Semi-insulating Liquids

Charge separation near a liquid/solid interface is explored by the electrohydrodynamic pumping of Freon TF, transformer oil, and silicone oil using high voltage traveling waves. Liquid head displacement measurements show strong dependencies on peak voltage amplitude (1.0-2.5 kV), frequency (0.1-20 Hz), liquid viscosity (0.5-20 cS), and liquid conductivity to 10-9 0-lm-l ). Reverse pumping, opposite to the traveling wave direction, occurs when the ratio of the voltage to the frequency is small, but switches to forward pumping when the ratio is large. The largest measured head was of order 7 cm, which can drive an average flow velocity of 18 cm/s. A charge transport analysis coupled to viscous dominated flow is used to describe the pumping process. The time-average coulombic force on the migrating charge can result in pumping in either direction. In the regime where ion migration dominates charge relaxation, a ‘universal’ curve for the pumping dependence on traveling wave voltage and frequency is deduced experimentally and predicted by the model. Measurements of average volume charge density show that undoped liquids have a positive net charge density, while additives used to increase the conductivity lead to bipolar injection and an augmentation of the idection as well as the conductivity.