Transient Effects in Evaporating Sessile Drops: With and Without Heating

Abstract

The evaporation phenomenon of sessile drops has been recently subject to an extensive interest by industry and researchers. This is stimulated by new developments in exploiting this basic process for more industrial technologies and biological applications. The underlying mechanisms to this apparently simple, yet elusive phenomenon as well as its complete description are still far from being achieved. Many theoretical models describe the phenomenon by neglecting some important physical aspects of the problem. Transient thermal effects can indeed be very crucial, nonetheless very often neglected. In a recent work, a new approach was adopted to model the physical process taking into account the thermal resistance of the substrate. This was, however, limited to the investigation of cases where steadystate assumption is adopted. In such pseudo steadystate, a controlling nondimensional SB number was identified. The evaporation of sessile drops deposited on a substrate is found to exhibit various regimes. These latter are related to the wetting and spreading behavior of the drop, depending on whether the drop is pinned with a decreasing contact angle, with a receding contact line and constant angle or a mixed behavior. Most modeling attempts have considered vapor diffusion in the gas phase as the limiting mechanism for evaporation. However, the heat and mass transfer in the solid, liquid, and gas phases describe the problem and predict droplets evaporation. It is worth noting that most theoretical and numerical models proposed so far assume the quasi steadystate hypothesis and neglect transient effects. It is essential to acknowledge that not only the three phases (gas, solid, and liquid) take part in mass and energy transport but also the interfaces between these phases are equally important. The liquid–vapor interface, for instance is the surface through which phase change takes place. This interface is subjected to evaporative cooling effects, depending on the physical dimensions, properties as well as experimental conditions. In the present paper, we propose to extend this approach to account for transient effects. The results of this investigation demonstrate that in some cases transient effects can extend beyond the lifetime of the drop, making the entire process transitory. These effects are quantified and the implications for modeling wetting drops are discussed.