Abstract
Abstracts Evaporating water droplets on a heated substrate are investigated in this work. Specifically, the influences of electric fields are studied in the context of the heat flux distribution beneath the droplets as well as the droplet mechanics and resulting shapes and forces. To facilitate a deeper understanding of the problem, both hydrophilic and superhydrophobic droplets are considered for an entire evaporation period with and without electric field effects. Both wetting scenarios show that the net radial directed electric force is directed inward, resulting in a compressive force which influences the droplet shape in such a way that it appears elongated. Conversely, the net vertically directed electric force is determined to be downwardly directed for hydrophilic droplets, pressing the droplet to the surface, whereas it is upwardly directed for the superhydrophobic droplets, representing a lifting force. With regard to the heat transfer to the droplets, only a pronounced electric field effect was observed for the superhydrophobic droplet. For all droplets, the contact line density, representing the ratio of the contact line perimeter to the total base area of the droplet, is determined to be a parameter that unifies the average heat flux from the heater to the droplets. This suggests that the heat transfer to the base of the droplet in the presence of an electric field is dominated by the electric fields influence, or lack thereof, on the contact line density.
Highlights
Phase change phenomena underpin the performance of several engineered systems across a broad spectrum of applications
The hydrophilic and superhydrophobic data presented here demonstrate that the electric field is an important parameter only if it significantly influences the droplet shape in such a way that it augments the contact line density of the evaporating droplet
If the electric field acts to change the Contact Line Density (CLD), a notable change in the heat transfer characteristics of the droplet is observed, and this is due to the shape augmentation and not specific EHD induced heat transfer enhancement
Summary
Phase change phenomena underpin the performance of several engineered systems across a broad spectrum of applications. The proximity of the heated surface and the liquid-gas interface at the contact line region combined with the high vapour diffusion to the ambient creates an overall lower thermal resistance pathway between the heat source (heated substrate) and the heat sink (the ambient air) It has been shown in the literature that when an ambient temperature water droplet is placed on a heated substrate, Marangoni convection initially dominates due to the temperature gradient from the base of the droplet to the droplet apex. In their first study [33], for an applied voltage of 300 V, they reported an evaporation time enhancement of ~3 × for water, ~20 × for ethanol, and ~1.3 × for cyclohexane in comparison with the non-electric field case. The droplet interfaces are experimentally and numerically analysed and contrasted to investigate the impact of the electric field on geometric properties and subsequent heat transfer and droplet mechanics
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