Freeze Fracture Approach
to Directly Visualize Wetting
Transitions on Nanopatterned Superhydrophobic Silicon Surfaces: More
than a Proof of Principle
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Abstract
Freeze fracturing is applied to make the wetting behavior
of artificially
nanopatterned Si surfaces directly visible. For this purpose, almost
hexagonally arranged nanopillars of fixed areal density (127 μm<sup>–2</sup>) and diameters (35 nm) but varying heights (40–150
nm) were fabricated on silicon. Measurement of contact angles (CAs)
including hysteresis allowed to distinguish between the Wenzel (W)
and the Cassie–Baxter (CB) states with droplets completely
wetting the pillars or residing on top of them, respectively. Providing
additional depth contrast by evaporating the ice replica with thin
carbon and (typically 3 nm) platinum layers under 45° allowed
resolving 3D features of 5 nm within the ice replica. In this way,
laterally sharp transitions from CB- to W-states could be revealed,
indicating the formation of zero-curvature water surfaces even on
the nanoscale