AC Electrokinetic Phenomena Generated by Microelectrode Structures

In the last several years, the lab on a chip community has embraced an exciting technology known as AC Electrokinetics. This group of phenomena can be used to manipulate particles and fluids in the micron to nanometer scale in rapid and versatile waves, and is quickly becoming essential for many bio chip applications. In this video, we're going to go over the basics of AC electro kinetics in enough detail to help other scientists and engineers who may benefit from using these phenomena in their research.

Hi, my name is Robert Hart and I'm a PhD student here at Drexel in the department of Biomedical Engineering Science and Health Systems. We'll start this video off with a brief description of the physics behind AC electric kinetics. Next, we'll move into device fabrication, and finally we'll show some videos of AC electro kinetics and explain what's happening.

The first of the three forces we'll be describing is known as di electrophoresis. Here we see an electric field generated between two submerged electrodes. If we had a dielectric particle into this electric field, it becomes polarized.

As you can see, the charges on the particle are balanced by the charges within the liquid. Whether the particle is more polarizable than the liquid or less, polarizable can be determined by the classiest MoSo factor in a uniform electric field. The particles experience known nut force.

However, in a non-uniform electric field, such as the one shown here, particles that are more polarizable move towards areas of high electric field as they experience positive di electrophoresis changing the frequency to switch the polarizability results in the opposite effect known as negative di electrophoresis and particles move away from areas of high electric field strength. The second force is AC electro osmosis at the foundation of AC electro osmosis as the formation of the electric double layer due to the electric potential. At the surface, this region is divided into the stern layer, which consists of immovable ions rigidly bound to the surface and the diffuse layer, which contains ions that while bound are still free to move laterally.

If we inspect one of the ions near the edge of the electrode, we observe that it experiences a coolant force from the electric field. The Y component of the force is balanced by the existence of charges at the surface. Therefore, the ion experiences a net lateral force directed towards the center of the electrode ions on both sides of the electrode move and mass towards the center of the electrode and enough numbers to drag the fluid along.

The convergence of these two flows causes the fluid in the center to move upward and a rotational fluid pattern emerges. Switching the potential does not affect the direction of the fluid pattern because the counter ions have switched as well. The third and final phenomena is the AC hydrothermal effect.

When an electric field is passed through a liquid, JUUL heating causes temperature gradients. As shown in the simulation, the electrical properties of water change. As a result, these perturbations in electrical properties interact with the electric field to cause a body force.

The resulting motion like AC electro osmosis is rotational in nature despite the different nature of its origins. We mentioned the AC hydrothermal effect briefly for completeness, but the effects of the hydrothermal effect are subtle. Under the operating conditions of our experiments From the mathematical principles behind each of the three forces, a finite element numerical simulation was created, which shows the total combined force acting on a two micron polystyrene particle At each position in the channel, the finite element simulation we have run takes a two dimensional cross section of the electrodes and centers on just one.

The first simulation shows low conductivity media and progresses from 100 hertz to one megahertz at low frequencies. A C electro osmosis is dominant as can be seen by the rotational force pattern. As we progress, positive di electrophoresis takes over as illustrated by the attractive forces leading to each electrode corner.

As the frequency increases past a threshold, positive DEP gives weight to negative DEP and particles will be repelled to a certain height where they're balanced by the force of gravity. Now, we'll run through the same frequencies at high conductivity. At high conductivity, the AC electros mo force is generally less strong than that at low conductivity and the peak velocity occurs at a higher frequency.

Also, note that there's no positive DEP because the conductivity is too high. AC electro osmosis gives way directly to negative DEP with a higher conductivity and higher voltage. The electrodermal effect will be much more clearly shown.

In this section, we will talk about device fabrication and assembly. The devices themselves consist of gold electrodes patterned onto a substrate. In this case glass.

We'll show a wet etch method for achieving this, but the well-known liftoff procedure is used routinely as well and will be shown later. The four designs we're using are parallel interdigitated, parallel castellated, potential well and quadruple. A brief description of the process is as follows.

First, a layer of chrome and gold are deposited onto the glass substrate. Next, the substrate is coated in photoresist and the electrode pattern is transferred from the mask to the substrate. With UV contact exposure.

After development, the chrome and gold are etched away and the photoresist is stripped. In order for good adhesion, the glass slides must be very clean. This is commonly done with heated piranha solution, which consists of sulfuric acid and hydrogen peroxide.

Great care must be taken when working with this dangerous combination. After cleaning, the substrates are dried and ready for metal deposition. This step is performed in an electron beam evaporator.

The glass slides are loaded onto the sample holder with cap on tape, which is specially suited to withstand the deposition conditions. Next, the samples are loaded into the machine and vacuumed down. The process consists of a short two minute deposition of chrome and a 30 minute deposition of gold resulting in approximately 20 and 200 nanometers respectively.

When the samples are removed, the gold surface is clearly seen. Photolithography Starts with a coating of photo resist using a spin coating machine. The photo resist is pipetted onto the substrate which sits on a chuck within the machine.

A consistent layer of photo resist is created by spinning the glass at a specific speed, which removes most of the excess photo resist. This process is followed by a soft bake for two minutes at 100 degrees C.This hardens the photo resist and prepares it for the UV Exposure. Next, the photo mask is placed in contact with our substrate and exposed to UV light for approximately eight seconds.

This transfers the pattern to the photo resist. The development step removes all areas of photo resist that have been exposed to light. This process completes the photolithography stage and we are ready For gold and chrome etching.

Those areas on our substrate That have been exposed in the development process are now free to be etched. The photo resist effectively protects the rest of the surface, but like in all steps, the etch time should be carefully controlled. Here we see the substrates being placed in the dark, iodine based gold etching.

After rinsing And water, the chrome is removed with the chrome etching. Note the transformation that takes place as the glass has become transparent again. Once the chrome is removed, A comparison of the Etched to un etched substrates shows the results.

A quick inspection under the microscope shows the success Of the process. Here We see a successfully fabricated device with electrical connections made. Next to it sits a PD DS channel with tubing connections.

When the PDMS channel is placed onto a device, a very effective seal is made with a glass and liquid may flow through the channel. This is done carefully with a forceps. Since fingerprints and dust can prevent good adhesion, the opposite sides of the forceps may be used to ensure good attachment.Filling.

The channel is done by attaching a syringe to one side, placing the other in a polystyrene microsphere suspension and applying gentle suction. Once it is placed in the microscope and focus, the electrical connections are made to the function generator. With the samples loaded and the connections made are devices now ready For an experiment.

All of the experimental videos we will show consist of injecting an aqueous suspension of two micron polystyrene microspheres into the channel and applying a signal to The electrodes. Initially, the particles are randomly distributed and exhibit browning motion. When the one killer, her signal is applied, particles rapidly align on the center of the electrode.

Keep in mind, since we're using an AC field, we are not witnessing KIC force. This fascinating behavior is due to the fluid patterns generated as well as the attractive forces of di electrophoresis. As the frequency is increased, particles start to spread out along the width of the electrode.

As AC electro osmotic velocity decreases and di electrophoresis begins to take over at 56 kilohertz, the particles migrate to the electrode edge. As AC electro osmosis forces die out and positive di electrophoresis predominates. As shown in this diagram, this behavior continues at 100 kilohertz and the particles are now firmly rooted to the electorate edge.

When the frequency is increased further to 250 kilohertz, the particles begin to line up across the gap and the so-called pearl chain behavior, which is caused by particle particle interactions at 500 kilohertz particles are repelled from the electrode edge as negative DEP predominates. This can be explained by the K clausius MoSo factor, which changes from positive to negative with a frequency increase causing a transition from positive di electrophoresis to negative di electrophoresis at one megahertz. Negative DEP is near its maximum value and particles are levitated above the electrode.

An increase in the conductivity causes an important change in CM factor. As you can see, there is no more positive DEP, which drastically changes the particle behavior. Keep this in mind as we sweep through the same frequency range when we apply one kilohertz signal particles orbit out of plane along the electrode edge.

The top view provided by the microscope shows only lateral particle movement as demonstrated in this animation. This view which shows the particles moving back and forth, hides the true motion of the particles when viewed from the side. The true nature of their motion can be more readily seen.

They are in fact orbiting the reason they orbit and are not trapped at the center of each electrode is believed to be because the DEP component is reversed. As the frequency continues to increase, particles begin to coalesce into clumps while maintaining the same orbital notion. This clumping is due to particle particle interaction.

The origins of this interaction are thought to be due to the slight electric field distortions caused by the particles themselves. The distortions around the particles create DEP forces, which attract nearby Particles. As we continue Increasing the frequency, a dramatic change occurs at approximately 250 kilohertz.

Particles largely stop orbital motion and form pro change another manifestation of particle particle interaction. Eventually, as the frequency gets even higher. At this point, one megahertz repulsion due to negative DEP propels the particles upwards and out of the microscope focal Plane.

Next, we'll show A cast elated electrode type operating at low conductivity. This electrode design is similar to that of the last type in that it is interdigitated, but the straight fingers have been replaced with a more complicated shape. At one kilohertz, particle collection takes place at the center of the intersections and rapidly forms a diamond shape.

As the frequency increases, we see the same spreading out of the collected particles. As AC electro osmosis begins to die out and DEP takes over, Like Before 56 kilohertz causes the particles to slowly migrate to the electrode edge. Interestingly, almost all of the particles move to one side, which may be due to some hydrostatic pressure.

They move much faster. At 100 kilohertz as a CEO has disappeared almost completely. At 250 kilohertz, the particles begin to form pro chains.

The negative DEP caused by shifting to 500 kilohertz forces the particles away from the electrode edge. Increasing the frequency even more to one megahertz causes particles to move upwards out of the focal plane as they're repelled even more by negative di electrophoresis. Next, we'll show a castellated electrode type operating at high conductivity.

The rotational pattern that is generated with this electrode type occurs most dramatically at the inside electrode corners, and this is where particles eventually migrate. The diamond shape chopping behavior we saw earlier does not exist here because there is no positive di electrophoresis at this conductivity. As the frequency increases, the fluid velocity slowly decreases.

As AC electro osmosis forces die out at 56 kilohertz, the motion is very slow and in some places particles start clumping and forming pearl chains at 100 kilohertz. Pearl chains are quite clear. Slowly as the frequency is increased, clumps of particles coalesce and form X shapes at each of the corners.

Finally, at one megahertz, the pro chains are overcome by negative DEP and particles are repelled from the surface. The Quadruple design shown here causes an area of low electric field strength at the center of the electrode pattern and is designed to use negative dielectrophoresis to focus particles. When we apply 10 volts to the electrodes, we see dramatic particle focusing.

We'll speed the time up a bit so that we can see what the particles look like in equilibrium. If we reduce the voltage to one volt, we see the focused area start to expand. As dielectrophoresis loses ground against brownie and motion, increasing the voltage again causes the particles to move back in towards the center.

Like the quadruple pattern, the potential well creates areas of low electric field in order to trap particles. The electrodes are interdigitated, so other effects we've already seen can be observed here as well. When the signals applied, we see rapid trapping of particles due to a CEO and DEP.

The more interesting effect though, is what's happening in the hollow squares. Particles here are being collected due to negative di electrophoresis after some time has gone by. We also see some collection on either side of the potential well in the shape of triangles.

We have just shown some of the many interesting physics behind AC electrokinetics, how to fabricate these devices and how to interpret experimental results based on numerical simulations and the underlying physics. These phenomena, which deal with moving particles are quite difficult to understand without visual aids. AC electro kinetic phenomena can be used in many areas of research.

For example, particle collection for biosensor applications, separation of particles with different properties such as size and shape for sample processing and active mixing for assay improvement. We hope that this video helps scientists and engineers use and fabricate AC electric kinetic devices, one of the most important and growing areas of the lab on a chip community. Well, that's it.

Thanks for watching and good luck with your experiments.