The overall goal of the following experiment is to show device fabrication operation and subsequent flow visualization of a surface acoustic, wave driven microfluidic counterflow device. This is achieved by first fabricating the two layer device comprised of a gold patterned lithium ovate surface acoustic wave layer, and a polymethyl xin microchannel layer. As a second step, the device is tested with a network analyzer to ensure the proper functioning of the interdigital transducer, as well as to find the resonant frequency at which to run the counterflow device.
Next, the inlet of the device is loaded with microsphere seeded fluid and an RF signal is applied to the interdigital transducer at the outlet. The resulting channel flow is recorded with a high speed microscope for analysis results are obtained to show various flow schemes, which are primarily dependent on the device geometries and applied power. While this method is targeted for the fabrication and operation of the acoustic control flow devices, the procedure from this video can also be applied for the fabrication and operation of other surface acoustic wave driven microfluidic devices such as microfluidic, mixer pumps, and optimizer.
Generally, individuals new to this method may struggle as there are many different aspects of fabrication operation and analysis that all need to be mastered when working with surface acoustic wave microfluidics. To begin, create a microfluidic device as described in the accompanying written protocol using two separate photo masks, one for the surface acoustic wave layer shown in pink and a second for the microchannel mold shown in yellow. Once the basic device has been fabricated, add a negative photo resist, such as ARN 43 40, and expose the microchannel area using optical lithographic techniques.
Then activate the exposed sample surface using two minutes of oxygen plasma at 0.14 millibar and 100 watts to give a bias voltage of approximately 450 volts. Next mix, 35 milliliters of he aec, 15 milliliters of carbon tetrachloride, and 20 microliters of Okta desal trichlorethylene into a beaker inside a fume hood. Place the device into the solution to ize the surface and make it hydrophobic.
Leave the device in the solution covered for two hours and then rinse it with isopropanol and dry it under nitrogen. Once dried, check that the contact angle of water on the surface is above 90 degrees. If the contact angle is insufficient, clean the sample and resize the surface again.
Otherwise, remove the residual photoresist with acetone, then rinse in isopropanol and dry with a nitrogen gun. Next, after mounting the sample on a printed circuit board with radio frequency wave guides and standard coaxial connectors, put an acoustic absorber polymer on the sample edges and connect the interdigital transducers by wire bonding or using POGO connectors as shown here. Next, prepare the patterned PDMS layer containing a channel with the desired dimensions as described in the accompanying text protocol.
Once cured, cut around the channel using a surgical blade, being careful not to damage the master mold and peel it off. Then refine and straighten the replica edges using a razor blade. Make sure to leave at least two millimeters clearance on the lateral side of the channel and no clearance at the channel outlet by cutting right through the sample.
Next, punch a hole in the micro chamber using a Harris unicorn puncher to form the fluid loading inlet and take off the cylinder. Then bond the PDMS channel with the lithium ovate substrate by simple conformal bonding using a microscope for alignment. In this way, the bond will hold through the fluid testing stage while remaining reversible.
Test the RF device by first connecting the surface acoustic wave delay line to the ports of a spectrum analyzer and measure the scattering matrix of the device. The transmission for a pair of single electrode transducers will resemble the absolute value of a sync function centered at the operating frequency of the interdigital transducer.Sir. Next, observe the reflection spectrum.
A dip in the reflection spectrum occurs at the same frequency as the peak in the transmission in devices operating at 100 megahertz along the major axis. Typical values are minus eight decibels for S 11 and S 22 and minus 20 decibels for S 12 without PDMS channels. In order to best visualize the flow dynamics, place the sample under a microscope.
The specific optical setup depends on the surface acoustic wave microfluidics phenomena to be observed. To study fluid filling dynamics, use a simple inverted microscope equipped with a Forex objective and a 30 frames per second video camera. However, to investigate more complex microparticle dynamics, use a 20 x objective and a 100 FPS or higher video camera.
It is important that both the objective and frame rate are high enough to capture any spatially and temporally important flow features. Next, connect the interdigital transducers in front of the channel outlet to an RF signal generator and amplifier and operated at the resonant frequency observed in the scattering matrix measurements. If necessary, use a high power UHF amplifier.
Here we're using a mini circuits zw five amplifier with a maximum output of 37 DBM and a gain of 40 decibels. It's important that the total output power can reach at least 25 DBM for investigating acoustic counterflow. Observe acoustic streaming and atomization phenomena without acoustic counterflow while running the device.
At lower power, typically acoustic streaming recirculation begins at zero DBM, and atomization occurs above 14 DBM. Next, prepare the microbead loading fluid for visualization by adding 500 nanometer micro beads at 10 to the 10th particles per milliliter and vortexing the solution for 15 seconds prior to the experiments. Then load 60 microliters of the microbead suspension into the chamber.
Slowly using a micro pipette fluid will passively diffuse into the micro chamber to avoid possible particle adhesion on the substrate, apply a zero DBM signal to the device while loading. Start recording the video through the microscope and increase the operating power in order to observe acoustic counter flow. Different flow schemes will be determined by input power, chip design and particle diameter.
In order to qualitatively capture the dynamics, record the fluid flow in proximity of the meniscus and inlet at different stages of channel filling, using markers as a spatial reference. Then quantitatively, measure the particle dynamics by microparticle image, VE assymetry or spatial temporal image correlation spectroscopy as referenced in the accompanying text protocol for accurate results. Record the fluid flow at the point of interest with a fixed field of view for at least 100 frames at a frame rate imposed by the particle dynamics.
For more complicated analysis to obtain streamlines and velocity field measurements, use customized microparticle image mal symmetry or spatial temporal image correlation spectroscopy code shown. Here are typical S 11 and S 12 spectra taken prior to bonding of the lithium ate layer. The resonance frequency in the two spectra can be seen at 95 megahertz.
The depth of the valley at central frequency in the S 11 spectrum is related to the efficiency of conversion of RF power in surface acoustic wave mechanical power. Hence, a reduction in the valley minimum will result in a reduction of the power required to operate the device. The maximum of the S 12 spectrum is related to the efficiency of conversion of RF power and surface acoustic wave mechanical power, and the attenuation of wave along the delay line.
Reduction of this value can stem from defects such as misalignment of the wave delay line or cracks.Shown. Here are four different characteristic flow patterns observed using 500 nanometer latex speeds. The top two images show different results at the channel inlet.
On the left, two symmetrical vortices are observed due to the acoustic streaming phenomena at the beginning of the channel filling after some time when the channel is partially filled, laminar flow is achieved. The images shown here are from the proximity of the meniscus. When the channel is partially filled on the left particles are observed accumulating in lines and moving at the same speed as the meniscus.
This occurs when particle dynamics is dominated by the acoustic radiation force on the right. The dominance of drag force and acoustic streaming are observed here. Particles swallow two vortices and accumulate only in bands within 300 microns from the meniscus and are located close to the substrate surface.
While attending these procedures, it's very important to remember to keep the device very clean at all the stages of the experiment from fabrication to operation. After watching this video, you should have a good understanding of how to fabricate and test surface acoustic wave devices. Work with PDMS microfluidics and visualize and analyze macrofluidic flows.I.
