The accurate measurement of streaming potentials in either capillaries or fiat plate systems requires Poiseuille flow, i.e., flow must be steady, incompressible, laminar, and established. The established flow stipulation is rarely addressed yet it is of critical importance. Our findings suggest that while the onset of turbulence causes no abrupt change in the streaming potential, flow must be established throughout at least 90% of the flow field for accurate streaming potential measurement. The development of a flat plate flow system based on (1 × 25 × 75) mm plates is discussed in light of the hydrodynamic requirements. The electrokinetic equivalency between plates and capillaries of the same material is discussed and the small discrepancy is attributed to surface roughness and possible differences in surface chemical composition. The flat plate system offers substantial advantages over capillaries in that both surface treatments and analyses via a variety of quantitative techniques are greatly facilitated. © 1980 by Academic Press, Inc.
