Showing papers by "Friedrich Gunther Mugele published in 2008"

Direct Observation of a Nonequilibrium Electro-Osmotic Instability

Abstract: We present a visualization of the predicted instability in ionic conduction from a binary electrolyte into a charge selective solid. This instability develops when a voltage greater than critical is applied to a thin layer of copper sulfate flanked by a copper anode and a cation selective membrane. The current-voltage dependence exhibits a saturation at the limiting current. With a further increase of voltage, the current increases, marking the transition to the overlimiting conductance. This transition is mediated by the appearing vortical flow that increases with the applied voltage.

Electrowetting-based microdrop tensiometer.

Abstract: We performed electrowetting (EW) contact angle measurements to determine the interfacial tension between aqueous drops laden with various inorganic and organic solutes and various ambient oils. Using low frequency AC voltage, we obtained interfacial tensions from 5 to 72 mJ/m2, in close agreement with macroscopic tensiometry for drop volumes between 20 and 2000 nL. In addition to the conventional EW geometry, we demonstrate the possibility of performing “contact-less” measurements without any loss of accuracy using interdigitated coplanar electrodes.

Geometry-controlled droplet generation in head-on microfluidic devices

Abstract: We investigated the generation of droplets in a head-on microfluidic device operated with the two identical channels as inlets and the “long leg” as a constriction channel leading to a wider outlet section. For capillary numbers (Ca) of approximately 10−5 or less, we find a Ca-independent droplet volume equal to the volume of the constriction channel, which decreases at higher Ca when shear forces become relevant. The droplet generation mechanism is explained in terms of a global capillary instability involving surprisingly stable intermediate surface configurations.