Interfacial Phase Change Phenomenon during Crystallization and Boiling in Droplets

Abstract

Two-phase interaction of droplets on heated substrates is ubiquitous and has applications in power plants, industrial cooling, and particle drying. At temperatures significantly higher than the saturation temperature of the liquid, the vigorous nucleate boiling is replaced by intermittent contact between the liquid and the substrate which is called the transitional boiling regime. The transitional boiling regime is marked by an increase in the total evaporation time of a constrained droplet with an increase in substrate temperature. At even higher substrate temperature, the liquid contact with the solid is eliminated due to the formation of an intervening vapor layer over which the droplet levitates. This Leidenfrost (LF) phenomena, while detrimental to cooling, is useful in applications such as formation of supra-particles in pharmaceuticals and food processing industries, as well as in facilitating the self-cleaning of substrates due to enhanced droplet mobility. In cooling applications utilizing water as the heat transfer medium, scaling or mineral fouling occurs on heat exchanger surfaces, leading to a decrease in thermal efficiency. This scaling is primarily caused by dissolved impurities such as salts, which adhere to the substrate due to high contact line pinning and make the removal of deposits challenging. In the first part of the thesis, we investigate the dynamics of evaporative crystallization in a saline droplet on substrates of different wettability with an objective to develop dependable strategies for scale mitigation. During evaporation of saline droplets on hydrophobic substrates with low contact line pinning, we observed out-of-plane crystallization that allowed for easy removal of the inorganic scales. The lift-off behavior of crystalline deposits from the substrate is shown to remain consistent across different salt types regardless of hydrophobic coating or initial droplet volumes. This observation holds promise for reduction of inorganic fouling in numerous industries using deionized water as their primary coolant. In the second part of the thesis, we explore the dynamics of unconstrained water droplets on substrates with different roughness in the transitional boiling regime. The bouncing dynamics of the droplet leads to a non-monotonic behavior in terms of the total duration of evaporation with increase in substrate temperature. A millimetric droplet is observed to trampoline on the substrate to heights 6-7 times its diameter and the height of bounce is observed to increase further as the droplet becomes smaller. We develop a model to explain the droplet trampolining in terms of the bubble nucleation and growth at the liquid-substrate interface prior to take-off. In the last part of the thesis, we study the bouncing dynamics of Leidenfrost droplets and unveil the underlying relationship between the internal flow field inside the droplet and the intervening vapor layer. We observe that LF droplet trampolines, i.e. the bouncing height of the droplet increases with each impact; an observation consistent across different substrates and liquids. We propose a resonance-driven phenomenon, where the increase in amplitude of droplet bouncing occurs when the frequency of the vapor layer oscillations is in multiples of the natural frequency of the droplet. Additionally, we examine the interplay between the liquid-vapor interface and internal flow dynamics in the LF state through simultaneous interferometric characterization of the vapor layer and particle image velocimetry measurements. We observe that as the LF droplet evaporates and the size of the droplet is less than the capillary length, a single directional internal convection roll resembling solid body rotation is developed. The interferometric characterizations reveal inclined droplet base, i.e., an asymmetric liquid-vapor interface. We demonstrate the influence of internal flows in the droplet on the shape of the vapor layer. The internal flow velocity is ~10 times lower than that of water in 25 wt% water-glycerol mixtures and consequently, the shape of the vapor layer stays symmetric.