“Phase Diagram” of Surface Temperature Distribution of Sessile Droplets and the Effects of Evaporative Cooling

The temperature distribution along the surface of evaporating droplets can affect significantly the flow field inside the liquid and consequently the deposition pattern on the substrate. Although a “phase diagram” for the temperature distribution along the droplet surface was revealed by the numerical simulations, its experimental verification has still not been reported. In this paper, the surface temperature of evaporating droplets has been observed by using an infrared (IR) camera. The experimental observations show that three different patterns of temperature distribution along the droplet surface occur in succession with the change of the contact angle during the evaporation process, which is in good agreement with the theoretical predictions by the “phase diagram” of the surface temperature distribution. Furthermore, the effects of evaporative cooling on the “phase diagram” of sessile droplets have been explored. The numerical results indicate that the evaporative cooling effect can alter the size of the phase regions in the “phase diagram”. These results may provide a better understanding of the evaporation process of drying sessile droplets.

A modeling study of different kinds of sessile droplets on the horizontal surface with surface wettability and gravity effects considered

Droplet phase change is important for energy storage and saving technology.The initial profile of the droplet is extremely important for its vaporization or solidification on a horizontal surface.To understand the effect of liquid physical properties on droplet profile,a theoretical model was developed in this study,based on the Young-Laplace equation with gravity effect specially considered.After the model was experimentally validated by comparing the geometric shape of water droplets,it was further used for predicting droplet shapes of other materials,and thus analyzing the influence of different physical properties,such as temperature,pressure,surface wettability,etc.Results show that the results of this model agree well with the experimental data.The maximum and average deviations are less than 4.5%and 1.5%,respectively.For all kinds of droplets on the fixed surfaces,when the temperature increases,the droplet contact radius increases and height decreases.The droplets of nitro-gen and carbon dioxide are more sensitive to temperature than ethanol and ethylene glycol droplets.For 20𝜇L droplets on the surface of contact angle 150°,when the temperature changes from 273.15 K to 293.15 K,the droplet contact radiuses increase by 30.6%for carbon dioxide,1.2%for ethanol and 0.67%for ethylene glycol,and the droplet heights decrease by 42.9%,2.5%,1.1%,respectively.Results of this study are meaningful for predicting the phase change process of droplets on the horizontal surface by controlling their initial profiles.

A Review on the Evaporation Dynamics of Sessile Drops of Binary Mixtures:Challenges and Opportunities

The wetting and evaporation dynamics of sessile droplets have gained considerable attention over the last few years due to their relevance to many practical applications,ranging from a variety of industrial problems to several biological systems.Droplets made of binary mixtures typically undergo complex dynamics due to the differential volatility of the considered components and the ensuing presence of thermocapillary effects.For these reasons,many research groups have focused on the evaporation of binary droplets using a variegated set of experimental,numerical,and purely theoretical approaches.Apart from reviewing the state-of-the-art about the existing experimental,analytical,and computational techniques used to study the evaporation dynamics of binary sessile droplets,we also provide some indications about possible future research directions in this specific area.