Regensburg 2007 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 7: POSTER: INTERNAL SYMPOSIUM Scattering Experiments

CPP 7.29: Poster

Monday, March 26, 2007, 16:00–18:00, Poster B

Optical diffraction measures the filling of superhydrophobic surfaces. — •Helmut Rathgen and Frieder Mugele — University of Twente, Physics of Complex Fluids, Postbus 217, 7500AE Enschede, The Netherlands

A hydrophobic surface with a periodic texture of period of order 1µm is a superhydrophobic surface and an optical grating at the same time. A drop deposited on such a superhydrophobic optical grating can float on the texture like a fakir (Cassie-Baxter state) or it can penetrate the texture, soaking the grooves of the grating completely (Wenzel state). In the prior case, micrometer size liquid-gas interfaces are formed at the rims of the grooves. The diffractive properties of the optical grating (diffraction efficiencies of all diffraction orders) now depend on position and curvature of these micro menisci. Realspace information can be obtained by comparing measured diffraction data to a theoretical model based on Multilayer Rigorous Coupled Wave Analysis. We use this to investigate filling state and microscopic curvature at a superhydrophobic surface for a true water-vapor two phase system as a function of external pressure. We find: 1. The surfaces fill (transition from Cassie-Baxter to Wenzel state) at a critical pressure that depends on geometry and hydrophobicity in agreement with a prediction based on capillary theory. 2. Filling cannot be reversed by decreasing the external pressure to the vapor pressure. Our method is relevant as a technique of optical detection for a range of problems in micro- and nanofluidics, such as tracer free liquid detection in nanochannels or surface cavitiation.