Fabrication and Visualization of Capillary Bridges in Slit Pore Geometry

The overall goal of this procedure is to form and image capillary bridges in slit poor geometry. This is accomplished by first making long raised PDMS pillar substrates using standard photo lithographic molds. The second step is to functionalize the top of the pillars to introduce a wetting contrast between the top and the sides using imprint transfer lithography, and self-assembled monolayers.

Next, the two pillars are placed facing each other in a four axis micro stage device. The translational and rotational degrees of freedom allow the investigator to properly align the two facing pillars and to control the separation between the pillars during the experiment. The final step is to introduce liquid between the two raised pillars and image the resulting capillary bridge with the CCD camera.

As the height of the slit pour has changed, ultimately the change in morphology of the liquid bridge as the slit pour height is increased, is used to show that the lalo pressure of the liquid bridge switches sign from attractive to repulsive as the slit pour height increases. Though this method can provide insight into understanding the characteristics of partially pinned wetting phenomenon. It can also be applied to problems such as flip chip design, oil recovery in porous materials and bio-inspired adhesion.

Prior to starting this procedure, clean a four inch silicon wafer with piha solution following this Spinco SU eight 2002 onto the surface of the water for 40 seconds at 500 RPM. Next Spinco SU 8 20 50, onto the wafer with a two step spin coder program for 40 seconds at 500 RPM followed by one minute at 1500 RPM. Once the spin coated wafer is dried, place a transparency mask over it, then place it under an ultraviolet lamp and expose it for 30 seconds at 200 watts.

After drying the wafer on a 95 degrees Celsius hot plate, place it in SU eight developer's solution and lightly agitated until all the unexposed SU eight has been removed. Then rinse the wafer in a stream of isopropyl alcohol for 30 seconds and blow dry it with nitrogen. At this point vigorously mix a 10 to one mass ratio of P-D-M-S-S guard, 180 4 base to curing agent in a beaker using a disposable stir rod Degas, the PDMS in a vacuum chamber until all the bubbles are gone.

After placing the SU eight fabricated mold in a large four inch plastic Wang dish, pour the PDMS mixture over it. After degassing the mold and PDMS mixture, place the entire dish in an oven at 75 degrees Celsius for at least two hours. Once the sample is cooled to room temperature, cut away the dish from the PDMS and the PDMS from the silicon wafer with a straight razor blade.

Then cut out the PDMS region with the pillars from the bulk and store it in a clean Petri dish. After evaporating 20 nanometers of gold directly onto a clean silicon wafer, place it into a plasma reactor and clean it using oxygen plasma at a pressure of 300 milour with a power of 50 watts for 10 minutes. When finished, place the wafer in a Pyrex Petri dish filled with 200 proof ethanol for at least 10 minutes.

After removing the wafer from the dish, rinse it with ethanol and blow dry it with nitrogen. Next spin coat a previously prepared MPTS toluene solution onto the wafer at 500 RPM for 30 seconds, followed by 2, 750 RPM for one minute. Once the wafer has been rinsed and dried, place it into a Pyrex Petri dish that contains enough 16 millimolar hydrochloric acid solution to fully submerge the wafer.

After at least five minutes of submersion, remove the wafer from the solution and blow dry it with nitrogen. Then place the PDMS pillars into the plasma chamber and perform oxygen plasma at a pressure of 300 milour and a power of 50 watts for 30 seconds. After removing the PDMS substrates from the plasma reactor, bind the back of each one to a clean glass slide by applying light pressure to them.

Flip the glass backed PDMS substrates and press the pillars down onto the MPTS functionalized gold films. Once moderate pressure has been applied, put an approximately 100 gram weight on the glass slide to ensure conformal contact After at least 12 hours, separate each PDMS substrate from the wafer and use a straight razor blade. If the PDMS substrate is stuck, then use an optical microscope to verify that the gold film is not cracked or that there are no parts missing along the pillar.

Next, immerse the PDMS substrate in a previously prepared one. Millimolar MHA dimethyl sulfoxide solution for at least 24 hours. After rinsing with deionized water and drying place the PDMS substrate in a vacuum chamber with the pressure of less than 100 milour at 25 degrees Celsius for at least 12 hours using two pillar substrates.

Place one in the top and one in the bottom holders of the four axis micro stage assembly. Secure the substrates using side tension screws. At this point, assemble the device by attaching the top substrate stage to the breadboard such that the top substrate is roughly above the bottom substrate.

Decrease the height between the two facing pillars to about one millimeter using the x, y and rotation knobs. On the bottom substrate stage. Align the gold strips for the two substrates so that they are parallel.

Then position the camera to look down the length of the PDMS pillar using the live camera feed on the computer screen further, adjust the position of the bottom substrate so the pillars are parallel. Following this, move the camera to the opposite side of the device and repeat the previous two steps. Using the live camera feed, decrease the separation between the two pillars until the top pillar makes contact with the bottom pillar.

Then zero the digital micro stage and increase the pore height to approximately 200 micrometers. Mount a syringe containing an 80%glycerol water solution to the syringe XY, Z translation stage with the mechanical clamp. Then adjust the micrometers on the syringe positioning stage so that the 30 gauge needle fits into the slit pour.

Decrease the slit pour height so that the top and bottom surfaces gently contact the needle after slowly dispensing the liquid from the syringe into the slit pour. Use the micrometers on the syringe positioning stage to remove the needle from the slit pore. The experimental device can be broken up into four main parts.

The top substrate stage, the bottom substrate stage, the syringe XY, Z translation stage, and the camera optics and camera holder. See the text protocol for details on the experimental device 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 shown Here is a microscope image of A-P-D-M-S pillar with gold after a successful transfer pictured.

Here is an image that 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 wafers surface to prevent additional gold foil from sticking to the edge of the substrate.Shown.

Here is an image showing how cracks can form in the gold layer after transfer if the PDMS substrate undergoes significant shear or bending. Once the fabrication process is finished, it is important to verify the quality of the MHA monolayer by testing its water contact angle. Shown here is a liquid water drop on a gold PDMS substrate after being functionalized with MHA.

The low contact angle on the PDMS indicates that the process was successful. The inset shows a liquid water drop placed on one of the raised pillars after the completed procedure. The 140 degree contact angle demonstrates that the combination of physical and chemical heterogeneities allow the drop to be pinned on the sides of the pillars.

Once the substrates have been fabricated and installed into the micro stage holders, the channels can be filled using the syringe XY, Z translation Stage shown here is a filled slit pore with a perspective perpendicular to the width of the pillar pictured. Here is a perspective orthogonal to the first image that is perpendicular to the length of the slit pore. The process of filling the channel from the same perspective as the second image is shown here, it is critical during the filling stage to dispense the liquid from the syringe.

Slowly, the force from sudden large flow rates can deep pin 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 in the filling process repeated. After watching this video, you should have a good understanding of how to form PDMS pillars.

Functionalize them with a self-assemble monolayers and align them to form capillary bridges. Don't forget that working with piran can be extremely hazardous and precautions such as a working fume hood. Safety goggles are a face shield.

A lab coat in full neoprene gloves should be used whenever performing this procedure.