This study is aimed at elucidating the mechanism by which rising air bubbles induce shear stress on a vertically set flat plate. Shear stress was directly measured changing air flow rate and viscosity of water. The time-series data showed strong fluctuation sometimes showing downward direction values (opposite to bubble flow direction). Time-averaged shear stress values were approximately double the water flow induced shear stress values in all experimental conditions. Histograms of the shear stress variation data were fitted well to the normal distribution curve and mean plus three times of standard deviation was chosen as “maximum” shear stress value, which means “effective” shear stress for the detachment of substances off the membrane surface. The “effective” shear stress value increased with the increase of air flow flux and increased also with the increase of viscosity. Especially in the highest viscosity condition (15 mPa s), the value showed quite large values probably due to the interacting force between the wall surface and large bubbles that were produced by the effect of highly viscous liquid.

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