Dielectrophoretic Microfluidic Devices

On the microscale, the dielectrophoretic (DEP) force may be used to control and manipulate sub-microliter volumes of liquid water using co-planar electrodes of very simple design. The basic phenomenology of liquid DEP on the microscale is familiar; water tends to collect in regions where the electric field is strongest. However, because capillarity and surface wetting become stronger as dimensions are reduced, certain important distinctions between microDEP and centimeter-scale DEP are observed. One important difference is that, in certain cases dependent on electrode geometry and dimensions, the water can assume either of two distinct, and apparently stable, cross-sectional profiles, "narrow" or "wide". For the narrow profile, the advancing water finger is confined to the immediate vicinity of the electrode gap, while for the wide profile, the liquid spans the entire width of the electrodes. This indeterminate behavior depends on the width of the electrodes, though the ratio of the electrode spacing to the width may be just as important. The existence of two stable hydrostatic equilibria seems to be related to the surface wetting. An hypothesis is presented that local electric field intensification near the edges of the electrodes is responsible for the behavior.