Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs
July 2nd, 2020
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.
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.
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Overview
This paper describes a protocol along with a comparative study of two microfluidic fabrication techniques suitable for high-pressure conditions. The techniques enable direct observation of fluid flow in surrogate permeable media and fractured systems under reservoir conditions.
Key Study Components
Area of Science
- Microfluidics
- High-pressure systems
- Fluid dynamics
Background
- Collaboration between the University of Kansas and the University of Wyoming.
- Development of a protocol for testing polyelectrolyte complex nanoparticles.
- Utilization of state-of-the-art facilities for high-pressure microfluidics.
- Support from NSF and DOE for the project.
Purpose of Study
- To compare microfluidic fabrication techniques.
- To enable testing under high-pressure conditions.
- To facilitate direct observation of fluid flow in complex media.
Methods Used
- Photolithography
- Wet-etching
- Thermal-bonding
- Selective Laser-induced Etching (SLE)
Main Results
- Comparison of two microfluidic fabrication techniques.
- Demonstration of their applicability under high-pressure conditions.
- Insights into fluid flow behavior in surrogate permeable media.
- Establishment of a collaborative framework for future research.
Conclusions
- The study provides valuable protocols for high-pressure microfluidics.
- Both techniques show promise for future applications in fluid dynamics research.
- Collaboration enhances the capabilities of both research groups.
Frequently Asked Questions
Executive Industry Relevance
This work addresses a critical gap in microfluidic platform capabilities by enabling direct visualization of multiphase flow under high-pressure and high-temperature conditions relevant to subsurface reservoirs. The described fabrication techniques provide a pathway for biopharma R&D teams to develop robust lab-on-chip systems for studying complex fluid behavior in surrogate permeable media, supporting mechanistic de-risking in early discovery and assay development workflows. By establishing protocols for chemically and physically resistant microfluidic devices, the study enhances predictive confidence in modeling transport phenomena that inform target validation and preclinical decision-making.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses through direct observation of fluid-structure interactions in complex geometries under physiologically relevant pressure conditions.
- Operational Value: Supports biological de-risking by providing reproducible platforms for functional target validation in surrogate systems that mimic in vivo microenvironments.
- Predictive Value: Enhances portfolio triage by generating quantitative data on foam stability and transport dynamics that inform lead identification and mechanistic de-risking.
Screening & Assay Development
- Scientific Value: Prepares validated biological systems for downstream workflows by enabling high-resolution observation of multiphase flow at 10 µm resolution, critical for assay standardization.
- Operational Value: Addresses assay reproducibility and scalability through fabrication techniques that yield chemically resistant devices tolerant of extreme conditions, supporting reliable compound evaluation.
- Platform Reuse: Highlights scalability and platform reuse potential, as both photolithography/wet-etching and SLE techniques produce devices suitable for repeated high-pressure testing cycles.
Translational & Preclinical Research
- Translational Continuity: Discusses disease relevance and translational biomarker alignment by linking scCO2 foam behavior observations to enhanced oil recovery processes, offering a proxy for studying complex fluid transport in heterogeneous media.
- Risk-Adjusted Advancement: Describes continuity from discovery through preclinical validation by establishing protocols that withstand reservoir-like conditions, enabling risk-adjusted decisions in preclinical model selection.
- Mechanistic De-risking: Focuses on predictive de-risking value when applied mechanistically, as the method clarifies pathway-level interactions in fractured systems under controlled variables.
Pipeline & Workflow Integration
The method integrates into the discovery continuum from Early Discovery through Lead Identification to Preclinical work, supporting hypothesis testing, assay readiness, and quantitative analytics that inform translational continuity and enterprise reuse.
- Discovery Biology: Explains how the method supports hypothesis testing, pathway clarification, and biological de-risking by enabling direct visualization of complex fluid interactions in surrogate permeable media under high pressure.
- Screening: Describes assay readiness, reproducibility, and quantitative outputs through the use of high-resolution monochromatic sensors and image-based analysis of foam microstructure and stability.
- Analytics: Highlights measurements, readouts, and statistical outputs such as bubble diameter distribution via ImageJ, enabling teams to compare conditions and assess foam generation under varying flow rates.
- Translational Research: Connects the method to preclinical continuity by linking observed scCO2 foam transport to reservoir-scale processes, supporting biomarker alignment in studies of fluid recovery from unconventional formations.
- Enterprise Reuse: Frames the method as a reusable capability rather than a single-use technique, as both fabrication pathways yield devices capable of withstanding repeated high-pressure and thermal cycling.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence, target validation, reduction of mechanistic ambiguity in multiphase flow studies.
- Operational Value: Standardization, reproducibility, and scalability of microfluidic platforms for high-pressure applications.
- Strategic Value: Better go/no-go decisions, capital efficiency, and reduced late-stage biological risk through early-stage mechanistic insights.
- Portfolio Impact: Risk-adjusted prioritization and advancement decisions based on quantitative transport data generated under reservoir-mimicking conditions.
Implementation Considerations
- Required scientific expertise in microfluidic fabrication, photolithography, wet-etching, and thermal bonding processes.
- Instrumentation and analytical infrastructure needs including UV exposure systems, etching solutions, pressure-resistant holders, and high-resolution imaging systems.
- Cross-team standardization requirements for protocol execution across multidisciplinary teams involving chemistry, engineering, and imaging specialists.
- Adaptation considerations across model systems, noting that while optimized for glass substrates and scCO2 foam, the techniques may require modification for soft biological materials or aqueous-based assays.
- Practical limitations supported by source material: photolithography/wet-etching enables complex channel networks challenging for laser etching, but both techniques require careful handling of hazardous chemicals such as piranha solution and NMP during fabrication.
