Is free surface free in micro-scale electrokinetic flows?
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Updated: Jun 13, 2026

Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions
Published on: September 7, 2018
WooSeok Choi1, Ashtosh Sharma, Shizhi Qian
1Department of Mechanical Engineering, POSTECH, Pohang, Republic of Korea.
Abstract:
When a rigid boundary is replaced by a free surface, or by a liquid-liquid interface, the shear stress is reduced near the boundary because the no-slip on the rigid boundary is relaxed. A nonzero fluid velocity exists on the free surface, and the flow rate would increase by the replacement. For electrokinetic flows, however, the replacement from rigid to free boundary can lead to an increase (decrease) in shear stress (flow rate). This counterintuitive effect of free surface is presented in this note for a generic electroosmotic flow in a microchannel.
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