Preparation of Janus Particles and Alternating Current Electrokinetic Measurements with a Rapidly Fabricated Indium Tin Oxide Electrode Array

The overall goal of this procedure is to rapidly fabricate indium tin oxide electrodes and prepare metal-coated particles for electrokinetic experiments. This method can facilitate experiment in electrokinetics field, such as electrorotation, dielectrophoresis, and microfluidics. The main advantage of this technique is that it provides a rapid and convenient method to fabricate ITO electrode and prepare metal coat particles.

To begin fabricating the electrode, place a clean piece of 25 millimeter by 50 millimeter ITO-coated glass on the stage of a pulsed fiber laser engraving machine. Position the laser 279.5 millimeters above the ITO glass. Import the electrode pattern into the instrument control software.

Select Frame, Fill, and Fill First. Set the engraving speed to 800 millimeters per second, the laser power to 60%and the frequency to 40 kilohertz. Preview the engraving pattern, and manually adjust the glass position to center the pattern on the glass.

Once the pattern is centered, perform the engraving to divide the conductive surface into quadrants. Use polyimide tape to affix an insulated stranded wire to each conductive quadrant of the engraved ITO glass. Then, split each channel output of a two-function generator with double BNC connectors.

Equip one output of each channel with an inverting operational amplifier. To prepare an electrode array for electrorotation measurements, first connect the upper left electrode directly to the double BNC connector of the first channel. Connect the corresponding inverter output to the non-adjacent electrode.

Then, connect the lower left electrode to the second channel. Connect the corresponding inverter output to the last wire to produce an electrode array with a 90 degree phase shift between adjacent electrodes. To prepare an electrode array for dielectrophoresis, instead connect the inverted first channel to the lower left electrode.

Connect the lower right electrode to the second channel. Connect the last electrode to the corresponding inverter output to produce an array with a 180 degree phase shift between adjacent electrodes. To begin preparing the Janus particles, centrifuge a 10%by weight aqueous suspension of two micrometer silica particles at 2, 200 times g for one minute.

Transfer two microliters of the resulting sedimentary silica particles to a 1.5 milliliter microcentrifuge tube. Add 500 microliters of 99.5%ethanol to the silica particles. Sonicate the suspension for one minute, and then centrifuge the suspension at 2, 200 times g for three minutes.

Decant the supernatant, and add another 500 microliters of ethanol to the sample. Repeat the sonication and centrifugation three more times, refreshing the ethanol each time. Then, decant the supernatant and add eight microliters of ethanol to the sample.

Sonicate the sample for three minutes. Pipette two microliters of the resulting suspension onto a glass slide. Use a cover slide to slowly spread the droplet over the slide to form a monolayer of silica particles.

Once the solvent has evaporated, place the slide in a sputter coater with a gold target. Sputter coat the slide at 15 milliamps for 200 seconds to prepare the Janus particles. Apply 20 microliters of deionized water to the slide.

Carefully dislodge the Janus particles with a 200 microliter pipette tip so that the particles are suspended in the water droplet. Then, transfer the Janus particle suspension to a 1.5 milliliter microcentrifuge tube. Dilute the suspension with deionized water as needed to achieve a suitable Janus particle concentration for the experiment.

To begin preparing fully-coated metallic particles, mix together a polydimethylsiloxane polymer base and its curing agent in a 10 to one weight ratio. Apply tape to the edges of a glass slide to form a container. Pour the PDMS mixture into the tape-walled container to a depth of two to three millimeters.

Degas the PDMS under vacuum for 30 minutes, and cure the PDMS at 70 degrees Celsius for two hours. Then, separate the cured PDMS from the tape and the slide to obtain a smooth PDMS stamp. Prepare a monolayer of Janus particles on a glass slide.

Then, lay the PDMS stamp smooth side down on the slide and apply even pressure to transfer the Janus particles to the stamp. Carefully separate the slide from the PDMS stamp to expose the silica faces of the Janus particles. Sputter coat the particles on the PDMS stamp with another thin layer of gold to obtain the fully-coated particles.

Use a droplet of deionized water and a pipette tip to dislodge the particles from the stamp surface to be suspended in the droplet. Transfer the suspension to a microcentrifuge tube, and dilute the suspension as needed. To begin the experiment, cover the ITO four-phase electrode array with five sheets of paraffin laboratory film.

Use a heat gun to fix the paraffin layers to the electrode array, forming a 500 micrometer spacer. Then, place the electrode array on the stage of an inverted microscope with a 40X 0.6 NA objective lens. Place eight microliters of a particle suspension at the center of the array, and cover it with a cover glass.

Set the function generator phase shift to 90 degrees for an electrorotation experiment or zero degrees for a dielectrophoresis experiment. Select a sine wave as the waveform, and set the peak-to-peak voltage and frequency. Start the signal generator, and use the inverted microscope camera to capture images of particle motion and rotation.

Use image processing software to determine the particle velocities for further analysis. Alternating current electrokinetic property measurements were performed on silica particles with varying degrees of gold coating. Janus particles were found to rotate in the same direction as the electric field at low frequencies and counter-field at higher frequencies.

The maximum angular speed was observed at a characteristic frequency. Electrorotation measurements of bare silica particles showed co-field rotation at all measured frequencies. The maximum angular speed was observed at the lowest measured frequency.

In contrast, gold-coated silica particles showed counter-field rotation at all measured frequencies. The maximum angular speed for the gold-coated particles was observed at a lower characteristic frequency than in the Janus particles. These results, overall, suggested that the Janus particle behavior could not be attributed to a simple, two-hemisphere superposition model.

Dielectrophoresis measurements of the fully gold-coated particles showed an N-dielectrophoresis response at low frequencies and a P-dielectrophoresis response at high frequencies. The crossover frequency closely matched the characteristic frequency at which the maximum angular speed had been observed. After its development, these techniques paved the way for the researchers in the field of soft matter to explore electrode canopy for laminar in microfluidic synthesis.

After watching this video, you should have a good understanding of how to fabricate ITO electrode with a laser marking machine and prepare metal coat particle for electrokinetic experiments.