AC Electrokinetic Phenomena Generated by Microelectrode Structures

Summary

Manipulating fluids and suspended particles in the micro- and nano-scale is becoming more of a reality as enabling technologies, like AC electrokinetics, continue to develop. Here, we discuss the physics behind AC electrokinetics, how to fabricate these devices and how to interpret the experimental observations.

Abstract

The field of AC electrokinetics is rapidly growing due to its ability to perform dynamic fluid and particle manipulation on the micro- and nano-scale, which is essential for Lab-on-a-Chip applications. AC electrokinetic phenomena use electric fields to generate forces that act on fluids or suspended particles (including those made of dielectric or biological material) and cause them to move in astonishing ways^{1, 2}. Within a single channel, AC electrokinetics can accomplish many essential on-chip operations such as active micro-mixing, particle separation, particle positioning and micro-pattering. A single device may accomplish several of those operations by simply adjusting operating parameters such as frequency or amplitude of the applied voltage. Suitable electric fields can be readily created by micro-electrodes integrated into microchannels. It is clear from the tremendous growth in this field that AC electrokinetics will likely have a profound effect on healthcare diagnostics^3-5, environmental monitoring⁶ and homeland security⁷.

In general, there are three AC Electrokinetic phenomena (AC electroosmosis, dielectrophoresis and AC electrothermal effect) each with unique dependencies on the operating parameters. A change in these operating parameters can cause one phenomena to become dominant over another, thus changing the particle or fluid behavior.

It is difficult to predict the behavior of particles and fluids due to the complicated physics that underlie AC electrokinetics. It is the goal of this publication to explain the physics and elucidate particle and fluid behavior. Our analysis also covers how to fabricate the electrode structures that generate them, and how to interpret a wide number of experimental observations using several popular device designs. This video article will help scientists and engineers understand these phenomena and may encourage them to start using AC Electrokinetics in their research.

Protocol

Fabricating Cr/Au Electrodes on Glass Substrates

Part 1A: Wet Etch Method

*For the highest quality devices, the fabrication process should be performed in a clean room environment or under laminar flow hoods so that dust and other contaminants won't affect the pattern. 

1. 2-inch by 4-inch Glass slides are placed in a heated (80°C) Piranha solution (5:7 H₂O­₂:H₂SO₄) for 30 minutes to remove contaminants (especially organic) and then rinsed in DI water and dried with compressed air.
2. 20 nm Cr and 200 nm Au are deposited onto the substrates with....

Discussion

In this video, we have shown a wide variety of particle and fluid manipulation behaviors caused by AC electrokinetic phenomena. The electrodes that generate these phenomena are easy to fabricate and can be easily integrated into many other systems. As we have shown, there are numerous applications for the use of AC electrokinetics. The versatility of these devices, as well as the rapid nature of manipulation, makes them particularly attractive. As healthcare and other industries begin to embrace lab-on-a-chip systems, we.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2" by 4" Pyrex Glass Slide	Substrate			Pyrex 7740
chrome mask	material			This photomask will have the micr–lectrode patterns on them and can be ordered from a variety of microfabrication centers.
PDMS Microchannels	material			These may be fabricated and used in-house or a simple microscope slide will suffice.
Hydrogen Peroxide 30%	Reagent	Fisher Scientific	7722-84-1	Certified ACS, Fisher Scientific
Sulfuric Acid	Reagent	Fisher Scientific	A300-212	Certified ACS Plus
Acetone Electronic Grade	Reagent	Fisher Scientific	A946-4
Shipley 1827 Positive Photoresist	Reagent	MicroChem Corp.
Shipley 351 Developer	Reagent	MicroChem Corp.
Gold Etchant	Reagent	Transene Company, Inc.	Type TFA
Chrome Photomask Etchant	Reagent	Cyantek Corporation	CR-7S
NR-7 1500 PY Negative Resist	Reagent	Futurrex
RD6 Developer	Reagent	Futurrex