Electrothermal flows generated by alternating and rotating electric fields in microsystems

Abstract

Electrothermal motion in an aqueous solution arises from the action of an electric field on inhomogeneities in the liquid induced by temperature gradients. The temperature field can be produced by the applied electric field through Joule heating, or caused by external sources, such as strong illumination. Electrothermal flows in microsystems are usually observed at applied signal frequencies around 1 MHz and voltages around 10 V. In this work, we present self-similar solutions for the motion of an aqueous solution in a constant temperature gradient placed on top of: (a) two coplanar electrodes subjected to an a.c. potential difference, and (b) four coplanar electrodes subjected to a four-phase a.c. signal, generating a rotating field. The first case produces two-dimensional rolls whereas the second case produces a liquid whirl. Finally, we present experimental results of electrothermal liquid flows generated by alternating and rotating electric fields under strong illumination, and these experiments are compared to the analytical solutions. The induced rotating flow could be used in the mixing of analytes and of liquids in microsystems.

Joule heating effects on electroosmotic flow in insulator‐based dielectrophoresis

“…With the assumption of non-conducting PDMS and glass substrates, the applied electric field, E, is confined within the fluid and is governed by the following two equations [40]:…”

“…Combining Eqs. (1) and (2) gives rise to a quasi-electrostatic equation for the electric potential [40], f…”

“…The steady-state flow field of the fluid is governed by the Navier-Stokes equation along with the continuity equation [40],…”

“…where p is the pressure, Z is the dynamic viscosity of the fluid, and /f e S is the time-averaged electrical body force or the so-called electrothermal force [40]. The general form of /f e S is given as [51] hf e i ¼ hr e E À 0:5E 2 Hei ð 8Þ…”

“…It was reported that a pair of counter-rotating fluid circulations could form near the microelectrodes [39]. This so-called electrothermal flow is a consequence of the interactions between the applied AC electric field and the Joule heating-induced gradients of fluid conductivity and permittivity [40,41]. Its magnitude is proportional to the fourth power of the local electric field.…”