AC Electrothermal Effect in Microfluidics: A Review

Abstract

The electrothermal effect has been investigated extensively in microfluidics since the 1990s and has been suggested as a promising technique for fluid manipulations in lab-on-a-chip devices. The purpose of this article is to provide a timely overview of the previous works conducted in the AC electrothermal field to provide a comprehensive reference for researchers new to this field. First, electrokinetic phenomena are briefly introduced to show where the electrothermal effect stands, comparatively, versus other mechanisms. Then, recent advances in the electrothermal field are reviewed from different aspects and categorized to provide a better insight into the current state of the literature. Results and achievements of different studies are compared, and recommendations are made to help researchers weigh their options and decide on proper configuration and parameters.

Keywords: electrothermal; microelectrode; microfluidics; micromixing; micropump.

Figure 1

Schematic of different electrode geometries. (a–e) 2D Electrodes: (a) asymmetric rectangular, (b) orthogonal, (c) meandering, (d) concentric, and (e) triangular. (f–i) 3D Electrodes: (f) 3D electrode array, (g) microgrooved configuration, (h) electrodes facing each other, and (i) spiral design. Reproduced with permission from [56,60,62,69,77,85,93,94,95].

Figure 2

Schematic of different mechanisms for imposing asymmetry to the electric field in alternating current electrothermal effects (ACET) devices. (a) Direct current (DC) biased configuration. The left electrode is always at a positive potential and subject to faradaic charging, while the right electrode is always at a negative potential and subject to capacitive charging. (b) Travelling wave configuration. The induced temperature gradient generates a charge density which is moved with the travelling electric field. Note that the direction of propagation of travelling wave is opposite to the direction of fluid flow. (c) Two-phase asymmetric configuration. The narrow electrodes in two adjacent pairs have different polarities. The superposition of the two configurations enhances the electric field in region B. Reproduced with permission from [6,68,69].