Back-and-forth horizontal vibration of a membrane can produce a high shear rate in fluid near a membrane surface. The present study investigated the relationship between the flow structure near the membrane surface and the characteristics of separation performance. A numerical calculation was performed to analyze the flow field near the membrane surface. Results revealed that the velocity of fluid near the membrane surface lags that of the membrane. The delay depends on the magnitudes of the inertia and viscosity of the fluid. Additionally, the thickness of the velocity boundary layer on the membrane surface obtained from numerical calculation was found to be identical to that provided by the boundary layer theory of horizontal vibration. Furthermore, the permeate flux measured using emulsion solution was well correlated with the shear rate as a function of amplitude and frequency. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.