Electrohydrodynamic velocity and pumping measurements in water and alcohols

Bubble and particle velocities in water and alcohols, under the influence o f an electric field, were investigated in this work. Air bubbles were injec ted into the liquids through an electrified metal capillary insulated by gl ass with its tip left exposed. The end of the capillary from which the bubb les were released was conical in shape. Due to an electric field formed bet ween the noninsulated capillary tip and a ground electrode immersed in the solvent, small bubbles were formed and used as tracers for the electrohydro dynamic (EHD) flow held. The pressure inside the capillary was measured for all liquids used in this study. For water, ethanol, and n-propanol, it was found that, at relatively low applied voltage, the pressure increases with voltage, reaches a maximum (pressure breakpoint), and then sharply decreas es. This behavior is a result of the competition between the electric force appearing at the interface and the force due to the Ef-LD flow near the ca pillary tip. The electric force tends to increase the pressure inside the c apillary, while the EHD flow tends to decrease this pressure. For isopropan ol and butanol, the pressure breakpoint was not observed in the range of vo ltage applied in the experiments. The EHD flow velocity was measured by usi ng microbubbles and particles as flow tracers. An adaptive phase-Doppler ve locimeter was employed to measure the velocity of bubbles, while the veloci ty of particles was measured by trajectory visualization of fluorescent par ticles. A discrepancy was observed between the two methods because of the l ocation at which the measurements were made. It was found that average velo cities of both bubbles and particles increase linearly with applied voltage . Experiments were also conducted to investigate pumping of water, which is a result of the EHD velocity near the capillary tip. The pumping flow rate was linearly related to the applied voltage and ag reed well with EHD velo city measurements obtained from particle trajectories. (C) 2000 Academic Pr ess.