gaussian parameter, &#945;NG, can be calculated to determine the extent of structural heterogeneity of a system,
<img alt="Equation 2" src="/files/ftp_upload/57429/57429eq2.jpg" />
where &#916;x(t) is the one-dimensional particle movement in the x direction. Using MPT, we can characterize a single phase transition, but by characterizing materials with MPT in a microfluidic device, we are able to manipulate the surrounding fluid environment and collect data of several phase transitions on a single gel sample.
This microfluidic device is designed to investigate the critical transitions of a single gel sample that undergoes phase changes in response to changes in the surrounding fluid environment. The device exchanges fluid surrounding the sample when it is either in the gel or sol state by locking the sample in place to induce a phase transition while minimizing shear. A solvent basin is located directly above the sample chamber, which are connected by six symmetrically spaced inlet channels. This symmetry allows for the exchange of fluid from the solvent basin to the sample chamber while creating equal pressure around the sample, locking it in place. There have been several studies that use this technique for single particle and DNA trapping, but this work scales up the volume from single molecules to samples that are approximately 10 &#181;L24,25,26. This unique design also enables real-time microrheological characterization during phase transitions.
&#181;2rheology is a robust technique that is applicable to many soft matter systems. The technique described in this paper was designed for colloidal gels, but it can be easily adapted to other materials such as polymer or micellar solutions. With this technique, we determine not just how phase transitions affect the equilibrium material properties, but also how different processing steps can have lasting effects on the rheological evolution of the material and the final scaffold structure and properties.

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			<td height="21" style="height:21px;width:205px;">150 x 15 mm Petri Dish</td>
			<td style="width:195px;">Corning, Inc.</td>
			<td style="width:131px;">351058</td>
			<td style="width:183px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">75 x 50 x 0.15 mm glass slide</td>
			<td style="width:195px;">Fisher Scientific</td>
			<td style="width:131px;">Custom</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">75 x 50 x 1.0 mm glass slide</td>
			<td>Fisher Scientific</td>
			<td>12-550-C</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">75 x 25 x 1.0 mm glass Slide</td>
			<td style="width:195px;">Fisher Scientific</td>
			<td style="width:131px;">12-550-A3</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">22 x 22 Glass cover slips</td>
			<td style="width:195px;">Fisher Scientific</td>
			<td style="width:131px;">12-542-B</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Acetone, 99.5%</td>
			<td style="width:195px;">VWR Analytical</td>
			<td style="width:131px;">67-64-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Low intensity UV source</td>
			<td style="width:195px;">UVP</td>
			<td style="width:131px;">UVL-56</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Chloroform, 99.9%</td>
			<td style="width:195px;">Fisher Chemical</td>
			<td style="width:131px;">C298-500</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Cotton Swabs</td>
			<td style="width:195px;">Q-tips</td>
			<td style="width:131px;">83289205</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Ethanol, 90%</td>
			<td style="width:195px;">Fisher Chemical</td>
			<td style="width:131px;">A962-4</td>
			<td></td>
		</tr>
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			<td height="42" style="height:42px;width:205px;">Fluoresbrite&#174; YG Carboxylate Microspheres 0.50&#181;m</td>
			<td style="width:195px;">Polysciences, Inc.&#160;</td>
			<td>15700-10</td>
			<td></td>
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			<td height="21" style="height:21px;width:205px;">High-Intensity UV Lamp</td>
			<td style="width:195px;">Spectroline Corp.</td>
			<td style="width:131px;">SB-100P</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Hot plate</td>
			<td style="width:195px;">Corning, Inc.</td>
			<td style="width:131px;">PC-420</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Hydrochloric Acid, 6N</td>
			<td style="width:195px;">Ricca Chemical Company</td>
			<td style="width:131px;">3750-32</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Methyltriethyoxysilane, 98%</td>
			<td style="width:195px;">Acros Organics</td>
			<td style="width:131px;">174622500</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Microcentrifuge</td>
			<td style="width:195px;">Eppendorf</td>
			<td style="width:131px;">5424</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Plasma cleaner</td>
			<td style="width:195px;">Harrick Plasma, Inc.</td>
			<td style="width:131px;">PDC-32G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Polydimethylsiloxane (PDMS)</td>
			<td style="width:195px;">Robert McKwown Company</td>
			<td style="width:131px;">2065622</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Sonicator</td>
			<td style="width:195px;">Branson, Emerson Electric</td>
			<td style="width:131px;">1800</td>
			<td></td>
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		<tr height="42">
			<td height="42" style="height:42px;width:205px;">Steel connectors, ID 0.023 inch</td>
			<td style="width:195px;">New England Small Tube Corp.</td>
			<td style="width:131px;">Custom</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Tetraethoxysilane, 98%</td>
			<td style="width:195px;">Alfa Aesar</td>
			<td style="width:131px;">A14965</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Thiol-ene Resin (UV curable)</td>
			<td style="width:195px;">Norland Products, Inc.&#160;</td>
			<td style="width:131px;">NOA81</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:205px;">Transparency</td>
			<td style="width:195px;">Staples Inc.&#160;</td>
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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.

A two-layered microfluidic device is constructed with PDMS (Figure 1a,b), which is patterned on a microfluidic stamp. The design of the stamp is shown in Figure 1c. Improper experimental setup can result both in errors in passive microrheology and microfluidic flows during surrounding fluid exchange (Figure 2). Examples of improper experimental setup are detailed in ...

Watch this Scientific Journal Video about Combining Microfluidics and Microrheology to Determine Rheological Properties of Soft Matter during Repeated Phase Transitions at JoVE.com

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

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 ...

The overall goal of this procedure is to use microfluidics to induce consecutive phase transitions in a single soft matter sample by exchanging the surrounding fluid while simultaneously measuring changes in the rheological properties using multiple particle tracking microrheology. This method can answer key questions in the soft matter field, such as the impact of phase transitions, including molecular or colloidal rearrangement on dynamic rheological properties. This technique's main advantage is that the sample remains in place during multiple phase transitions, enabling real-time characterization.This allows quantification of rheological properties, especially unique properties in heterogeneous systems. To begin the procedure, print the microfluidic device negative on a clear acetate sheet, and warm up a high-intensity UV lamp. On a second clear acetate sheet, trace the corners of a 75 millimeter by 50 millimeter slide.Place one millimeter high glass spacers on the corners. Pour about five milliliters of UV-curable thiol-ene resin in the center of the spacers. Carefully lay a 75 millimeter by 50 millimeter by one millimeter glass slide on the spacers to cover the resin.Ensure that there are no air bubbles between the slide and the resin. Place the printed negative on the glass slide. Carefully drag the assembly under the UV lamp, and allow the resin to cure for 45 seconds.Then, slide the assembly away from the UV lamp, and remove the patterned glass slide or microfluidic stamp. Immerse the microfluidic stamp in acetone for no more than 10 seconds, and then immerse the stamp in ethanol for the same length of time. Repeat this immersion sequence once.Then, submerge the stamp in distilled water. Working with minimal hand movement, use a cotton swab to remove the remaining unreacted resin without touching the cured resin. Replace the cotton swab when it becomes saturated with water.Cure the stamp under UV light for another 30 minutes. Then, mix 70 grams of PDMS base with cross-linking agent in a one-to-10 ratio, and degas the PDMS for one hour or until the PDMS is no longer turbid. Place the microfluidic stamp face up in a 150 millimeter plastic Petri dish.Slowly pour the PDMS into the dish, pouring close to the dish surface to minimize bubble formation, to completely cover the stamp. Cover the Petri dish, and cure the PDMS at 55 degrees Celsius overnight. Use a knife to cut away the excess PDMS.Separate the patterned PDMS from the stamp by hand. Remove the stamp and the excess PDMS from the Petri dish. Then, place the dish on a sheet of paper printed with the locations of the symmetric outer holes for the sample chamber.Align the patterned PDMS with the guide, and use a 5 millimeter biopsy punch to punch holes through the PDMS at the marked locations. The key feature of this device is the symmetry between inlet channels and the same chamber. This enables exchange of fluid without loss of sample.It is important to use a guide pattern to ensure accuracy when cutting the channels. Lastly, punch a hole in the center of the sample chamber, at the inner edge of the suction chamber, and at each end of the long outer channels. Cover the Petri dish when storing the patterned PDMS.One day before the device assembly, prepare the preconverted fluid that will be used to create glass walls in the microfluidic device. To begin the device assembly, plasma treat the patterned PDMS and a thin glass slide for 40 seconds. Carefully press together the plasma-treated faces of the slide and the patterned PDMS.Apply UV-curable resin to the seam between the PDMS and the slide, and cure the resin under low-intensity UV light for five minutes. Next, insert thermoplastic tubing fitted with stainless steel connectors into all but one corner hole of the patterned PDMS. Connect a three milliliter syringe of preconverted fluid to the last hole using an 18 gauge needle, thermoplastic tubing, and a connector.Fill the microfluidic device with preconverted fluid. Then, place the device on a hot plate heated to 100 degrees Celsius, and, over the course of 10 seconds, flow three milliliters of preconverted fluid through the device to fabricate the glass walls. Then, remove the device from the hot plate.Use a 30 milliliter syringe of air to expel excess preconverted fluid. Slowly flow 15 milliliters of chloroform through the device, followed by 30 milliliters of ethanol, each over the course of about one minute. Then, push air through the microfluidic device until dry.Remove the tubing from the holes. Next, use UV-curable resin to attach one millimeter thick glass strips to the undersides of the long edges of the device. Cut a 30 by 30 millimeter piece of excess cured PDMS, and punch a hole wider than the sample chamber in the center to form the solvent basin.Plasma treat the basin and the microfluidic device for 40 seconds. Place the plasma-treated face of the basin on the device with the sample chamber centered in the hole. Apply gentle pressure to fix the basin to the device.Lastly, cut six five millimeter squares from the excess PDMS. Use a 5 millimeter biopsy punch to cut a hole halfway through each square. Insert a stainless steel connector into each hole to form the PDMS stoppers.After loading the soft matter sample into a one milliliter syringe, insert thermoplastic tubing into three of the device's corner inlet channels and the suction chamber outlet. Fill another syringe with deionized water. Attach thermoplastic tubing to the syringe, and expel any air bubbles.Connect the syringe to the fourth corner inlet channel. Fill the device and the solvent basin with deionized water. Ensure that there are no bubbles in the sample chamber or in the microfluidic channels.Top off the water in the solvent basin with a transfer pipette if needed. It is essential that the microfluidic device is set up correctly. Most importantly, ensure that there are no air bubbles in the device, as they can disrupt both the microfluidic flow and the microrheology data.Then, inject 10 microliters of the soft matter sample into the sample chamber via the center inlet at about two microliters per second. Stopper the center inlet after loading the sample. Using an inverted microscope and a high-speed camera, record at least 20 videos of the Brownian motion of the probes in the sample mixture over time intervals appropriate for the duration of the phase transition.For gelation, continue recording videos until probe movement has stopped completely. For degradation, continue recording videos until the probes are completely diffusive. Then, remove the device from the microscope stage.To exchange the water for a higher density fluid, withdraw the water from the solvent basin using a transfer pipette, and pipette four milliliters of the higher density fluid into the basin. Return the device to the microscope stage, and continue recording videos. To exchange a higher density fluid for water or another lower density fluid, connect a syringe to the suction chamber channel via an 18 gauge needle and thermoplastic tubing.Mount the syringe on a syringe pump set to withdraw at one milliliter per minute. Pipette most of the high-density fluid from the solvent basin, and rinse the basin with the low-density fluid three times, never allowing the basin to drain completely. Fill the basin with low-density fluid after rinsing.Fill a transfer pipette with the low-density fluid. Then, use the syringe pump to withdraw the high-density fluid for one minute. Add the low-density fluid dropwise to the basin as needed during the fluid exchange.Then, stopper the suction chamber, and return the device to the microscope stage for recording. Acquire videos of degradation gelation cycles in this way for the desired number of cycles or until there are insufficient probes for measurement. The ensemble averaged mean-squared displacement of probe particle trajectories was tracked during successive phase transitions of a hydrogenated castor oil system between gel and sol.Gelation was induced by gravity flow exchange of water for a higher density gelling agent. Degradation was induced by suction flow exchange of the gelling agent for water. The sample chamber was reflushed with water when the phase behavior suggested that the gelling agent had not been fully removed from the chamber.No changes to the rheological properties were observed when the sample was left in the sol phase with no fluid exchange for about 600 minutes. Repeated phase transitions did not induce observable changes in equilibrium rheological properties, as shown by the logarithmic slopes of the MSD curves or alpha returning to the same values for each phase. The equilibrium alpha value of the sol phase was less than 1.0, indicating that the material retained some structure even after degradation.The gel underwent an increase in structural heterogeneity during the gel to sol phase transition, as indicated by the non-Gaussian parameter peaks during degradation cycles. Colloidal rearrangement could occur immediately after fluid exchange, as indicated by increases in alpha. We first had the idea for this method to determine whether phase transitions were causing permanent changes in rheological properties and microstructure of a soft material.In the HCO system, this device enabled measurement of two possible mechanisms for these permanent changes, colloidal rearrangement during phase transitions or shear induced during sample preparation. Once mastered, this technique can be used to identify the rheological properties of a soft matter system over many phase transitions in response to changes in the surrounding fluid environment.

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