Laser Doppler anemometer measurements of particle-fluid motion in dielectric liquids under uniform electric field

Electrohydrodynamic (EHD) flow velocities have been measured using a laser Doppler anemometer in Voltesso 35 and Norpar 12 contaminated with ionic and particulate impurities. The unsteadiness in the observed flow, with field ranging from 6.6 to 20.8 kV/cm, is probably due to secondary injection in the presence of both ionic and particulate impurities. The large difference (more than one order of magnitude) between ν_max reached during transient state and mean velocities suggests that the injection in nonpolar liquids is weak and depends on initial charge density at the electrode surface. The influence of carbon black (50-70 nm) on EHD instabilities seems to be enhanced by small eddy formation associated with enhanced electroconvective charge transport. In Norpar 12 the addition of carbon black (2.22 p.p.m.) raised the steady-state velocity and current from 0.16 to 2.60 cm/s and 1.7 to 2.8 nA, respectively. In less transparent Voltesso 35, addition of carbon particles above a critical concentration tends to cause an underestimate of the EHD flow. This is due to increased optical absorption masking the Doppler effect from the seeding particles