Understanding drop evaporation mechanisms is important for many industrial, biological, and other applications. Drops of organic solvents undergoing evaporation have been found to display distinct thermal patterns, which in turn depend on the physical properties of the liquid, the substrate, and ambient conditions. These patterns have been reported previously to be bulk patterns from the solid-liquid to the liquid-gas drop interface. In the present work the effect of ambient temperature and humidity during the first stage of evaporation, i.e., pinned contact line, is studied paying special attention to the thermal information retrieved at the liquid-gas interface through IR thermography. This is coupled with drop profile monitoring to experimentally investigate the effect of ambient temperature and relative humidity on the drop interfacial thermal patterns and the evaporation rate. Results indicate that self-generated thermal patterns are enhanced by an increase in ambient temperature and/or a decrease in humidity. The more active thermal patterns observed at high ambient temperatures are explained in light of a greater temperature difference generated between the apex and the edge of the drop due to greater evaporative cooling. On the other hand, the presence of water humidity in the atmosphere is found to decrease the temperature difference along the drop interface due to the heat of adsorption, absorption and/or that of condensation of water onto the ethanol drops. The control, i.e., enhancement or suppression, of these thermal patterns at the drop interface by means of ambient temperature and relative humidity is quantified and reported.

The effect of ambient temperature and relative humidity on the dynamics of ethanol drop evaporation is investigated. Although drop evaporation of mixtures and pure fluids has been extensively studied, very little is known about the transition from a pure fluid to a binary mixture following transfer of a second component present in the atmosphere. This is of importance for industrial, biological and medical applications where the purity of the solvent is paramount. Adsorption-absorption and/or condensation of water into ethanol drops during evaporation is presented through direct quantification of the drop composition in time. In particular, we combine drop profile measurements with Gas Injection Chromatography (GIC) to directly quantify the amount of ethanol evaporated and that of water intake over time. As expected, drops evaporate faster at higher temperatures since both the ethanol saturation concentration and the vapor diffusion coefficient are directly proportional to temperature. On the other hand, increases in the ethanol evaporation rate and in the water intake are observed at higher relative humidity. The increase in ethanol evaporation at higher relative humidity is interpreted by the greater diffusion coefficient of ethanol into humid air when compared to dry air. Moreover, as ethanol evaporates in a high humidity environment, the drop interfacial temperature falls below the dew point due to evaporative cooling and water condenses compared to lower humidity conditions. As a consequence of the heat released by adsorption-absorption and/or condensation, a greater temperature is reported at the liquid-vapor interface as confirmed by IR thermography, inducing a greater ethanol saturation concentration at the surface and hence a greater driving force for evaporation. By coupling the drop profile and the composition of ethanol and water within the drop, we propose a combined evaporation-adsorption/absorption and/or condensation empirical correlation. The proposed correlation accounts for: the decreases in ethanol concentration due to water adsorption-absorption and/or condensation, the diffusion coefficient dependence on relative humidity, and the amount of water intake during evaporation. The proposed empirical correlation agrees remarkably well with experimental observations.

We report on experimental observations/visualization of thermocapillary or Marangoni flows in a pure water drop via infrared thermography. The Marangoni flows were induced by imposing a temperature gradient on the drop by locally heating the substrate directly below the center with a laser. Evidently, a temperature gradient along the liquid-air interface of ca. 2.5 °C was required for the Marangoni flows to be initiated as twin vortices and a subsequent gradient of ca. 1.5 °C to maintain them. The vortices exhibited an oscillatory behavior where they merged and split in order for the drop to compensate for the non-uniform heating and cooling. The origin of these patterns was identified by comparing the dimensionless Marangoni and Rayleigh numbers, which showed the dominance of the Marangoni convection. This fact was further supported by a second set of experiments where the same flow patterns were observed when the drop was inverted (pendant drop).

Influence of wettability contrasts and contact angle hysteresis on drop velocity and surface energy analysis describing the drop motion.

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.