Water Drop Friction on Superhydrophobic Surfaces
- Publication date
- Publisher
Abstract
To
investigate water drop friction on superhydrophobic surfaces,
the motion of water drops on three different superhydrophobic surfaces
has been studied by allowing drops to slide down an incline and capturing
their motion using high-speed video. Two surfaces were prepared using
crystallization of an alkyl ketene dimer (AKD) wax, and the third
surface was the leaf of a Lotus (Nelumbo Nucifera). The acceleration of the water droplets on these superhydrophobic
surfaces was measured as a function of droplet size and inclination
of the surface. For small capillary numbers, we propose that the energy
dissipation is dominated by intermittent pinning–depinning
transitions at microscopic pinning sites along the trailing contact
line of the drop, while at capillary numbers exceeding a critical
value, energy dissipation is dominated by circulatory flow in the
vicinity of the contacting disc between the droplet and the surface.
By combining the results of the droplet acceleration with a theoretical
model based on energy dissipation, we have introduced a material-specific
coefficient called the superhydrophobic sliding resistance, <i>b</i><sub>sh</sub>. Once determined, this parameter is sufficient
for predicting the motion of water drops on superhydrophobic surfaces
of a general macroscopic topography. This theory also infers the existence
of an equilibrium sliding angle, β<sub>eq</sub>, at which the
drop acceleration is zero. This angle is decreasing with the radius
of the drop and is in quantitative agreement with the measured tilt
angles required for a stationary drop to start sliding down an incline