UCLA

Sessile droplet evaporation is an omnipresent phenomenon both in nature and technologies such as biodiagnostics, microfabrication, inkjet printing, spray cooling, and agriculture irrigation. Evolution of single component sessile droplets has been extensively studied under various parameters. However, in real applications, sessile droplets usually consist of two or more components. It has been shown that the environmental conditions such as humidity and temperature substantially change the behavior of sessile droplets. The strong tendency of organic fluids to absorb water is an important factor in evaporation of these fluids in humid environment. The water vapor present in the surrounding adsorbs/absorbs and possibly condenses into the droplet transforming the droplet into a binary system. While the humidity of surrounding is typically an imposed condition resulting in unwanted effects for many industries, we have proposed that these undesired effects can be controlled and eliminated by tuning the temperature of the substrate. We have studied the combined effect of relative humidity of surrounding and substrate temperature on evaporation of methanol droplets. Our results demonstrated that the diffusion of water into the droplet can be limited by changing temperature of the substrate by both shortening the lifetime of droplet or increasing the temperature of the liquid-gas interface above the due point. Additionally, we have developed machine learning, classification and regression, models to analyze the behavior of droplet under different conditions. We have shown that the regime of droplet evaporation can be accurately classified by analyzing the profile of the evolution of droplet macroscopic parameters. We have also demonstrated that the humidity of surrounding can be accurately estimated by analysis of droplet profile. Furthermore, the time evolution of diameter and contact angle are estimated by the regression model. As the number of components in the droplet increases, the underlying mechanisms become more complex. The proposed approach to analyze the dynamics of sessile droplet evaporation through data-driven techniques opens up ways to better understand the complicated physics behind multi-component droplet evaporation and in general intricate interfacial fluid mechanics problems. The combined effect of surrounding humidity and substrate temperature has been experimentally studied on the behavior of ternary droplet consisting of methanol, anise oil, and water. The simultaneous optical microscopy and infrared thermography revealed different mechanisms in the droplet such as hydrothermal waves, oil microdroplet nucleation, etc. Our results showed three stages in the evolution of hydrothermal waves during droplet lifetime. The experimental procedures and results in this work introduce easy and inexpensive method to control sessile droplet behavior which are crucial for the final product resolution in numerous applications.