Assembly and Characterization of an External Driver for the Generation of Sub-Kilohertz Oscillatory Flow in Microchannels

Our technique brings oscillatory flows to biologists and chemists using microfluidics. This device is quick to assemble, easy-to-use, and is a plug and play method for producing high fidelity oscillatory flows. Demonstrating the procedure will be Giridar Vishwanathan, a PhD student from my laboratory.

To begin, clamp the alligator clip ends of a pair of alligator to pin wires to the terminals of a 15 watt speaker with an eight centimeter cone. Place the aux controller chip on an insulating component. Insert the pin ends into the screw sockets of the aux chip.

Tighten with a screwdriver to ensure connectivity. Connect one end of an aux cable to the controller chip, and the other end to an aux port on a computer. Connect a 12 volt direct current adapter to the power supply.

Power the controller chip on by connecting the coaxial end of the DC adapter to the power socket. Using an internet browser, navigate to an online tone generator website. Type in the desired frequency between 5 to 1200 Hertz in the online application and scroll the volume bar to the required amount.

Click on the wave type generator symbol and select the desired wave form like sign, square, triangle, or sawtooth. The default setting is a sign wave form. Press Play to actuate the speaker.

Tape the speaker and the controller chip onto the 3D printed speaker mound for positioning on the microscope stage. Place the 3D printed adapter concentrically on the speaker cone. Apply silicone sealant generously along the edges of the adapter and let cure for two hours.

Cut a 200 microliter micropipette tip approximately two centimeters from its narrow end and dispose the wider half of the tip where the narrow conical end will serve as a wedge seal for reversible attachment. Connect the polyethylene tubing to the microchannel output by first threading through the micropipette tip, and then through the adapters coaxial end, and finally, out through the side. Firmly wedge the narrow end of the pipette tip into the adapters coaxial end to create a detachable tight seal.

Add tracer particles into a vial of 22%weight by weight glycerol solution to produce a neutrally buoyant suspension with a volume fraction of 0.01%to 0.1%polystyrene in liquid at 20 degrees Celsius. Mix vigorously by shaking to produce a homogenous suspension. Load a one milliliter inlet syringe with one milliliter of sample.

Mound and fasten the loaded syringe onto an automatic syringe pump. Insert the syringe needle into the inlet tubing of the device to create a water-tight seal. Ensure the outlet tube is rooted through the adapter assembly and into a reservoir.

Turn on the syringe pump, using the touch screen select the syringe type as Becton-Dickinson 1 mL, then select Infuse. Then select the required flow rate of flow volume. Initiate the steady flow using the syringe pump.

Wait until the outlet tube is filled with liquid up to the speaker. Select a required frequency amplitude and wave form in the tone generator application, and press Play to generate oscillatory flow inside the microchannel. Mount the device on the microscope.

Set up the optical configuration by selecting an objective lens with a magnification between 10X and 40X, and adjusting the focal plane and positioning the stage. To obtain measurements in a well-defined focal plane, ensure that the depth of field of the objective lens is smaller than the channel depth by a factor of five or more. To observe the oscillatory flow, use a high speed camera with a frame rate of at least twice the oscillation frequency.

For useful resolution of the wave form, measure at least 10 points per time period with 10 times greater frame rate than the oscillation frequency. Alternatively, to observe longtime effects of positive flows perform stroboscopic imaging by setting the observation to any perfect divisor of the oscillation frequency. For both direct and stroboscopic imaging, use a camera equipped with a global shutter to avoid the jello effect.

In either case, keep the exposure time considerably smaller than the oscillation time period by a factor of 10 or more to prevent streaking. To measure the oscillation amplitude without a high speed camera, record at a frame rate maintained close to but not to the stroboscopic frame rate, which results in a highly slowed down oscillation from which the amplitude can be accurately measured, observe, and record the amplitude measurements. The tracked displacement of tracer particles at the channel mid plane showed a harmonic signal for the oscillation frequencies 100, 200, 400, and 800 Hertz.

In a plot of oscillation amplitude versus frequency for all speaker volume settings, the characteristic curve had a resonant peak at around 180 Hertz beyond which the amplitude decreases with increasing frequency. The effect of different parameters on the oscillatory amplitude over the range of operational frequencies with respect to the reference case, showed that when the viscosity of the working liquid is increased, the amplitude decreases by a factor of nearly two. When the microfluidic tubing diameter for the same material is increased, the amplitude increases compared to the reference case by a factor between 1.5 to 3 depending on the frequency.

When the tube length for the same material is increased the amplitude increases significantly near the resonant frequency. Particle displacement tracks for non-sinusoidal wave forms showed that very sharp changes in position associated with square and sawtooth wave forms are not possible in real systems. Nonetheless, the Fourier spectra were in good agreement with the ideal spectra, at least up to the third harmonic.

It is important to confirm that the outlet tube is fully filled with liquid. This ensures that the amplitude is maximum and that it is constant with time. A camera with a global shutter must also be used.

We have used this technique to precisely observe and measure how micron sized particles behave after they travel a very long distance inside a microchannel. This has allowed us to implement new microfluidic manipulation techniques.