Pinning-Free Evaporation of Sessile Droplets of Water from Solid Surfaces

Armstrong, Steven, McHale, Glen, Ledesma Aguilar, Rodrigo and Wells, Gary (2019) Pinning-Free Evaporation of Sessile Droplets of Water from Solid Surfaces. Langmuir, 35 (8). pp. 2989-2996. ISSN 0743-7463

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Abstract

Contact-line pinning is a fundamental limitation to the motion of contact lines of liquids on solid surfaces. When a sessile droplet evaporates, contact-line pinning typically results in either a stick-slip evaporation mode, where the contact line pins and de-pins from the surface in an uncontrolled manner or a constant contact area mode with a pinned contact line. Pinning prevents the observation of the quasi-equilibrium constant contact angle mode of evaporation, which has never been observed for sessile droplets of water directly resting on a smooth, non-textured, solid surface. Here, we report the evaporation of a sessile droplet from a flat glass substrate treated with a smooth, Slippery Omni-phobic Covalently Attached Liquid-like (SOCAL) coating. Our characterization of the surfaces shows a high contact line mobility with an extremely low contact angle hysteresis of ~1°, and reveals a step change in the value of the contact angle from 101° to 105° between a relative humidity of 30% and 40%, in a manner reminiscent of the transition observed in a type V adsorption isotherm. We observe the evaporation of small sessile droplets in a chamber held at a constant temperature, T=(25.0 ± 0.1) °C and at constant relative humidity (RH) across the range RH=10-70%. In all cases a constant contact angle mode of evaporation is observed for most of the evaporation time. Furthermore, we analyze the evaporation sequences using the Picknett & Bexon ideal constant contact angle mode for diffusion-limited evaporation. The resulting estimate for the diffusion coefficient, DE, of water vapor in air of DE=(2.44 ± 0.48) x 10-5 m2s-1 is accurate to within 2% of the value reported in the literature, thus validating the constant contact angle mode of the diffusion-limited evaporation model.

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