Cassie-Levitated Droplets for Distortion-Free Low-Energy Solid–Liquid Interactions

Despite
the rapid advent of superomniphobic materials, there is a lack of
methodologies to accurately investigate the ultralow-energy interactions
taking place on these interfaces. For instance, universally employed
models such as the pendant droplet often fail to provide representative
information on the wetting properties of superomniphobic surfaces.
The delicate balance between the forces acting at the droplet–surface
and droplet–needle interfaces can easily result in heavily
distorted droplet profiles. Here, we introduce a Cassie-levitating
droplet model which overcomes the limitations of the pendant droplet
model, allowing a distortion-free assessment of the interactions between
super­(amphi)­omniphobic materials and low surface tension liquids.
Comparative analysis in wetting of low surface tension fluids such
as hexadecane (∼27.47 mN/m) on superamphiphobic surfaces via
the Cassie-levitating and pendant droplet models reveals up to 70°
(800%) deviations in the estimated contact angle hysteresis. A theoretical
framework is developed to assess experimentally observed profile distortions
against ideal gravity-induced sagging of droplet shapes during dynamic
droplet expansion and contraction cycles. Notably, pendant droplets
resulted in up to 50% distortion while the Cassie-levitating ones
achieved less than just 10%. We believe that the Cassie-levitating
droplet model bears ample potential for the characterization of the
rapidly emerging family of superomniphobic materials, setting the
basis for their future engineering in numerous emerging applications.