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 depins 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, nontextured, 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 coating. Our characterization of the surfaces shows 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 (RH) 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 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 and 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) × 10-5 m2 s-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.
Highlights
Evaporation of liquids occurs when the atmosphere surrounding the liquid is not saturated with the vapor of the liquid.[1]
slippery omniphobic covalently attached liquid-like (SOCAL) surfaces were created on 25 × 75 mm glass slides, using the method detailed by Wang and McCarthy, adapted to our specific equipment and with process parameters iteratively developed until a reproducible and low contactangle hysteresis method was achieved.[29]
The final stage of evaporation appears to be correlated to the observation of mineral deposit formation when the droplet radius reduced to ∼0.5 mm, which is the radius at which the contact angle first begins to decrease
Summary
Evaporation of liquids occurs when the atmosphere surrounding the liquid is not saturated with the vapor of the liquid.[1] It is a widely observed natural phenomenon and is important for many applications, including inkjet printing,[2] fuel delivery,[3] and heat exchange.[4] In these applications, droplets rest on a solid surface and this introduces two fundamental differences as to how evaporation occurs compared to spherical droplets in free space far from any surface. Nonuniform particle deposition causes problems in a diverse range of applications, from nonuniform delivery of the active components in aerosols used in pesticides to nonuniform fluorescence in spotted microarrays.[1,5−8] While, the effect on diffusion of a surface can be modeled, contact-line pinning is usually an unavoidable consequence of the contact between a droplet and the surface to which it is attached
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