Towards accurate evaluation of pressure-induced Cassie-to-Wenzel wetting transition on superhydrophobic surfaces.

HYPOTHESIS

Pressure-induced Cassie-to-Wenzel wetting transition is one of key impediments to implementation of superhydrophobic interfaces in practical applications, yet it remains largely overlooked in surface engineering, primarily due to absence of standardized evaluation procedures. Several studies demonstrated that the stability of Cassie-Baxter wetting regime can be evaluated by compressing a water droplet against a superhydrophobic surface and calculating the Laplace pressure from the droplet's curvature. However, their treatment of droplet geometry involved various simplifications, adversely affecting reliability of pressure estimation. Therefore, we hypothesize that accounting for actual droplet geometry will improve the accuracy of transition pressure evaluation.

EXPERIMENTS

Pressure-induced wetting transition was evaluated by compressing a water droplet against hydrophobized silicon samples with micropillars, whilst measuring the force exerted by the droplet onto the surface and capturing side-view images of the compression process. The Laplace pressure, at which the droplet transitions to homogeneous wetting, was obtained by fitting the droplet profile from the captured images based on the Young-Laplace equation, without adopting the most common simplifications found in literature.

FINDINGS

The accuracy of pressure calculation was validated by strong agreement between simultaneous side-view backlit imaging and micro-force sensor measurements, with average root mean square error value of 27.71 μN for measured forces up to 2.5 mN, a significant improvement compared to available literature. The results of our experimental evaluation of silicon samples with different micro-topography indicate that the transition pressure scales with pillar height and interpillar distance; furthermore, the individual scaling factors are independent of other pillar geometric parameters.