Dynamic wetting and spreading and the role of topography

McHale, G., Newton, M.I. ORCID: 0000-0003-4231-1002 and Shirtcliffe, N.J., 2009. Dynamic wetting and spreading and the role of topography. Journal of Physics - Condensed Matter, 21 (46).

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Hoffman-de Gennes law, which relates the edge speed, ve, to the dynamic and equilibrium contact angles q and qe by ve µq(q2 -qe 2 ). When the liquid wets the surface completely and the equilibrium contact angle vanishes, the edge speed is proportional to the cube of the dynamic contact angle. When the droplets are non-volatile this law gives rise to simple power laws with time for the contact angle and other parameters in both the capillary and gravity dominated regimes. On a textured surface the equilibrium state of a droplet is strongly modified due to the amplification of the surface chemistry induced tendencies by the topography. The most common example is the conversion of hydrophobicity into superhydrophobicity. However, when the surface chemistry favors partial wetting, topography can result in a droplet spreading completely. A further, frequently over-looked consequence of topography is that the rate at which an out-of-equilibrium droplet spreads should also be modified. In this report, we review ideas related to the idea of topography induced wetting and consider how this may relate to dynamic wetting and the rate of droplet spreading. We consider the effect of the Wenzel and Cassie-Baxter equations on the driving forces and discuss how these may modify power-laws for spreading. We relate the ideas to both the hydrodynamic viscous dissipation model and the molecular-kinetic theory of spreading. This suggests roughness and solid surface fraction modified Hoffman-de Gennes laws relating the edge speed to the dynamic and equilibrium contact angle.

Item Type: Journal article
Publication Title: Journal of Physics - Condensed Matter
Creators: McHale, G., Newton, M.I. and Shirtcliffe, N.J.
Publisher: Institute of Physics
Date: 2009
Volume: 21
Number: 46
Rights: © IOP Publishing 2009
Divisions: Schools > School of Science and Technology
Record created by: EPrints Services
Date Added: 09 Oct 2015 10:10
Last Modified: 09 Jun 2017 13:20
URI: https://irep.ntu.ac.uk/id/eprint/8733

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