Experimental verification of an equivalent circuit for the characterization of electrothermal micropumps: high pumping velocities induced by the external inductance at driving voltages below 5 V.

Electrothermal micropumps (ETμPs) use local heating to create conductivity and permittivity gradients in the pump medium. In the presence of such gradients, an external AC electric field influences smeared spatial charges in the bulk of the medium. When there is also a symmetry break, the field-charge interaction results in an effective volumetric force resulting in medium pumping. The advantages of the ETμP principle are the absence of moving parts, the opportunity to passivate all the pump structures, homogeneous pump-channel cross-sections, as well as force plateaus in broad frequency ranges. The ETμPs consisted of a DC-heating element and AC field electrodes arranged in a 1000 μm × 250 μm × 60 μm (length × width × height) channel. They were processed as platinum structures on glass carriers. An equivalent-circuit diagram allowed us to model the frequency-dependent pumping velocities of passivated and nonpassivated ETμPs, which were measured at medium conductivities up to 1.0 S/m in the 300 kHz to 52 MHz frequency range. The temperature distributions within the pumps were controlled by thermochromic beads. Under resonance conditions, an additional inductance induced a tenfold pump-velocity increase to more than 50 μm/s at driving voltages of 5 V(rms). A further miniaturization of the pumps is viewed as quite feasible.