Droplets of water vapor condensing on a cool surface may grow more quickly or slowly around certain discontinuities such as edges and cavities. The authors study this phenomenon experimentally and theoretically in various configurations. In certain cases, the growth can be enhanced by nearly 500% on edges or corners.

Gravity-driven drainage of small volumes of condensates, such as natural dew, is a challenge because small drops usually remain pinned to inclined surfaces. We report that submillimetric grooves substantially reduce dew retention by modifying the repartition of liquid: Because of a long-range coalescence mechanism mediated by grooves imbibition, the growth and shedding of large drops are accelerated. Such findings can be applied to increase the passive harvesting of dew as well as to accelerate the drainage of other condensates.

Acoustic waves can generate steady streaming within a fluid owing to the generation of viscous boundary layers near walls of typical thickness $\delta$. In microchannels, the acoustic wavelength $\lambda$ is adjusted to twice the channel width $w$ to ensure a resonance condition, which implies the use of MHz transducers. Recently, though, intense acoustic streaming was generated by acoustic waves of a few kHz (hence with $\lambda \gg w$), owing to the presence of sharp-tipped structures of curvature radius at the tip $r_c$ smaller than $\delta$. The present study quantitatively investigates this sharp-edge acoustic streaming via the direct resolution of the full Navier-Stokes equation using the finite element method. The influence of $\delta ,r_c$, and viscosity $\nu$ on the acoustic streaming performance is quantified. Our results suggest choices of operating conditions and geometrical parameters, in particular the dimensionless tip radius of curvature $r_c/\delta$ and the liquid viscosity.