Behavior of bubbles moving along horizontal flat plates with different surface wettability

This paper presents experimental findings on the behavior of bubbles moving along upper flat plates with different surface wettability in a horizontal channel. The size and velocity of the bubbles are measured by image processing to evaluate the wettability dependence in the range of channel Reynolds number, Re (2000 <= Re <= 5000) and bubble Weber number, We (1 < We < 10). When the bubbles move along a hydrophilic flat plate, their wall-sliding velocity increases with the bubble diameter, and no bubbles adhere to the plate even at low Re. By contrast, their behavior depends strongly on Re when a hydrophobic plate is used. At Re = 5000, an intrinsic cycle of coalescence and breakup of bubbles takes place. In this cycle, large wall-creeping bubbles become stretched by shear, and then each tends to break up into a small bubble that adheres to the plate surface and a medium-sized bubble that is detached from the plate. Because the small, adhered bubbles grow to large creeping bubbles through coalescence with bubbles supplied upstream, this intrinsic cycle repeats downstream. As a consequence, the hydrophobic plate produces two groups of bubbles within the turbulent boundary layer of the plate: one consisting of medium-sized free bubbles and one consisting of small wall-adhering bubbles to large wall-creeping bubbles.