Analyzing Mixing Inhomogeneity in a Microfluidic Device by Microscale Schlieren Technique

The overall goal of this procedure is to non-invasively measure the concentration gradient in a microfluidic device by using the microscale sch larin technique. This is accomplished by first constructing the microscale sch larin system from a Hoffman modulation contrast microscope. Remove the slit plate of the condenser and replace the modulator at the rear focal place of the objective with a knife edge.

The second step is to mount a T microchannel and start a calibration mixing procedure. Image the T channel in order to obtain gray scale micro schlein images. Next, compare the micros larin images with a computational fluid dynamic simulation.

To find the relationship between gray scale and concentration gradients, the final steps are to load a target microfluidic device. Take a micro schlein image of mixing and use the calibration curve to convert the gray scale values to concentration gradients. Ultimately, microscale schlein technique is used to show mixing in homogeneity quantitatively in a microfluidic device in real time.

The main advantage of this method over the existing techniques like fluorescence imaging or shadow, is that it enables unsteady fulfill measurements of concentration gradient that reveal the three dimensional in homogeneity in a microfluid device. Demonstrating this procedure will be Dr.A recent PhD graduate from a lab. The first step is to prepare the microfluidic device to be used in the experiment.

This t microchannel device was fabricated by molding poly dimethyl Sloane. The schematic of the device shows its feed channels and its confluence channel. Each channel is connected to a circular region that will be the site of fluid connection.

For the experiment, mount a microfluidic device on a glass slide and insert Teflon tubes. To make fluid connections, put the device aside. As the experiment is prepared, construct the microscale schlein system from a Hoffman modulation contrast microscope.

Have a knife edge ready to put in the optical path. This knife edge is custom made for use in this system and is blackened to reduce reflectivity. Keep the knife edge at hand and begin to modify the microscope burst.

Remove the slit plate in front of the focal plane of the condenser. Next, remove the modulator from the rear focal plane of the five x objective. Get the knife edge and insert it in place of the modulator.

To capture video, use a high speed camera mounted to the tri inocular tube of the microscope and connected to a computer. Turn on the light source and begin sending grayscale video to the computer. Next, capture and process an image.

Record an image from the video stream and use image processing software to obtain its grayscale value. Return to the microscope and remove the knife edge on the computer. Monitor the average gray scale readout while adjusting the illumination and the aperture.

Stop when the average gray scale readout of the image is about 10%less than the maximum value. This is the background intensity for a 0%cutoff. Now insert the knife edge to block the incident light completely.

At the computer. Record the average gray scale readout of the image. This is the background intensity for a 100%cutoff.

Next iteratively adjust the position of the knife edge while monitoring the average gray scale readout of captured images.Stop. When the average gray scale readout of the image is in the middle of the zero and 100%values, this sets the degree of cutoff to 50%Now return to the microscope to mount the T microchannel device. Place it on the specimen stage so that the confluent channel is parallel to the knife edge.

Roughly adjust the focus once the device is in place. Repair the fluids for the experiment. Select two transparent fluids with no refractive indices that are completely missable with each other.

For this video, water is used as the reference fluid and an aqueous ethanol solution is the other and s plus of the refractive index as a function of concentration. And choose the mass fraction of the aqueous ethanol so that it falls in the linear region. This experiment uses a mass fraction of 0.05 to dispense the fluids repair.

Two identical syringes filling one syringe with water, and the other would dilute aqueous ethanol connecting the syringes to the T microchannel. Use Teflon tubing to connect each of the two inlets of the T Microchannel to one of the syringes. Have two syringe pumps in place to deliver fluids to the microchannel inlets.

Load one pump with the reference fluid syringe. Load the other pump with the dilute ethanol syringe, but the end of the outflow tube into a container and fix it on the container's wall. Begin calibration to give them Reynolds number by setting the flow rate of the syringe pumps.

Start the syringe pumps to deliver the working fluids simultaneously at identical volume flow rate. Use the camera to observe the micro channel fine. Tune the focus and wait until steady flow stabilizes a signaled by a stationary SL and pattern.

Use a frame rate of 30 frames per second to record 20 frames of fluidic mixing. These frames are the acquired image. When done, prepare the pumps to get another set of frames.

Stop the syringe pump for the dilute ethanol and continue to pump water through one inlet. Observe the flow and wait until steady flow occurs with nole and pattern. Record 20 frames of images at 30 frames per second.

These frames are the reference image. Continue by capturing ethanol and water and then water only images for each Reynolds number of interest. When all the data is collected, turn to the computer to continue with calibration.

Use the image processing software to divide acquired images by the reference image for each Reynolds number error. Sample acquired images for when the aqueous ethanol enters from the channel at the top of the image and the channel at the bottom division by the reference image gives the gray scale ratio data at a given distance along the stream. Select points in the direction of the cross stream and extract the gray scale ratio at each.

Do this for several points along the stream and plot the data for comparison. Perform a computational fluid dynamic simulation of the experiment. Use this to compute the derivative of the mass fraction with respect to the cross stream derivative.

Next, exploit the expected linear relation between the gray scale ratio and the derivative by using linear regression. To find the regression line after calibration, use the micro schlein technique to study a target device. First, disconnect and remove the T microchannel device from the sample stage.

Replace it with a target microfluidic device of approximately equal depth. Connect the device to water and the aqueous ethanol solution with the camera recording input water and dilute ethanol at equal volume flow rates at the computer. Wait until the flow is stabilized and record 20 frames for the acquired image.

Next, stop the syringe pump for the aqueous ethanol and input only water into the target microfluidic device. Return to the computer to obtain the reference image. The gray scale ratio under different Reynolds numbers for positive and negative mass fraction gradients have a band appearing in the middle of the T microchannel at a low Reynolds number.

The tail of the Schlein band is expanded and blurred due to dispersion across the mixing interface. As the Reynolds number increases, the diffusion length shortens, which leads to a narrower band. In this video, the linear relation found between the grayscale ratio and the mass fraction derivative was used to capture the oscillating nature of the flow in a microfluidic oscillator at a Reynolds number of 250.

After watching this video, you should have a good understanding of how to implement the microscale layering technique for quantitation of concentration gradients in a microfluidic device.