Numerical simulation of travelling wave induced electrothermal fluid flow

Abstract

Many microdevices for manipulating particles and cells use electric fields to produce a motive force on the particles. The movement of particles in non-uniform electric fields is called dielectrophoresis, and the usual method of applying this effect is to pass the particle suspension over a microelectrode structure. If the suspension has a noticeable conductivity, one important side effect is that the electric field drives a substantial conduction current through the fluid, causing localized Joule-heating. The resulting thermal gradient produces local conductivity and permittivity changes in the fluid. Dielectrophoretic forces acting upon these pockets of fluid will then produce motion of both the fluid and the particles. This paper presents a numerical solution of the electrical force and the resulting electrothermal driven fluid flow on a travelling wave structure. This common electrode geometry consists of interdigitated electrodes laid down in a long array, with the phase of the applied potential shifted by 90o n each subsequent electrode. The resulting travelling electric field was simulated and the thermal field and electrical body force on the fluid calculated, for devices constructed from two typical materials: silicon and glass. The electrothermal fluid flow in the electrolyte over the electrode array was then numerically simulated.The model predicts that the thermal field depends on the conductivity and applied voltage, but more importantly on the geometry of the system and the material used in the construction of the device. The velocity of the fluid flow depends critically on the same parameters, with slight differences in the thermal field for glass and silicon leading to diametrically opposite flow direction with respect to the travelling field for the two materials. In addition, the imposition of slight external temperature gradients is shown to have a large effect on the fluid flow in the device, under certain conditions leading to a reversal of the fluid flow direction.

Joule heating effects on electroosmotic flow in insulator‐based dielectrophoresis

“…Similar to those previously studied in eDEP [34][35][36][37][38][39][40][41][42][43][44][45][46][47], electrothermal flows in iDEP devices also arise from the action of the electric field (both DC and AC) on fluid inhomogeneities (predominantly electrical properties including conductivity and permittivity) formed in the constriction region due to Joule heating-induced temperature gradients. Figure 4 shows the numerically predicted temperature and electric field contours in the constriction region at 100 V DC/500 V AC.…”

“…It has been long known in capillary electrophoresis that Joule heating can elevate the buffer temperature and disturb the electroosmotic flow causing significant sample dispersion [30][31][32][33]. The effects of Joule heating on fluid temperature and motion in eDEP have been investigated previously [34][35][36][37][38]. It was reported that a pair of counter-rotating fluid circulations could form near the microelectrodes [39].…”

Electrode Cooling Effect on Out-of-Phase Electrothermal Streaming in Rotating Electric Fields

“…In stark contrast, merely one dominating horizontal TWET whirlpool in the direction of signal-phase propagation is captured on the glass base at frequencies on the order of the inverse charge relaxation time of bulk fluid ( Fig.2(e)), and the rotating flow profile becomes more circular and less helically cascading with further increase in field frequency ( Fig.2(f)). Unexpectedly, however, the co-field induction vortex in current microfluidic device is opposite to the anti-field streaming of induction micropump using glass substrate from previous researchers [42]. According to preliminary mathematical analysis, this peculiar flow reversal phenomenon is originated by external natural convection on millimeter-scale highly-conductive electrode pads that substantially gives rise to an electrode cooling effect, which simultaneously makes the rotating flow velocity measured by experiments much closer to theoretical prediction compared to the traditional non-3 of 17 cooling models.…”

“…Perch-Nielsen et al have predicted that flow direction of induction pump using a glass substrate is against the signal-phase propagation [42]. The low thermal conductivity of glass material cannot effectively remove the internal electric heat generation to the ambient environment, so that the hot spots of the whole microfluidic system are positioned within the interelectrode gaps right on the channel bottom surface.…”

“…In the simulation analysis of Perch-Nielsen et al, Fourier heat conduction on the electrode array was ignored, taking into account a negligibly small thickness (100~200nm) of the thin electrode layer usually involved in lab-on-chip [42]. In typical microsystems, highly conductive materials, such as gold, platinum and so on, are routinely used for depositing microelectrode patterns on insulating substrates, those noble metals are only conducting for electric current but possess high thermal conductivities as well, e.g.…”