Abstract
The breakup dynamics of a capillary bridge on a hydrophobic stripe between two hydrophilic stripes is studied experimentally and numerically using direct numerical simulations. The capillary bridge is formed from an evaporating water droplet wetting three neighboring stripes of a chemically patterned surface. By considering the breakup process in a phase space representation, the breakup dynamics can be evaluated without the uncertainty in determining the precise breakup time. The simulations are based on the Volume-of-Fluid (VOF) method implemented in Free Surface 3D (FS3D). In order to construct physically realistic initial data for the VOF simulation, Surface Evolver is employed to calculate an initial configuration consistent with experiments. Numerical instabilities at the contact line are reduced by a novel discretization of the Navier-slip boundary condition on staggered grids. The breakup of the capillary bridge cannot be characterized by a unique scaling relationship. Instead, at different stages of the breakup process different scaling exponents apply, and the structure of the bridge undergoes a qualitative change. In the final stage of breakup, the capillary bridge forms a liquid thread that breaks up consistently with the Rayleigh-Plateau instability.
Highlights
Wetting of patterned surfaces is omni-present in nature
The simulations are based on the Volume-of-Fluid (VOF) method implemented in Free Surface 3D (FS3D)
Hartmann and Hardt [30] showed that an evaporating droplet wetting two hydrophilic stripes, with a hydrophobic one in between them, is stable as long as the pressure inside the liquid forming the capillary bridge above the hydrophobic stripe can be balanced in the liquid above the two hydrophilic stripes
Summary
Wetting of patterned surfaces is omni-present in nature. The Lotus effect [3] or the fog harvesting of the Stenocara desert beetle in the Namib Desert [45] are only two examples. The focus lies on chemically patterned striped surfaces that are wetted by water droplets with radii of the order of magnitude of the stripe width. For such kind of systems, static wetting behavior has been studied by using energy minimization techniques [12, 32, 42], lattice Boltzmann simulations [34, 35] and experiments [4]. While in the latter publication the focus is on the statics of an evaporating droplet, the subject of the present article is the dynamics of the breakup process itself
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