Session: 11-13-01 Fundamentals and Applications of Evaporation, Boiling and Condensation

Paper Number: 72622

Start Time: Wednesday, 10:15 AM

72622 - Surface Evaporation of Sessile Water Droplet on a Hydrophobic Surface 

Droplet evaporation is found in various engineering and medical applications such as spray cooling, fuel injection in combustion engines, DNA deposition, drug screening, and molecular assay. The evaporation of micro-scale sessile droplets on a solid substrate is governed by surface tension rather than gravity. The multi-dimensional transient heat and mass transport of three (gas, liquid, and solid) phases in the droplet evaporation is rather complex, and a comprehensive understanding of the underlying fundamental mechanisms is critical in utilizing the droplet evaporation process in the applications. In this paper, the surface evaporation of a sessile droplet on solid substrates was numerically and experimentally investigated. A Computational Fluid Dynamics (CFD) simulation was performed to analyze the simultaneous heat and mass transfer in the surface evaporation of a sessile water droplet on hydrophobic substrates in different ambient relative humidity. The results from the CFD simulation were validated using the experimental results for the water droplet evaporation on a hydrophobic-coated silicon substrate. The computational and experimental results were in good agreement. It is found known that a thermocapillary-driven flow (Marangoni convection) and natural convection play a great role in the droplet evaporation. To investigate the interplaying effect of the Marangoni and natural convection on the droplet evaporation, the CFD simulations were performed by varying key parameters: contact angle, substrate thermal conductivity, and ambient relative humidity. The droplet evaporation rate is greatly increased by the Marangoni convection in the droplet with small liquid volumes less than 2.4μL on a superhydrophobic substrate with a contact angle of 147.3°, while the contribution of the natural convection is negligible. The Marangoni convection causes a liquid flow on the liquid-gas interface of the droplet from the three-phase contact line (hot area) on the substrate toward the droplet apex (cold area) causing an internal liquid circulation in the droplet. On hydrophobic surfaces, as the contact angle decreases, the evaporation rate increases because the solid-liquid contact area increases and thus the heat transfer from the substrate increases. In contrast, on hydrophilic surfaces, as the contact angle decreases, the evaporation rate decreases due to the thick layer of high vapor concentration covering the isothermal droplet of a high curvature radius. The substrate thermal conductivity greatly affects the droplet evaporation rate. The peak mass flux of evaporation observed along the three-phase contact line on the solid substrate becomes more noticeable on the solid substrate with high thermal conductivities. The evaporation rate decreases linearly as the relative humidity in ambient air and thus the vapor concentration in the gas domain increase.

Presenting Author: Chanwoo Park University of Missouri

Authors:

Minwoo Lee University of Missouri
Chanwoo Park University of Missouri