Thermocapillary Effect in Heterogeneously-Formed Bubbles in Microsystems Available to Purchase

Bubble nucleation and growth in microsystems, and in microchannels supporting subcooled nucleate boiling, occur within liquids with extremely large temperature gradients. Non-uniform bubble-liquid interfacial temperatures may occur, leading to important thermocapillary effects. This analytical investigation is an attempt to demonstrate the potential impact of the thermocapillary effects on heterogeneously-generated micro-bubbles. Quasi-steady bubbles occupying heated microtubes are first modeled. It is shown that the temperature of the liquid-vapor interphase in these bubbles can be non-uniform. This temperature non-uniformity increases with increasing the heat transfer rate, and depends on the microtube size. A method for the prediction of liquid-vapor interphase geometry resulting from non-uniform bubble surface temperature is developed. It is shown that the aforementioned bubbles can be distorted from spherical geometry rather significantly. Quasi-steady bubbles attached to a heated microchannel surface supporting subcooled nucleate boiling are also modeled. The bubble-liquid interfacial temperature distribution is estimated based on assuming equal evaporation and condensation mass fluxes at the bubble base and top, and using the simple gas kinetic theory methods. The result shows that the thermocapillary effect tends to slightly distort and elongate the bubbles in the direction perpendicular to the heated surface, and leads to a thermocapillary force that resists bubble detachment. The geometric distortion of the bubble leads to an increase in drag force in comparison with a chopped-spherical bubble with equal volume.