HEAT FLUX DURING DIP-COATING OF A SUPERHEATED SUBSTRATE

We report transient heat flux calculations based on temperature measurements during dip-coating of a superheated substrate. During the withdrawal of the substrate from a pool of volatile liquid, a film of finite length forms on the substrate’s surface, locally reducing the substrate temperature due to evaporation. The surface temperature of the solid substrate is measured using high-resolution infrared thermography and used as a boundary condition to calculate the transient heat flux profiles at the interface between the superheated substrate and the fluid. The shapes of these heat flux profiles are analyzed with special focus on the local heat flux in the thin film region and near the three-phase contact line. It is shown how the heat flux in both regions is dependent on wall superheat and dewetting velocity. Two evaporation regimes, namely contact line evaporation and microlayer evaporation, can be clearly distinguished by their magnitude in overall heat flux. A temperature-dependent critical velocity separates both regimes. The local heat flux in the contact line region sharply increases, when the critical velocity is exceeded. Within the thin film, the local heat flux increases with growing wall superheat and decreases with growing dewetting velocity.