Breakup Dynamics of Capillary Bridges on Hydrophobic Stripes

Abstract

The breakup dynamics of a capillary bridge on a hydrophobic stripe between two hydrophilic stripes is studied experimentally and numerically. The capillary bridge is formed from an evaporating water droplet wetting three neighboring stripes of a chemically patterned surface. 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 adaptation of the Navier-slip boundary condition. By considering the breakup process in phase space, the breakup dynamics can be evaluated without the uncertainty in determining the precise breakup time. It is found that within an intermediate inviscid regime, the breakup dynamics follows a t 2/3 -scaling, indicating that the breakup process is dominated by the balance of inertial and capillary forces. For smaller bridge widths, the breakup velocity reaches a plateau, which is due to viscous forces becoming more important. In the final stage of breakup, the capillary bridge forms a liquid thread that breaks up consistent with the Rayleigh-Plateau instability. The critical wavelength is identical to the distance between the tips of two liquid cones between which the thread is arranged. The existence of satellite droplets in a regular pattern indicates that the primary breakup process is followed by self-similar secondary breakups.

Computing volume fractions and signed distances from triangulated surfaces immersed in unstructured meshes

“…A novelty of SMCI/A algorithms is their ability to compute volume fractions from arbitrary surfaces on Figure 20: Application of the SMCI-approach to a liquid-gas interface of a liquid drop on a coated surface. The interface is provided by the blue STL-data from [15] and intersected with the equidistant background mesh yielding the volume fractions indicated by the gray-scale in the inset. Image adapted from [15].…”

“…The interface is provided by the blue STL-data from [15] and intersected with the equidistant background mesh yielding the volume fractions indicated by the gray-scale in the inset. Image adapted from [15].…”

“…However, many problems have more complex initial conditions than, e.g. a sphere or a planar interface, especially when the shape of the fluid interface is prescribed from an experiment [15]. We present a new algorithm that calculates initial conditions for such problems in terms of signed distances as well as volume fractions from an arbitrarily shaped fluid interface on unstructured meshes that can contain mixed-shape cells.…”