(a) Scheme of the microfluidic channel (not to scale) consisting of two linear electrode structures on glass plates with different thicknesses, which are separated by a thick polymer spacer forming a flow channel. The sketch in the upper right corner shows the time course of the four phase-shifted signals generating the electric traveling wave. Ohmic heating leads to gradients in conductivity and permittivity. Thereby, volume charges are induced which interact with the electric field such that a longitudinal fluid flow occurs. An additional temperature field may be applied by the two Peltier elements. (b) Micrograph of the microchannel used with integrated electrodes.
Flow velocity in the direction as a function of for three different pumping modes: electrode layer on the thick glass (filled circles), on the thin glass (open circles), or both electrode layers (filled squares) are switched on (at and ). Negative velocities denote a flow against the propagation direction of the electric wave. The data were fitted to the Hagen-Poiseuille law for laminar flow.
Temperature profile along the direction between two electrodes calculated by finite element analysis and sketches illustrating the resulting flow direction. For channel geometry, applied signal, and fluid properties, see Fig. 2. The temperature distribution results from the ohmic heating of the electrode arrays and the heat dissipation. (a) The temperature fields for pumping with one electrode layer on thick or thin glass, respectively. The resulting flow direction is opposite to the travel direction of the electric field (left arrow in the sketch). (b) The temperature field for pumping with both electrode layers. Flow is in the same direction as the travel direction of the electric field. is the normal vector of the electrode plain.
Flow velocity vs externally generated temperature field. The abscissa represents the difference in temperature between the outer faces of the glass chips. Temperatures were controlled by Peltier elements. Pumping is induced by one electrode layer consisting of 36 electrodes on the upper thick glass. Negative temperature differences denote heating of the lower glass chip and cooling the upper one, and vice versa (conductivity of the fluid: ; applied signal: , ). Temperature differences are larger than those in Fig. 3 as a consequence of the relative thicknesses and heat conductivities of the layers of the sandwich glass/microchannel/glass between the Peltier elements.
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