Multiple frequency electrothermal induced flow: Theory and microfluidic applications

We put forward herein a unique physical mechanism of multifrequency electrothermal (MET) induced flow, in the context of a brand-new manipulation tool for liquid and colloid mixtures of microfluidic systems. Since the characteristic operating frequencies of standing-wave electrothermal (SWET) and traveling-wave electrothermal (TWET) convection are far from each other, the cross product of induced charge wave with local electrical field of another oscillation frequency always time-averages to zero. For this reason, we make use of a paradigmatic dual-frequency standing-wave/traveling-wave signal to engender the phenomenon of MET streaming, which subtly combines the respective feature of transversal SWET whirlpool and longitudinal TWET pump fluid motion under suitable excitation frequencies. The synthetic flow pattern in regards to MET is mathematically analyzed under the approximation of small temperature gradient, and it is discovered that the flow velocity of out-of-phase electrothermal pump and in-phase vortex shedding are in effect cross-influenced by the dual-frequency sinusoidal voltage waves, when taking into consideration the coaction of double-component thermal-electric coupling of electric heat generation in the liquid bulk. Meanwhile, we demonstrate MET can be fully exploited for dealing with solid particle samples suspended in buffer medium. By carrying out direct numerical simulation in full-scale 3D computational geometry, it is proved that MET can induce simultaneous transport and chaotic stirring of nanoscale objects, as well as spawn spontaneous dynamic separation of binary mixtures of microscale entities assisted by active dielectrophoretic effects in a straight fluidic channel even without external moving elements. Our physical demonstration with multifrequency signal control on electrothermal induced convection provides invaluable guidelines for innovative designs of multifunctional on-chip analytical platforms in the broad context of microfluidics, nanofluidics, and lab-on-a-chip technology.