Unidirectional Fast Growth and Forced Jumping of Stretched Droplets on Nanostructured Microporous Surfaces

Superhydrophobic nanostructured surfaces
have demonstrated outstanding capability in energy and water applications
by promoting dropwise condensation, where fast droplet growth and
efficient condensate removal are two key parameters. However, these
parameters remain contradictory. Although efficient droplet removal
is easily obtained through coalescence jumping on uniform superhydrophobic
surfaces, simultaneously achieving fast droplet growth is still challenging.
Also, on such surfaces droplets can grow to larger sizes without restriction
if there is no coalescence. In this work, we show that superhydrophobic
nanostructured microporous surfaces can manipulate the droplet growth
and jumping. Microporous surface morphology effectively enhances the
growth of droplets in pores owing to large solid–liquid contact
area. At low supersaturations, the upward growth rate (1–1.5
μm/s) of these droplets in pores is observed to be around 15–25
times that of the droplets outside the pores. Meanwhile, their top
curvature radius increases relatively slowly (∼0.25 μm/s)
due to pore confinement, which results in a highly stretched droplet
surface. We also observed forced jumping of stretched droplets in
pores either through coalescence with spherical droplets outside pores
or through self-pulling without coalescence. Both experimental observation
and theoretical modeling reveal that excess surface free energy stored
in the stretched droplet surface and micropore confinement are responsible
for this pore-scale-forced jumping. These findings reveal the insightful
physics of stretched droplet dynamics and offer guidelines for the
design and fabrication of novel super-repellent surfaces with microporous
morphology.