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Hybrid Surface Design for Robust Superhydrophobicity
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posted on 2012-06-26, 00:00 authored by Susmita Dash, Marie T. Alt, Suresh V. GarimellaSurfaces may be rendered superhydrophobic by engineering
the surface
morphology to control the extent of the liquid–air interface
and by the use of low-surface-energy coatings. The droplet state on
a superhydrophobic surface under static and dynamic conditions may
be explained in terms of the relative magnitudes of the wetting and
antiwetting pressures acting at the liquid–air interface on
the substrate. In this paper, we discuss the design and fabrication
of hollow hybrid superhydrophobic surfaces which incorporate both
communicating and noncommunicating air gaps. The surface design is
analytically shown to exhibit higher capillary (or nonwetting) pressure
compared to solid pillars with only communicating air gaps. Six hybrid
surfaces are fabricated with different surface parameters selected
such that the Cassie state of a droplet is energetically favorable.
The robustness of the surfaces is tested under dynamic impingement
conditions, and droplet dynamics are explained using pressure-based
transitions between Cassie and Wenzel states. During droplet impingement,
the effective water hammer pressure acting due to the sudden change
in the velocity of the droplet is determined experimentally and is
found to be at least 2 orders of magnitude less than values reported
in the literature. The experiments show that the water hammer pressure
depends on the surface morphology and capillary pressure of the surface.
We propose that the observed reduction in shock pressure may be attributed
to the presence of air gaps in the substrate. This feature allows
liquid deformation and hence avoids the sudden stoppage of the droplet
motion as opposed to droplet behavior on smooth surfaces.