Electrochemical detection of localized mixing produced by ultrasonic flexural waves

Localized fluid mixing caused by both standing and traveling flexural waves in 4 μm thick membranes of low-stress silicon nitride, aluminum, and piezoelectric zinc oxide has been observed. Initial work on quantifying the mixing action using a ferri-ferrocyanide electrochemical cell is presented. One electrode is fabricated directly on the membrane; the cell current, limited by diffusion, increases from the enhanced ion transport due to the mixing. For a cell bias of 0.5 V, an increase in the limiting current from 4.2 mA without mixing to 6.6 mA with standing waves driven by 10 V at 5.0 MHz was observed. This is roughly equivalent to a rotating disk electrode spinning at 140 rpm. Possible applications include increasing the speed of amperometric sensors and as components in integrated microflow systems