Detachment of bubbles in slit microchannels by shearing flow

This paper reports an experimental study of the detachment of air bubbles adhered to both upper and lower surfaces of a slit microchannel by shearing flow. The liquid used in the experiments was a base engine oil (MCT-30, Imperial Oil). Microchannels were formed between two parallel glass slides (30 x 15 x 1 mm). The channel height ranges from 34 to 164 mum. The glass slides were coated with FC-725 Fluorocarbon coating before construction to produce smooth and hydrophobic surfaces to facilitate bubble adhesion. Once the air bubble's adhesion had completed, the flow rate, starting at zero, was incrementally increased and the system was allowed to reach a steady state. At each steady state, a computer imaging system was used to record the bubble's position and to measure the bubble's contact diameter. For the steady state in which bubble motion is detected, the flow rate, pressure drop, and bubble contact diameter were recorded as the critical values. The independent variables in the experiment were channel height, h, and bubble contact diameter, D-c. For a given channel height h, the required Capillary number, Ca-cr, to generate a detaching shearing force was found to decrease linearly as D-c/h increased. The slope of the Ca-cr vs D-c/h curves approaches zero as h decreases. Therefore, decreasing the channel height lessens the dependence of the Ca-cr upon the bubble contact diameter. The results are discussed in terms of the fluid shear force on the bubble, causing the bubble motion and the surface force holding the bubble to the surfaces. (C) 2001 Academic Press.