This method can help answer key questions in the microfluid dynamics field, such as the effects of nanostructures on a surface to the thin film profile and wicking velocity. The main advantage of this technique is that it provides a method of fabricating various wicking pillar arrays in a cost and time-efficient manner. Demonstrating the procedure will be Thomas Germain, a graduate student from our laboratory.
To start, prepare the stamping device for the protocol. The essential details of this stamping device are in this schematic. A backing plate to support the mold is mounted on an XY stage.
To stamp the plastic, a Theta-Z stage moves a micro sized bit. A laser heats the plastic mold at the bit tip. Here are the backing plate and the support for the stamping bit on their respective stages.
A video camera fitted with a 10x objective provides a view of the stamping from a vantage to the side of the bit. Get the backing plate from the device, along with a stamping mold. The mold is an acrylic disc, one inch in diameter and one-eighth inch thick.
Secure the stamping mold to the backing plate for use in the device. Next, manually translate the stamping bit out of the way to avoid damaging it. Then secure the backing plate with the mold to the motorized XY stamping stage.
With the computer controls, move the plastic mold to be aligned and centered with the axis of the stamping bit. Now manually translate the stamping bit until it is almost in contact with the plastic mold. Use the computerized stamping control program and monitor the bit.
Translate the stamping bit in small increments until the tip is in contact with the plastic. After that, translate the stamping bit away from the sample. Always ensure the plastic mold is normal to the bit.
Or else the pillars will not have uniform height once the PDMS mold is created. Non-uniform pillar heights could affect hemiwicking of the fluid. Next, assign parameters for creating the pattern, including pixel length, cavity depth, and initial position.
Continue by uploading a prepared patterning map. The grayscale value indicates the desired cavity depth, with black being the set maximum cavity depth. Start the stamping process.
The software will move the bit to the proper locations using the pixel length. The bit will stamp a cavity according to the set parameters. Once the cavities have been created, remove the stamped plastic mold.
Here is the stamped mold immediately after the stamping process. The mold is complete after its surface is polished with 9, 000 grit wet dry sandpaper, as with this example. Move on to use the mold to create a molding.
In a beaker, put PDMS elastomer and curing agent in a 10 to 1 ratio. Then, mix them together thoroughly for three minutes. After mixing, place the beaker in an evacuated chamber to release any trapped air bubbles.
Before proceeding, ensure that the mixture does not have any bubbles. Next, place the stamped plastic mold into a walled container. Ideally, the container would not be much larger than the outer diameter of the mold.
Start pouring the PDMS mixture into the center of the stamped area, and spiral outward to distribute it equally. Place the container in an evacuated chamber to release air bubbles. When done, transfer the container to a hot plate to heat at 100 degrees Celsius for 15 minutes.
After that, without allowing the hot plate to cool, heat it at 65 degrees Celsius for 25 minutes. Remove the container from heat and allow the PDMS to cool. Wait 20 minutes for cooling and curing before cutting the molding from the container wall.
Remove the PDMS from the mold and store the plastic mold and PDMS sample in covered containers. However, before using the plastic mold, be sure to clean the surface of any residual PDMS by placing it in an ultrasound water bath for five minutes. This bitmap defines a rectangular wicking structure.
Each pixel represents a square with a 100 micrometer side length. The uniformly black pixels mean each pillar in the PDMS will have the same height set to 100 micrometers. This is a top view of the pillars in the PDMS created with the bitmap.
This side view from an edge demonstrates the relatively consistent height of the wicking structure. Here are side and top views of a PDMS structure with an approximately 70 micrometer thick layer of deposited aluminum. With the application of ethanol to the surface, these same structures show evidence of the fluid along the base of the pillars.
Though this method can provide insight into hemiwicking dynamics, it can also be applied to other systems, such as heat pipes and nanoscale heat transfer. Following this procedure, other methods like interferometry can be performed in order to answer additional questions, like how the curvature of the meniscus is determined by the wicking structure and how that affects the heat flux in the region. After it's development, this technique paved the way for researchers in the field of microscale heat transfer to explore the role that the shape of the thin film region has on the heat blocks.
Don't forget that working with lasers can be extremely hazardous, and that laser safety glasses should always be worn while the stamping process is performed.
