DYNAMICS OF BUBBLES AND DRY SPOTS UNDER THE HEATED DOWNWARD-FACING SUBSTRATE

A comprehensive methodological approach is used to study bubble dynamics under the downward-facing substrate including high-speed imaging and interferometry simultaneously with shadow technique and infrared thermometry. A transparent indium-tin-oxide heater deposited on a sapphire substrate is utilized. This approach allows bubble and dry-spot dynamics, bubble coalescence, and the formation and evaporation of the microlayer to be investigated. The fluorocarbon liquid FC-72 is used as the working fluid. The study revealed significant nonlinear bubble and dry-spot growth rates with increasing heating power. The motion of the bubble relative to the heated substrate facilitates the formation of a microlayer at the bubble's periphery. It was observed that bubbles moving at higher velocity along the substrate at the same heating power experienced faster growth, apparently due to an increase in the area of the microlayer. It is established that the coalescence of bubbles results in the formation of a microlayer under the resulting bubble. The dynamics of the microlayer after coalescence is studied using interferometry. A significant increase in the evaporation rate of the microlayer with increasing heating power is observed. The considered processes play an important role in the microgravity conditions where, due to the lack of buoyancy force, the bubble does not detach from the heater. The presented experimental results can be useful for a deep understanding of bubble dynamics under the heated downward-facing substrate and for the future model development.