A procedure for creating and imaging capillary bridges in slit-pore geometry is presented. The creation of capillary bridges relies on the formation of pillars to provide a directional physical and chemical heterogeneity to pin the fluid. Capillary bridges are formed and manipulated using microstages and visualized using a CCD camera.
A procedure for creating and imaging capillary bridges in slit-pore geometry is presented. High aspect ratio hydrophobic pillars are fabricated and functionalized to render their top surfaces hydrophilic. The combination of a physical feature (the pillar) with a chemical boundary (the hydrophilic film on the top of the pillar) provides both a physical and chemical heterogeneity that pins the triple contact line, a necessary feature to create stable long but narrow capillary bridges. The substrates with the pillars are attached to glass slides and secured into custom holders. The holders are then mounted onto four axis microstages and positioned such that the pillars are parallel and facing each other. The capillary bridges are formed by introducing a fluid in the gap between the two substrates once the separation between the facing pillars has been reduced to a few hundred micrometers. The custom microstage is then employed to vary the height of the capillary bridge. A CCD camera is positioned to image either the length or the width of the capillary bridge to characterize the morphology of the fluid interface. Pillars with widths down to 250 µm and lengths up to 70 mm were fabricated with this method, leading to capillary bridges with aspect ratios (length/width) of over 1001.
The study of the shape and resulting forces caused by capillary bridges has been the subject of extensive studies2-7. Initially most efforts were focused, due to their simplicity, on axisymmetric capillary bridges. Often capillary bridges occurring in natural systems, such as those found in granular and porous media8,9 and bridges employed in technological applications, such as for capillary self-assembly in flip chip technologies10-15 are asymmetric with nonuniform wetting properties on the interacting surfaces. The combination of improved lithography techniques along with the accessibility of simple numerical tools to model fluid interfaces allows for the creation and modeling of capillary bridges with increasing complexity.
Capillary bridges in slit-pore geometry offer an interesting compromise: the directional wetting properties lead to nonaxisymmetric bridges that retain some symmetry planes (which simplifies the analysis). They have been studied theoretically and numerically as a case study for porous media. Systematic experimental studies of capillary bridges in slit-pore geometry have, however, been limited. Here we present a method to create and characterize capillary bridges in slit pore geometry. Briefly, the method consists of 1) the fabrication of pillars to create a chemical and physical heterogeneity, 2) the design of a microstage to align and manipulate the bridges, and 3) the imaging of the capillary bridges either from the front or the sides to characterize their morphology. The characterization of the bridge morphology, along with comparisons to surface evolver simulations are provided in a separate publication1.
The protocol text is broken up into three main sections: 1) the fabrication of the of the PDMS (polydimethylsiloxane) pillars, 2) the functionalization of the tops of the pillars, and 3) the formation and characterization of the capillary bridges.
1. Fabrication of the PDMS Pillars
This section details the fabrication of the PDMS pillars using die casting with a silicon/SU-8 mold.
2. Functionalization of the Tops of the Pillars
This three-step process involves first the evaporation of a gold film on a silicon wafer, followed by imprint transfer lithography16 of the gold film onto the PDMS pillars (fabricated in section 1), and lastly the functionalization of the gold film with a self-assembled monolayer to render it hydrophilic.
Note: To verify that the functionalization process was successful, step 2 can be performed on a bulk piece of PDMS (without pillars) and the wetting angle can be tested in a goniometer. The MHA gold films should have advancing and receding water contact angles of <15° and ~0°, respectively.18
3. Formation and Characterization of the Capillary Bridges
This section details how a liquid bridge can be introduced between two substrates followed by its characterization via imaging at different heights and fluid volumes.
Description of the experimental device
The experimental device can be broken up into four main parts: 1) the top substrate stage, 2) the bottom substrate stage, 3) the syringe/ syringe xyz-translation stage and 4) the camera/optics and camera holder. The details of each follow:
Transfer of Au foil to PDMS pillars
In the transfer of the gold to the PDMS substrate, it is important to separate the PDMS device from the silicon wafer smoothly and carefully (see step 2.2.6). Figure 3a shows a microscope image of a PDMS pillar with gold after a successful transfer. Figure 3b shows excess gold foil from the wafer that was transferred to the pillar due to poor transfer. To facilitate the transfer of the gold film a sharp safety razor can be used to gently pry one edge of the PDMS pillar from the silicon wafer. Additionally, the PDMS substrate should be pulled in a direction normal to the wafer's surface (avoid lateral motion) to prevent additional gold foil from sticking to the edge of the substrate. Figure 3c shows how cracks can form in the gold layer after transfer if the PDMS substrate undergoes significant shear or bending.
Characterization of the MHA monolayer
Once the fabrication process (step 2) is finished, it is important to verify the quality of the MHA monolayer by testing its water contact angle. Figure 2 shows a liquid water drop on an Au/PDMS substrate after being functionalized with MHA. The low contact angle on the PDMS indicates that the process was successful. The inset of Figure 2 shows a liquid water drop placed on one of the raised pillars after the completed procedure. The 140° contact angle demonstrates that the combination of physical and chemical heterogeneities allow the drop to be pinned on the sides of the pillars.
Visualization of capillary bridges
Once the substrates have been fabricated and installed into the microstage holders, the channels can be filled using the syringe/ syringe xyz translation stage. Figure 4a shows a filled slit pore with a perspective perpendicular to the width of the pillar (looking "down the barrel" of the channel). Figure 4b shows a perspective orthogonal to Figure 4a, that is, perpendicular to the length of the slit pore. Figure 4c shows the process of filling the channel from the same perspective as Figure 4b. It is critical during the filling stage to dispense the liquid from the syringe slowly. The force from sudden large flow rates can depin the liquid from the top of the pillar, causing it to spread onto the hydrophobic PDMS regions. If this happens, the substrates must be cleaned and dried and the filling process repeated.
Figure 1. Picture of complete experimental setup. The PDMS substrates are held at a variable distance apart though a combination of x,y,z and rotation stages. A separate set of microstages (far right) holds the syringe to introduce the liquid into a narrow gap to create the capillary bridge in a slit-pore geometry. A CCD camera (pictured left) is used to image the resulting capillary bridges as the pore separation is changed. The resulting images can then be analyzed in the open source image analysis software ImageJ. Click here to view larger image.
Figure 2. PDMS substrate with 20 nm Au layer functionalized by a MHA self-assembled monolayer. The low water contact angle shows that the procedure was successful. The inset shows a drop on a raised functionalized PDMS/Au pillar. Click here to view larger image.
Figure 3. Raised PDMS pillar after transfer of 20 nm Au layer. a) Successful transfer. b) Tearing due to lateral motion of the PDMS substrate during the transfer process. c) Cracking caused by the bending of the PDMS substrate during the transfer process. Click here to view larger image.
Figure 4. Images of capillary bridges on pillars in the experimental device. a) Field of view parallel to the length of the pillar. b) Field of view perpendicular to the length of the pillar. c) Shows the filling process of the slit pore (same perspective as b). The minor graduation of the ruler in b) and c) is 500 µm. Click here to view larger image.
The method presented here provides a way to create capillary bridges in slit pore geometry, and also a method for imaging these bridges so that their morphology can be analyzed and compared to simulation and theory.
This method incorporates physical relief as well as selective chemical patterning to create asymmetric wetting properties. If only a chemical heterogeneity is present, a liquid drop will stay pinned on the heterogeneity until the contact angle exceeds that of the less wettable (lower surface energy) region. When PDMS is the region of lower surface energy, the maximum achievable contact angles at the hydrophilic/hydrophobic boundary is around 100°. Adding a physical heterogeneity in the form of a pillar allows for significantly larger water contact angles at the edge of the pillars (>140°), as seen in Figure 2 (An alternate method for creating similar substrates is presented by Ferraro et al.19). Higher contact angles imply that liquid drops or bridges can be confined to specific areas and sustain higher pressures than would be possible for a purely chemical heterogeneity.
Since the gold on top of the PDMS pillars is functionalized with a self-assembled monolayer, different functionalizations are possible using different thiol precursors. Also, in addition to being able to adjust the height of the slit pore, the combination of microstages allows for real time adjustment of both lateral and rotational offsets. This functionality would make such a device ideal for imaging dynamic capillary bridges systems, such as those relevant to ink jet or gravure printing.
Critical steps within the protocol
To obtain reproducible capillary bridge morphologies precautions must be taken in the preparation of the chemical and physical heterogeneities. For example, pillars with a thickness gradient lead to bridges located at the end of the pore where the PDMS is thicker. Thickness gradients can arise if the plastic weighing dish holding the liquid PDMS during the molding step is not lying perfectly flat. The change in height along the length of the pore can also lead to a change in the curvature of the liquid, skewing image data. The extent of this thickness variation can be evaluated when the zero point is set in step 3.6. The tilt of the top substrate holder can be reduced by placing a soft spacer between the top substrate holder and the substrate holder-z stage connector piece (a few layers of masking or foam tape works well for this). By varying the tension on the screws that attach the connector piece to the substrate holder, the tilt can be eliminated from the system.
It is also important to ensure that no excess DMSO is left on the substrates after the 24 hr DMSO/MHA soak. It is possible that a small amount of residual DMSO can be present on the substrate even after rigorous rinsing with DI water. If the substrates are used at this point, the excess DMSO can leach into the capillary bridge. Excess DMSO can be evaporated from the sample by placing it in a vacuum chamber (pressure <100 mTorr, 25 °C) for at least 12 hr.
Limitations of the technique
A chief limitation of using raised pillars to form high aspect ratio capillary bridges becomes apparent during imaging. When the height of the pore is changed at constant volume, the liquid recedes away from the ends towards the center of the pore1. As a consequence, the bridge can become out of focus when imaging normal to the width of the strip. This loss in focus happens when the distance between the end of the slit pore and the liquid bridge exceeds the depth of field of the camera. It is therefore important to use the shortest possible slit pore lengths required for a given experiment. The depth of field can be extended by changing optics, or by decreasing the magnification, but these come at a cost to resolution.
The tops of the PDMS pillars are functionalized to have a high surface energy (low water contact angle). As a consequence they are susceptible to contamination, coming either from the ambient environment or the fluid. Our experiments were performed in a cleanroom (class 1000) which allowed us to test the samples 5-10 times before any degradation of the surface was noticeable. Contamination leads to pinning of the wetting angle for the contact line parallel to the width of the pillars.
The authors have nothing to disclose.
The authors are grateful for the support of the National Science Foundation under Grant No. CMMI-00748094 and the ONR N000141110629.
99.999% Gold wire | Kurt J. Lesker | EVMAU40040 | |
Acetone | Pharmco-AAPER | C1107283 | |
Dimethyl sulfoxide | Fisher | D128-500 | |
Ethanol (200 proof) | Pharmco-AAPER | 111000200 | |
Hydrochloric acid | EMD | HX0603-4 | |
Hydrogen peroxide (30%) | EMD | HX0635-3 | |
Isopropyl alcohol | Fisher | L-13597 | |
Mercapto hexadecanoic acid (90%) | Sigma-Aldrich | 448303-1G | |
Mercapto-propyl-trimethoxy-silane (MPTS) | Gelest | Sim6476-O-100GM | |
Milli-Q DI water | Millipore | Milli-Q | |
Nitrogen (gas) | Airgas | UN1066 | |
Oxygen (gas) | Airgas | UN1072 | |
Silicon wafers (4 in) | WRS Materials | CC8506 | |
SU-8 2002 (negative photo resist) | MicroChem | SU82002 | |
SU-8 2050 (negative photoresist) | MicroChem | SU82050 | |
SU-8 Developer solution | MicroChem | Y020100 4000L1PE | |
Sulfuric acid | J.T. Baker | 9681-03 | |
Poly dimethy sulfoxide (PDMS) | Dow Corning | Sylgard -184 | |
Toluene | Omnisolv | TX0737-1 |