Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

This method can help answer key questions in the micro and nano fluidic research fields, such as how effectively leakage are transported in narrow spaces. The main advantage of this technique is that cations and anions, whose transport pathways are electrified by using an ion exchange membrane, drive the electrohydrodynamic flow. Demonstrating the procedures be Ayoko Yano an assistant professor of Gunma University, who graduated from our laboratory, and Fumika Nito, a PHD student from our laboratory.

First, adhere acrylic plates at both ends of a PTFE mold with a plastic adhesive, which will make slits in the reservoir to settle the bias electrodes. In a 50 milliliter tube, mix a silicone elastomer base in curing agent in a ten to one ratio. Following this, set a liquid PDMS in a vacuum vessel, and de-gas it using a rotary pump.

Remove the tube from the vacuum vessel. Then pour the PDMS into a 40 by 50 by 24 millimeter cubed plastic vessel, to mold the outer shape of the reservoir, and place the reservoir mold in it. Bake the whole body of the liquid PDMS on a hot plate at 80 degrees Celsius for about four hours.

After the bake, isolate the PDMS reservoir from the PTFE mold in the outer vessel by hand. Then make a slit across the center of the reservoir, using a surgical knife. Using tweezers, set glass plates, previously coated with a gold thin film, at both ends of the reservoir, to serve as the bias electrodes.

Next, cut an anion exchange membrane into a 20 by 18 millimeter squared rectangle, using scissors. Then cut a three by five point five millimeter squared rectangle from one edge of the membrane. Now, cut a solidified PDMS block with a square flow channel into a three by six by four point five millimeter cubed piece, using a surgical knife.

Make slits along the outer edges, and attach it to the membrane within the rectangular cutout. Following this, set the anion exchange membrane with the PDMS flow channel into the PDMS reservoir, with tweezers. Using a micropipette, fill the reservoir with four milliliters of sodium hydroxide solution.

Apply an electric potential of two point two volts, using a DC power source, in forward and backward directions, for two hours each in series, to improve the conductivity of the membrane before observation. Following this, pull the gold electrodes out with tweezers. Then, remover the solution from the reservoir, using a micropipette.

Set new gold electrodes in the reservoir with tweezers. Fill the reservoir with four milliliters of sodium hydroxide solution, using a micropipette. At this point, set the frame rate and the exposure time of a high-speed complimentary metal oxide semiconductor camera to 500 frames per second and one millisecond, respectively.

Remove any bubbles from the channel, by inserting the tip of a micropipette into the channel end to push or pull them out, before applying an electric potential. Now, externally apply an electric potential of two point two volts to the gold bias electrodes. Simultaneously monitor the electrical responses, using a potentiostat, then record the behavior of the tracer particles on the computer.

Form gold bias electrodes with a 26 by 10 millimeter squared surface on the bottom glass plate, according to procedures similar to those previously described. Using radio frequency sputtering, coat the glass surface with chromium exposed to argon plasma for two minutes at 75 watts, and deposit a gold, thin film for five minutes at 75 watts. Using a solder iron, solder a lead line at an edge of the electrodes.

From a large silicone rubber sheet, cut out two chambers, each made made of a one by one by one millimeter cubed flow channel placed between two reservoirs, using a surgical knife. Next, cut out a cation exchange membrane to 20 by 30 millimeter squared, using a surgical knife. Ultrasonicate each part in pure water for 15 minutes, by applying 100 watts.

Insert the cation exchange membrane between the chambers, using tweezers, then press and seal the stack of the chambers and cation exchange membrane, with glass plates. Using syringes, inject previously prepared Tris-EDTA polystyrene particle and Tris-EDTA potassium chloride solutions into the lower and upper chambers, respectively. Now, set the experimental device on the stage of an inverted microscope.

Connect the microscope to the high-speed complimentary metal oxide semiconductor camera to monitor the trajectories of the particle motions, and record the observation data on a computer. Finally, apply an electric potential difference of two volts per six seconds between the two electrodes, by using a function generator as a power source. A representative result of an EHD flow generation, resulting from the rectification of ion transport pathways and highly concentrated cations that induced a liquid flow in the channel, is presented here.

The PIV analysis, demonstrated that the velocity of the tracer particles quickly increased to a peak value, when an eclectic potential of two point two volts was applied. After that, the velocity decreased and converged to zero. A representative result of the EHD flow generated in an electrically polarized solution, under ionic current conditions, is shown here.

The velocity response of the EHD flow was analyzed by tracking the tracer particles, which responded to the electric field when two volts was applied. The particles quickly translocated in the backward direction, and, after a short time response, the flow changed to the forward direction, and the velocity became steady, until the electric potential was turned off. The EHD flow, dragged by sodium ions in the channel, is triggered by the transport of hydroxide ions in an anion exchange membrane.

In EHD flow, induced under cationic current conditions, potassium ions penetrate a cation exchange membrane, causing cation dominant conditions, and, as a result, EHD flow is induce along the cationic current. Once mastered, this technique can be done in two hours, if it is performed properly. Note taking into account the time for both the gold electrodes and waiting for the electride solution to stabilize.

While attempting this procedure, it is important to remember that incorporation of the electride solutions takes a considerable amount of time. When following this procedure, The electro neutral condition has to be moderated in ionic current conditions to drive the electrohydrodynamic flows. After its development, this technique paved the way for researchers in the field of micro and nano fluidic fanomer, to explore new flow control methods in various types of leakage.

After watching this video, you should have a good understanding of how to make electrohydrodynamic flow that are induced by electrified ionic currents. Don't forget that working with high-concentration sodium hydroxide can be extremely hazardous, and precautions, such as wearing safety glasses, gloves and a lab coat should always be taken, while performing this procedure.