Dresden 2014 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 18: Wetting, Superamphiophobicity, Micro- and Nanofluidics I

CPP 18.5: Talk

Tuesday, April 1, 2014, 10:30–10:45, ZEU 222

How superhydrophobicity breaks down — •Periklis Papadopoulos, Frank Schellenberger, Lena Mammen, Xu Deng, Doris Vollmer, and Hans-Jürgen Butt — Max Planck Institute for Polymer Research, D-55128 Mainz, Germany

Droplets on superhydrophobic surfaces roll off easily, leaving the surface dry and clean. This property is due to a surface geometry that favors the entrainment of air beneath the drop, leading to the so-called Cassie state. This state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. Despite the fact that thermodynamically the Wenzel state is often more stable, the geometry imposes an energy barrier that prevents the liquid from wetting the structure. The transition dynamics is still poorly understood due to difficulties in monitoring the temporal evolution of the liquid-air meniscus between the asperities with sufficient spatial resolution. Here, we study the Cassie-to-Wenzel transition of water drops evaporating on arrays of hydrophobic micropillars. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. As evaporation proceeds, the curvature of the drop and the Laplace pressure increase. The three-phase contact line remains pinned until the contact angle reaches the advancing contact angle of the material. Then the water slides down the pillars. Alternatively, at large pillar spacing the curved water surface can touch the bottom of the substrate triggering an instantaneous Cassie-to-Wenzel transition. [1] Papadopoulos P., Mammen L., Deng X., Vollmer D., Butt H.-J., PNAS 110, 3254-3258 (2013)