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Interfacial Phenomena and Heat Transfer
ESCI

ISSN Imprimer: 2169-2785
ISSN En ligne: 2167-857X

Open Access

Interfacial Phenomena and Heat Transfer

DOI: 10.1615/InterfacPhenomHeatTransfer.2018021373
pages 321-335

COMBINED EFFECT OF SUBSTRATE WETTABILITY AND THERMAL PROPERTIES ON EVAPORATION DYNAMICS OF A SESSILE DROPLET

Sachin K. Singh
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India
Mohit Gogna
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India
Krishnamurthy Muralidhar
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Sameer Khandekar
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur (UP) 208016, India

RÉSUMÉ

The prediction and manipulation of sessile droplet evaporation is an important problem with applications such as in microfluidics, printing, and microfabrication. The simple vapor diffusion model is frequently used to model droplet evaporation for the prediction of interfacial evaporation rates and evaporative flux, under the premise that vapor mass diffusion is the only physical transport mechanism responsible for droplet evaporation. However, droplet evaporation is known to be governed by additional transport mechanisms. Two of these are evaporative cooling at the air-liquid interface and heat conduction in the solid-liquid-gas domain. Furthermore, sessile droplet evaporation is also limited by the substrates' characteristics, specifically thermal properties and surface wettability. Hence, an extended vapor diffusion model has been built in COMSOL that incorporates the additional transport mechanisms and predicts evaporation rates, interfacial evaporative flux, and temperature distribution around the sessile droplet. This model has first been validated against controlled laboratory experiments for sessile droplet evaporation of water on copper and glass substrates, with distinct wettability. Subsequently, the model is used to discern the combined effect of substrate thermal properties and wettability on the evaporation dynamics of a sessile droplet. Results show that droplet evaporation on a highly hydrophobic or low thermal conductivity substrate is accompanied by significant lowering of temperature at the liquid-gas interface. This limits the capability of a simple vapor diffusion model to correctly predict droplet evaporation rates for such substrates. The extended vapor diffusion model, however, takes interfacial evaporative cooling and substrate properties into account and correctly predicts the ensuing evaporation rates. Specifically, when a sessile droplet evaporates on a low wettability and low thermal conductivity substrate, their combined effect causes a large drop in its evaporation rates, relative to what is estimated using a pure vapor diffusion model.


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