Influence of Local Heating on Marangoni Flows and Evaporation Kinetics of Pure Water Drops

Abstract

The effect of localized heating on the evaporation of pure sessile water drops was probed experimentally by a combination of infrared thermography and optical imaging. In particular, we studied the effect of three different heating powers and two different locations, directly below the center and edge of the drop. In all cases, four distinct stages were identified according to the emerging thermal patterns. In particular, depending on heating location, recirculating vortices emerge that either remain pinned or move azimuthally within the drop. Eventually, these vortices oscillate in different modes depending on heating location. Infrared data allowed extraction of temperature distribution on each drop surface. In turn, the flow velocity in each case was calculated and was found to be higher for edge heating, due to the one-directional nature of the heating. Additionally, calculation of the dimensionless Marangoni and Rayleigh numbers yielded the prevalence of Marangoni convection. Heating the water drops also affected the evaporation kinetics by promoting the "stick-slip" regime. Moreover, both the total number of depinning events and the pinning strength were found to be highly dependent on heating location. Lastly, we report a higher than predicted relationship between evaporation rate and heating temperature, due to the added influence of the recirculating flows on temperature distribution and hence evaporation flux.

Taming the Coffee Ring Effect: Enhanced Thermal Control as a Method for Thin-Film Nanopatterning

“… 49 Importantly, the simulated azimuthal relocation of the flow vortices within the droplets has also been confirmed and experimentally studied by Askounis et al. 50 …”

Nonuniform heating of a substrate in evaporative lithography

“…This led to the appearance of Marangoni flows. Various thermal patterns associated with hydrothermal waves have been observed [27]. The simulation results showed that the evaporation is jointly affected by the local heating of the droplet and the wettability of the substrate [28].…”

“…The thermocapillary rupture of the film in a cell with a built-in heater was experimentally investigated in [24,25,26]. The kinetics of evaporation under local heating of the droplet was studied in [27]. Point heating of the substrate with a laser was carried out in the area of the center or periphery of the drop.…”

Interplay of transport mechanisms during the evaporation of a pinned sessile water droplet

“…Convective transport mechanisms stem from the buoyant and thermocapillary forces [7]. Since the dominance of thermocapillary forces over buoyant forces is well established [8,9], convection mechanisms associated with thermocapillarity have been the subject of the studies [10,11]. Thermocapillary effect refers to the pulling of the liquid from a warmer region to a colder region on the interface.…”