Adapting Taylor Dispersion to Measure the Dispersion Coefficient of Electrolyte Solutions via an Accessible Microfluidic Setup

The scope of our work is to design and implement an accessible microfluidics experimental platform suitable to answer a broad range of fundamental fluids questions. The biggest challenge is to develop a reproducible and yet flexible manufacturing process for microchannels with sufficient precision utilizing low-cost equipment. Our research seeks to address the current lack of easily accessible and accurate experimental setups and protocols to measure the enhanced diffusivity of an electrolyte species.

Our platform also allows for visualization of multi-species ion interactions. Our experimental setup and protocol are inexpensive, easily accessible, and accurate. The low-cost microchannel manufacturing technique employed allows for the production of custom design chips in minutes.

To begin, launch the craft cutter design software on the connected computer. Design the microchannel top directly in the software, or import a compatible design from external software. Then attach a 21 centimeter by five centimeter polyester rectangle to the sticky side of the cutting mat.

Using masking tape, tape all four perimeter edges to secure the rectangle. Next, load the cutting mat into the craft cutter by aligning the marked edges with the arrow indicators on the device. Insert the blade in the first carriage slot of the craft cutter.

Click on Send located at the top right of the design page on the monitor to proceed to the review screen. Then set the blade depth to nine, force to 33, passes to one, and speed to one. Now click on Send to submit the job to the craft cutter and initiate the cutting process.

After removing the cutting mat from the cutter, use tweezers to remove the negative polyester material from the cut sheet. Next, design donut-shaped polyamide gaskets using the craft cutter design software or import the gasket design from compatible software. Attach a 21 centimeter long piece of polyamide tape with the sticky side facing up onto the cutting mat and secure it with masking tape along all four edges.

Enter the cut settings for the polyamide tape with a blade depth of nine, force of one, passes of one and speed of one. Click on Send to submit the gasket cutting job to the craft cutter. Then place the cut polyester sheet on a clean, flat surface with the protrusions facing upward.

Using tweezers, peel off one gasket from the cut polyamide tape and place it onto the flat underside of a 3D printed port. Align the port with the flow inlet hole, and using the gasket, attach it to the flat-laid polyester sheet. Now in a fume hood, apply a small amount of super glue along the perimeter of the port while pressing it downward to create a watertight seal.

For the fabrication of the polyamide microchannel body, design the microchannel body using the craft cutter design software or by importing a compatible external design. Attach the 21 centimeter long strip of polyamide tape with the sticky side up onto the cutting mat. Then, load the cutting mat into the craft cutter by aligning the marked edges with the arrow indicators on the device.

Click on Send at the top right of the design page to review the material and cut settings. Use the same cutting parameters as used for the gaskets. Click Send to submit the cutting job to the craft cutter.

Then remove the cutting mat from the cutter and using tweezers, remove the negative polyamide material from the channel design. Now place the polyamide tape with the sticky side facing upward on a flat, clean surface. Carefully position the polyester rectangle onto the exposed polyamide tape, centering the polyamide strip across the width of the polyester.

Using a roller, apply even downward pressure to eliminate large air bubbles and visually inspect for any debris or warping. Afterward, flip the polyamide tape assembly and remove the protective cover from the adhesive side. Align the top polyester sheet mounted with the 3D printed port to the inlet and outlet of the polyamide tape, then carefully lay the polyester sheet over the polyamide layer.

For the syringe pump setup, fill a 0.5 milliliter glass syringe with deionized water. Mount the syringe onto a programmable syringe pump and press the fast forward button until water begins to emerge from the syringe tip. Then, cut a 50 centimeter long piece of polytetrafluoroethylene tubing.

Using tweezers, connect the two ends of the tubing to 27 gauge syringe tips by inserting the tubing over the tips and pulling it downward. Fill the connected syringe tip and tubing with deionized water until a convex meniscus forms at the tip opening. Attach the tip to the prem mounted glass syringe on the pump, ensuring there are no air bubbles present in either the syringe or the tip.

Set the syringe pump to infuse mode only. Input the syringe type and size as 0.5 milliliters into the pump's interface. Using 2.54 centimeter wide masking tape, tape the fully assembled microfluidic tip to the light panel.

Next, mount a 20 millimeter F2 macro lens onto the camera and connect it to a remote trigger. Set up a tripod and mount the camera above the light panel, angled downward to face the experiment. Center the view on the capture point cut in the polyamide tape.

Program the camera via the remote trigger to take images every one second. Apply a layer of transparent tape over the tracer inlet hole to prevent liquid from escaping, ensuring one edge of the tape is folded over to form a small tab for easy removal. Connect and run the programmable syringe pump to gently flood the microchannel with deionized water at a very low flow rate.

Then fill a 0.5 microliter micro pipette tip with a prepared tracer solution. Using the folded tab, peel back the tape covering the tracer inlet hole. Using the corner of a low lint wipe, lightly wick away any excess deionized water from the inlet hole and wait 30 seconds for the waterfronts to stabilize.

After 30 seconds, dispense the tracer solution into the inlet hole using the pipette. Immediately smooth the tape back over the hole using minimal pressure and a continuous motion to reseal the inlet. After ensuring that the syringe pump is programmed to the target volumetric flow rate, start the syringe pump and trigger the remote camera simultaneously to begin imaging.

If the horizontal edges of the rectangle overlay are not aligned with the microchannel walls, hover the cursor over a rectangle corner, click and rotate the image until the horizontal walls align parallel to the channel walls. Press any key to proceed. The image popup will close and reopen with the corrected orientation.

Click and drag to select a square region with sides equal to the channel width, centered at the capture point. Press any key to continue and the image popup will close. Then extract the blue channel intensity at each pixel within the selected crop region from the RGB image.

Invert the values by subtracting each from 255, the maximum blue channel value. Compute the mean intensity value of the inverted blue channel across all pixels in the cropped region. Save each computed value to generate a time series of average inverted blue channel intensity at the capture point.

Use the nonlinear curve fitter toolbox in the code to input the full-time series of average inverted blue channel intensities. The averaged inverted blue channel intensities over time were plotted and showed close agreement between experimental data and the theoretical tailor dispersion fit, with time points at 140 seconds, 150 seconds and 200 seconds clearly shown. Dispersion factor results from experiments at three different aspect ratios showed good agreement with theoretical predictions.