Droplets Impacting and Migrating on Structured Surfaces With Imposed Thermal Gradients
01 Apr 2022-Journal of Tribology-transactions of The Asme (American Society of Mechanical Engineers Digital Collection)-Vol. 144, Iss: 4
TL;DR: In this paper, the dynamic process of oil droplets impacting and migrating on structured surfaces with imposed thermal gradients was investigated, and it was observed that on an isothermal smooth surface, a lubricant droplet would impact, spread to a maximum diameter, and retract.
Abstract: In this work, the dynamic process of oil droplets impacting and migrating on structured surfaces with imposed thermal gradients was investigated. It was observed that on an isothermal smooth surface, a lubricant droplet would impact, spread to a maximum diameter, and retract; while on a non-isothermal smooth surface, an asymmetric geometrical morphology of droplet was generated, accompanying with a migration process. Relevant dimensionless parameters were employed to evaluate the dynamic process, and the physical mechanism was revealed. Decorating surfaces with convergent microgrooves pattern could not only increase the maximum spreading diameter but also accelerate the migration process. These are beneficial for the heat exchange efficiency and lubrication performances. [DOI: 10.1115/1.4052779]
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TL;DR: In this paper, the authors present an attempt towards a unified picture with special emphasis on certain features of "dry spreading": (a) the final state of a spreading droplet need not be a monomolecular film; (b) the spreading drop is surrounded by a precursor film, where most of the available free energy is spent; and (c) polymer melts may slip on the solid and belong to a separate dynamical class, conceptually related to the spreading of superfluids.
Abstract: The wetting of solids by liquids is connected to physical chemistry (wettability), to statistical physics (pinning of the contact line, wetting transitions, etc.), to long-range forces (van der Waals, double layers), and to fluid dynamics. The present review represents an attempt towards a unified picture with special emphasis on certain features of "dry spreading": (a) the final state of a spreading droplet need not be a monomolecular film; (b) the spreading drop is surrounded by a precursor film, where most of the available free energy is spent; and (c) polymer melts may slip on the solid and belong to a separate dynamical class, conceptually related to the spreading of superfluids.
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TL;DR: In this article, experimental studies of wetting phenomena have helped to bridge the gap between the progress made in theory and simulation over the past decade, and the experimental evidence or verification of the theoretical predictions.
Abstract: Recent advancements in experimental studies of wetting phenomena have helped to bridge the gap between the progress made in theory and simulation over the past decade, and the experimental evidence or verification of the theoretical predictions. These developments include new measurements of the equilibrium thickness of precursor wetting films on solid and liquid substrates and at the liquid/gas interface, experimental studies of critical adsorption, as well as measurements of the dynamics of wetting and spreading and the nucleation of wetting layers in simple and complex systems. There have also been some recent results on dewetting of solid substrates by liquid films.
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"Droplets Impacting and Migrating on..." refers background in this paper
...Mathematically, the theoretical migration velocity (U ) can be simplified as [28,34]: U≈ΔTγTh/2μ, where h denotes the droplet height....
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TL;DR: By designing surfaces with tapered micro/nanotextures which behave as harmonic springs, the timescales become independent of the impact velocity, allowing the occurrence of pancake bouncing and rapid drop detachment over a wide range of impact velocities.
Abstract: When a water drop bounces back from a hydrophobic surface, its initial, spherical shape is usually restored. Now, experiments with a specially engineered superhydrophobic surface made from micrometre-sized tapered pillars covered with copper oxide ‘nanoflowers’ show that droplets can bounce back with a flat, pancake-like shape. Engineering surfaces that promote rapid drop detachment1,2 is of importance to a wide range of applications including anti-icing3,4,5, dropwise condensation6 and self-cleaning7,8,9. Here we show how superhydrophobic surfaces patterned with lattices of submillimetre-scale posts decorated with nanotextures can generate a counter-intuitive bouncing regime: drops spread on impact and then leave the surface in a flattened, pancake shape without retracting. This allows a fourfold reduction in contact time compared with conventional complete rebound 1,10,11,12,13. We demonstrate that the pancake bouncing results from the rectification of capillary energy stored in the penetrated liquid into upward motion adequate to lift the drop. Moreover, the timescales for lateral drop spreading over the surface and for vertical motion must be comparable. In particular, by designing surfaces with tapered micro/nanotextures that behave as harmonic springs, the timescales become independent of the impact velocity, allowing the occurrence of pancake bouncing and rapid drop detachment over a wide range of impact velocities.
698 citations
"Droplets Impacting and Migrating on..." refers background in this paper
...Previously, water droplet impact on solid surfaces has been widely investigated, researchers have successfully fabricated microstructures patterns [15–18], superhydrophobic coating [19,20], or wettability gradient [21,22] to control water droplets impact dynamic....
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621 citations
"Droplets Impacting and Migrating on..." refers background in this paper
...This is different from water droplet dynamic under a similar impacting velocity, of which the droplet would experience prompt splash within spreading process and break-up within retracting process [27]....
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