This protocol is a multi-disciplinary collaborative effort between the University of Kansas and the University of Wyoming. We had a need for testing of our patented, polyelectrolyte complex nanoparticles using high pressure microfluidics. And it started a collaborative effort between our research group and Dr.Aryana's research group to take advantage of his state-of-the-art facilities.
We would like to acknowledge NSF for funding part of this project, and thank the tertiary or recovery program for providing their facilities and support needed for this project. In addition, this work was in part supported by CMCUF and the Energy Frontier Research Center funded by the DOE. This protocol and its application to surrogate complex permeable media and their high pressure conditions, is the result of a collaboration between two research groups one from the University of Wyoming and the other from the University of Kansas.
This protocol details, the fabrication procedure for a robust microfluidic platform in the absence of clean room facilities. This platform enables direct visualization of flow in complex geometries under high pressure conditions. The described technique can be used to age complex channel networks in glass substrates that mimic a structures from subsurface media and withstand high pressure conditions experienced during CO2 utilization and storage.
This method may be used to provide insight into intersections and transport of complex fluids in impermeable media in the context of CCUS. When performing this protocol patients and extreme caution are required to minimize the risk of physical injury and failures in the fabrication process. Visual demonstration of this method enables effective communication of the steps that are critical in its successful execution.
Begin by pouring an adequate amount of chrome etching solution into a beaker and heating it to approximately 40 degrees Celsius. Then place the mask directly on the side of the borosilicate substrate that is covered with chrome and photo resist. Transfer the geometrical pattern into the layer of photo resist by exposing the stack of the substrate and the mask to UV light.
Remove the photo mask and substrate stack from the UV stage then remove the photo mask and submerge the substrate and the developer solution for approximately 40 seconds. Cascade rinse the substrate by flowing deionized water on top of the substrate and over all of its surfaces at least three times. After allowing the substrate to dry, submerge it in the preheated chrome etchant for approximately 40 seconds thereby transferring the pattern from the photo resist to the chrome layer.
Remove the substrate from the solution, cascade rinse it with deionized water and allow it to dry. Using a brush, apply several layers of HMDS to the uncovered face of the substrate and allow it to dry. Apply one layer of photo resist on top of the primer, then place the substrate in an oven at 60 to 90 degrees Celsius for 30 to 40 minutes.
Afterwards, leave the pattern substrate in the etchant solution for a predetermined amount of time, based on the desired channel depths. Remove the substrate from the etchant, using a solvent resistant pair of tweezers and cascade rinse it with deionized water. Expose the substrate to NMP solution preheated to 65 degrees Celsius for approximately 30 minutes to remove the photo resist.
Cascade rinse it with acetone followed by ethanol and deionized water. Place the clean substrate in chrome etchant and heated to approximately 40 degrees Celsius for about one minute. Then characterize the channel depth using laser scanning, confocal microscopy.
Mark the positions of the inlet and outlet holes on a blank borosilicate substrate by aligning the cover plate against the etched substrate. Use a micro abrasive sandblaster and 50 micro meter aluminum oxide micro sandblasting media to blast through holes in the marked locations. Cascade rinse both the etched substrate and the cover plate with deionized water.
Then bring a one to four hydrogen peroxide sulfuric acid piranha solution to a boil and submerge the substrate and the cover plate in the solution for 10 minutes. After rinsing the substrate and cover plate, submerge them in the buffer etchant for 30 to 40 seconds and rinse them again. Next, submerge the substrate and cover plate for 10 minutes in a six to one to one water, hydrogen peroxide, hydrochloric acid solution that is heated to approximately 75 degrees Celsius.
Press the substrate and cover plate tightly against each other while submerged then remove them from the solution, cascade rinse them and submerge them in deionized water. Make sure that the substrate and the cover plate are firmly attached to each other, press them against each other and carefully remove them from the water. Align the substrate and cover plate carefully while they are submerged in DI water and repeat the procedure until no air bubbles are observed between the two.
Place the stack substrates between two smooth 1.5 two centimeter thick glass ceramic plates for bonding. Place the glass ceramic plates between two metallic plates made of alloy x. Making sure that the glass wafers and the ceramic metallic holder are centered.
Hand tighten the nuts and place the holder in a vacuum chamber for 60 minutes at approximately 100 degrees Celsius. Then remove the holder from the chamber and carefully tighten the nuts. Place the holder inside a furnace and execute a heating program according to manuscript directions.
Remove the thermally bonded microfluidic device from the furnace and rinse it with water. Then bath sonicate it in hydrochloric acid for one hour. Fill the tanks of the carbon dioxide and water pumps with enough fluids for the experiment and use a syringe to fill the brine accumulator and flow lines with the surfactant solution.
Place the saturated microfluidic device in a pressure resistant holder and connect the inlet and outlet ports to the appropriate lines using 0.01 inch inner diameter tubing. Increase the temperature of the circulating bath, which controls the temperature of the brine and carbon dioxide lines to the desired temperature. Increase the back pressure and brine pump pressure simultaneously in gradual steps while maintaining continuous flow from the outlet of the back pressure regulator.
Increase the pressure up to 7.38 mega pascal and stop the pumps. Increase the carbon dioxide line pressure to a pressure above 7.38 mega pascal then open the carbon dioxide valve and allow the super critical carbon dioxide mixed with the high pressure surfactant solution to flow through an inline mixer and generate foam. Wait until flow is fully developed inside the device and the channels are saturated, monitoring the outlet for the onset of foam generation.
Turn on the camera to capture detailed images of flow inside the channels. Carbon dioxide foam transport and stability in the UV lithography microfluidic device, during the first 20 minutes of generation and isolation is shown here. The multi-phase moved across the micro cracks and foam was generated through the micro-fractures.
The foam was generated in a selective laser induced etching microfluidic device, starting from ambient condition with no flow to fully developed, super critical carbon dioxide foam at high and low flow rates. Foam distribution and stability was imaged under reservoir conditions during the first 20 minutes of generation and isolation. The distribution of bubble diameters was determined using image J.Quantification of the foam microstructure was performed using raw images, post processed images and their binarised equivalents.
When attempting this practical, it is important to carefully examine the pattern on the chrome layer under a yellow light to ensure the desired pattern is transferred correctly. Variations of these methods include high pressure injection of oil, polymer solutions and low salinity brines to develop an understanding of the physics of flow of complex fluids through direct visualization.
