Sweeping by sessile drop coalescence
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part of Springer Nature, 2020 https://doi.org/10.1140/epjst/e2020-900265-5
THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS
Regular Article
Sweeping by sessile drop coalescence? Jonathan M. Ludwicki and Paul H. Steena Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY, 14853, USA Received 9 November 2019 / Accepted 6 July 2020 Published online 14 September 2020 Abstract. During coalescence of liquid drops contacting a solid, the liquid sweeps wetted and solid-projected areas. The extent of sweeping dictates the performance of devices such as self-cleaning surfaces, antifrost coatings, water harvesters, and dropwise condensers. For these applications, weakly- and non-wetting solid substrates are preferred as they enhance drop dynamical behavior. Accordingly, our coalescence studies here are restricted to drops with contact angle 90◦ ≤ θ0 ≤ 180◦ . Binary sessile drop coalescence is the focus, with volume of fluid simulations employed as the primary tool. The simulations, which incorporate a Kistler dynamic contact angle model, are first validated against three different experimental substrate systems and then used to study the influence of solid wettability on sweeping by modifying θ0 . With increasing θ0 up to 150◦ , wetted and projected swept areas both increase as drop center of mass heightens. For θ0 ≥ 150◦ , coalescence-induced drop jumping occurs owing to the decreasing wettability of the substrate and a focusing of liquid momentum due to the symmetry-breaking solid. In this regime, projected swept area continues to increase with θ0 while wetted swept area reaches a maximum and then decreases. The sweeping results are interpreted using the mechanical energy balance from hydrodynamic theory and also compared to free drop coalescence.
1 Introduction For coalescing capillary drops, liquid advances and recedes along a solid surface evolving complicated patterns of wetted and projected areas. Instantaneous wetted W is the area on the surface enclosed by the contact line, while instantaneous projected P is the area of an imaginary shadow cast from above by parallel projection of light onto the surface. Projected exceeds wetted area for drops on non-wetting and hydrophobic surfaces where initial contact angle θ0 > 90◦ , Figure 1a. There is recent interest in the behaviors of drops on surfaces where wetted and projected areas differ [1–5], including sessile drop coalescence. During sessile drop coalescence, two or more drops in contact with a surface merge to form a final drop. This is a dynamic event where the liquid exhibits a complex shape evolution while progressing towards a final equilibrium configuration [6–8]. In some instances, wetted area must change topology ? Supplementary material in the form of one zip file available from the Journal web page at https://doi.org/10.1140/epjst/e2020-900265-5 a
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The European Physical Journal Special Topics
Fig. 1. (a) Sessile drop: initial contact angle θ0 > 90◦ , projected P and wetted W con
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