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 resistant to low molecular-weight oil components, unlike traditional polydimethylsiloxane (PDMS) devices. Here, we demonstrate a facile method for making a device with this property, and we use the product of this protocol for examining the pore-scale mechanisms by which foam recovers crude oil. A pattern is first designed using computer-aided design (CAD) software and printed on a transparency with a high-resolution printer. This pattern is then transferred to a photoresist via a lithography procedure. PDMS is cast on the pattern, cured in an oven, and removed to obtain a mold. A thiol-ene crosslinking polymer, commonly used as an optical adhesive (OA), is then poured onto the mold and cured under UV light. The PDMS mold is peeled away from the optical adhesive cast. A glass substrate is then prepared, and the two halves of the device are bonded together. Optical adhesive-based devices are more robust than traditional PDMS microfluidic devices. The epoxy structure is resistant to swelling by many organic solvents, which opens new possibilities for experiments involving light organic liquids. Additionally, the surface wettability behavior of these devices is more stable than that of PDMS. The construction of optical adhesive microfluidic devices is simple, yet requires incrementally more effort than the making of PDMS-based devices. Also, though optical adhesive devices are stable in organic liquids, they may exhibit reduced bond-strength after a long time. Optical adhesive microfluidic devices can be made in geometries that act as 2-D micromodels for porous media. These devices are applied in the study of oil displacement to improve our understanding of the pore-scale mechanisms involved in enhanced oil recovery and aquifer remediation.
The 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'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 µm. 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.
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's chemical safety guidelines.
1. Device Design
2. PDMS Mold Fabrication
3. Optical Adhesive Device Fabrication
4. Oil Displacement Experiment
5. Image and Data Analysis
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°) 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 completed foam generator with inlet and outlet tubes inserted. Figure 1b shows corresponding images for the heterogeneous optical adhesive model porous media with layered permeability contrast. Note the respective transparent and opaque portions of the photomask design. As shown in Figure 2, coarse foam is generated by a flow-focusing geometry into which gas and liquid are co-injected. The total flow rate selected for this demonstration is 0.8 mL/h with a nitrogen gas fractional flow of approximately 90%. The surfactant solution used is a 1:1 ratio of alpha olefin sulfonate C14-16 to lauryl betaine at a concentration of 1 wt%. A 1 wt% concentration of blue food grade dye is used in the aqueous phase to aid in distinguishing this phase from the device posts. A finer foam texture is produced as foam from the flow-focusing section propagated through the homogeneous foam-generator micromodel. Smaller bubbles are typically observed exiting the patterned matrix than those that are made by the flow-focusing geometry alone. Once steady foam generation is achieved, foam flow is then diverted to the optical adhesive micromodel to displace crude oil. Videos of the displacement process were captured at 50 fps by a high-speed camera, which allowed for frame by frame processing of the footage. In Figure 3, saturation profiles for each fluid phase were plotted as a function of total injected fluid pore volumes.
Image processing techniques also enable us to quantify the fluid diversion and foam phase separation in different layers. Capillary forces between different phases will drive more of the liquid to the lower permeability region and more of the gas to the higher permeability region. Figure 4 shows the saturation changes that occurred during the crude oil displacement experiment as a function of total injected fluid pore volumes. As predicted, at steady state, gas saturation was significantly higher in the high-permeability region compared to that in the low-permeability region.
In addition to analyzing the saturation change during the oil displacement experiment, a series of pore-level events such as foam destabilization, bubble generation, oil lamellae formation, and crude oil emulsification could also be easily identified. In Figure 5, some of these foam dynamics in presence of crude oil are shown. In this figure, the bubbles of interest are colored green. Foam is thermodynamically metastable and coalesces in porous media by mechanisms such as capillary suction (Figure 5c), gas diffusion (Figure 5e), thermal, or mechanical fluctuations. Crude oil also has a detrimental effect on foam (Figure 5b and Figure5d). The success of foam flooding depends on various mechanisms for bubble regeneration. We identify in-situ foam generation mechanisms like bubble pinch-off (Figure 5a) and lamella division (Figure 5f).
Figure 1: The fabrication of porous media micromodel devices. (a) PDMS-based Foam Generator: the CAD design, the photoresist mold on a silica wafer, and the completed device; (b) Optical adhesive-based heterogeneous porous media micromodel with layered permeability contrast: the CAD design, the photoresist mold, the PDMS mold, and the completed device. The scale bars indicate approximately one inch. Please click here to view a larger version of this figure.
Figure 2: Foam generation in the PDMS based homogeneous micromodel. Coarse foam is generated through the flow focusing device which becomes finer as foam passes through the device. The scale bar indicates 1 mm. Please click here to view a larger version of this figure.
Figure 3: Characterizing crude oil displacement by foam. (a) Initial condition: 100% oil saturation (crude oil in brown, posts in white); (b) binary background image for the micromodel; (c) a sample frame from the crude oil displacement video; (d) converted image after Matlab processing to distinguish distinct phases where green = gas, blue = aqueous phase, red = oil phase; (e) saturation history (Black arrow indicates the time when (c) was taken). The scale bar indicates 400 µm. Please click here to view a larger version of this figure.
Figure 4: Saturation history at different regions to show fluid diversion and foam phase separation. (a) high permeability region; (b) low permeability region; (c) fracture region. The label on the vertical axes stands for saturation of each phase (%).
Figure 5: Foam dynamics in the presence of crude oil. (a) Foam generation by bubble pinch-off mechanism; (b) bubble coalescence in presence of crude oil; (c) bubble coalescence by capillary suction; (d) foam destruction in the fracture region; (e) foam coarsening by gas diffusion; (f) foam generation by lamella-division. Gas bubbles of interest are colored green. The scale bar indicates 400 µm. Please click here to view a larger version of this figure.
This protocol for studying oil recovery processes in optical adhesive micromodels strikes a balance between the robustness of non-polymeric micromodels – such as glass or silicon – 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 gas19. In contrast, optical adhesive has shown much more stable wettability than PDMS, and it is much less permeable to gas20,21,22. Specifically, the water contact angle of optical adhesive remains stable for days after O2 plasma treatment, compared to hours for PDMS21. Therefore, with minimal extra effort, constructing micromodels of optical adhesive, rather than PDMS, grants better solvent resistance, more stable wetting properties, and lower permeability to gas. optical adhesive replaces neither glass nor silicon micromodels, however, as these materials can withstand much higher temperatures and pressures. Furthermore, optical adhesive microfluidic devices may exhibit bond degradation during long-term experiments14. Given the difficulty and expense of constructing glass and silicon micromodels, optical adhesive is still the material of choice for short-term ambient displacement experiments involving light organic substances. Therefore, employing optical adhesive micromodels for studying oil recovery processes with crude oil is a facile and cost-effective alternative to using labor-intensive glass and silicon micromodels.
Careful attention should be paid to several critical aspects of the photoresist-patterned silicon wafer master-mold preparation portion of the protocol to avoid unsuccessful results. First, best practice dictates ramping the temperatures slowly (5 °C per min) during all baking steps. Fast heating can cause thermal stress fractures in the wafer. Second, photoresist adhesion to the silicon wafer should be promoted as necessary. When using a new wafer, separation incidents should occur infrequently, but if separation of the cured photoresist from the wafer is a problem, then preventative measures can be taken. A quick isopropyl alcohol rinse followed by a pre-bake step at 110 °C for 10 min can result in better photoresist affinity for the surface of the wafer. Third, note that the parameters given in the procedure for UV dosage, baking times, baking temperatures, and developing times can be sensitive to changes in environmental conditions, instrument brand, and chemical batch number. Thus, resources should be allocated for several trials to tune these important parameters to eliminate issues such as over-polymerization, under-developed features, unresolved features, or poor adhesion to the wafer. Provided these tips are taken into consideration, silicon wafers should be successfully patterned with relative ease.
Later in the protocol, several nuances of the device fabrication and experimental steps of this procedure can contribute significantly to successful results. For example, the nonstandard PDMS component ratio offers a couple advantages. Commonly for PDMS cross-linking, a 10:1 elastomer to curing agent ratio is used; however, a 5:1 ratio allows for a tougher polymer that cures faster and can be reused more times. For the actual optical adhesive device preparation, one should note that the curing steps are all precisely tuned to avoid potential pitfalls. As such, partially curing the thin layer of optical adhesive on the substrate for the device is crucial for an extra strong bond to the cast portion. Furthermore, the optical adhesive is cured from both sides to ensure even curing throughout. If the optical adhesive has not fully cured, then the PDMS mold could be torn during removal from the cast. Conversely, if the optical adhesive is cured for too long, then the material becomes unfavorably tough. Over-cured epoxy can potentially break the punching tool used to make the port holes. If the cast is over-cured, the ports can be sand blasted or drilled with a 1 mm diameter drill bit on a drill press. Lastly, while conducting the displacement experiments, the displacing fluid should not be allowed to enter the micromodel before the crude oil. The wettability of the micro-channels is made initially oil-wet by first contacting the crude oil, but allowing components of the displacing fluid to alter the micromodel surfaces might change the performance of the displacement strategy. Following these steps carefully in the microfluidic device construction and displacement experiment will help ensure resources do not go to waste.
In the future, optical adhesive micromodels will continue to be a valuable tool for microfluidics research. These devices can serve as a robust screening platform for injection fluids tailored to specific crude oils. Additionally, these tools can be used to study fundamental mechanisms of oil recovery, mobility control, foam flow, or anaerobic microbial enhanced oil recovery (EOR) experiments. The cost effectiveness and favorable properties of optical adhesive micromodels naturally lend these tools an advantage in the microfluidic oil recovery field.
The authors have nothing to disclose.
We acknowledge the financial support from the Rice University Consortium for Processes in Porous Media (Houston, TX, USA).
3 mL Leur-Lok Syringe | Fischer Scientific | 14-823-435 | |
10 mL Glass Syringe | Fischer Scientific | 1482698G | |
Photomask | CAD/Art Services | ||
Silicon Wafer | University Wafer | 452 | |
Propylene-Glycol-Methyl-Ether-Acetate | Sigma Aldrich | 484431-4L | |
150 mm Glass Petri Dish | Carolina Biological Supply | #721134 | |
60 mm Plastic Petri Dish | Carolina Biological Supply | #741246 | |
Mask Aligner | EV Group | EVG 620 | |
1 mm Biopsy Punch | Miltex, Plainsboro, NJ | 69031-01 | |
Industrial Dispensing Tip | CML Supply | Gauge 23 | |
Inverted Microscope | Olympus | IX-71 | |
Plasma System | Harrick Plasma | PDC-32G | Plasma cleaner |
Polydimehtylsiloxane (PDMS) | Dow Corning, Midland, MI | SYLGARD 184 | |
Norland Optical Adhesive 81 (NOA81) or (OA) | Norland Products Inc. | 8116 | Optical adhesive |
Quick-Set Epoxy | Fisher Scientific | 4001 | |
Glass Slides | Globe Scientic Inc. | 1321 | |
SU-8 2015 Photoresist | MicroChem | SU-8 2015 | Photo resist |
Syringe Pump | Harvard Apparatus | Fusion 400 | |
Glass Capillary Tubing | SGE Analytical Science | 1154710C | |
High-Speed Camera | Vision Research | V 4.3 | |
Polyethylene Tubing | Scientific Commodities Inc. | #BB31695-PE/3 |