Pearl drops

If deposited on a hydrophobic rough substrate, a small drop of water can look like a pearl, with a contact angle close to $180^\circ$. We examine the conditions for observing such a phenomenon and show practical achievements where the contact angle can be predicted and thus quantitatively tuned by the design of the surface microstructure

Nanorheology of adsorbed polymer chains immersed in pure solvent

Long linear chains of polybutadiene are adsorbed on the two surfaces of a surface force apparatus and immersed in pure tetradecane. The hydrodynamic force was measured by drainage experiments and by frequency sweeps at constant distances. We related the hydrodynamic thickness to the chain dimension. The complex modulus encompasses the shear modulus and, at distances lower than the hydrodynamic thickness, a compression modulus. The compression term was related to the static force which appears when the two adsorbed layers are overlapped. The complex shear modulus was interpreted by a two-components hydrodynamic model proposed by P. Sens et al. We first complemented the theoretical model. Then, our experimental data fit the proposed viscoelastic expressions in the entire range of distances. The storage modulus is supposed to be affected by a residue of free chains and by the dispersion of the loop lengths