What Can Probing Liquid–Air Menisci Inside Nanopores Teach Us About Macroscopic Wetting Phenomena?

Solid surfaces with excellent nonwetting
ability have drawn significant
interest from interfacial scientists and engineers. While much effort
was devoted to investigating macroscopic wetting phenomena on nonwetting
surfaces, the otherwise microscopic wetting has received less attention,
and the surface/interface properties at the microscopic scale are
not well resolved and correlated with the macroscopic wetting behavior.
Herein, we first characterize the nanoscopic morphology and effective
stiffness of liquid–air interfaces inside nanopores (nanomenisci)
on diverse nonwetting nanoporous surfaces underneath water droplets
using atomic force microscopy. Detailed three-dimensional imaging
of the droplet-surface contact region reveals that water only slightly
penetrates into the nanopores, allowing for quantitative prediction
of the macroscopic contact angle using the Cassie–Baxter model.
By gradually increasing the scanning force, we observe incrementally
wetting of nanopores by water, and dewetting occurs when the force
is lowered again, exhibiting reversible wetting–dewetting transitions.
Further, nanoindentation measurements demonstrate that the nanomenisci
show apparent elastic deformation and size-dependent effective stiffness
at small indenting forces. Finally, we correlate the effective stiffness
of the nanomenisci with the transition from complete rebound to partial
rebound for impinging droplets on nanoporous surfaces. Our study suggests
that probing the physical properties of the liquid–air menisci
at the nanoscale is essential to rationalize macroscopic static and
dynamic wetting phenomena on structured surfaces.