Contact angles on hydrophobic microparticles at water-air and water-hexadecane interfaces

Advancing and receding contact angles on individual hydrophobic microsphere s at the air-water and liquid-liquid, namely hexadecane-water, interfaces w ere determined. For this purpose, spherical silanated silica particles (R = 2.35 mum) and polystyrene particles of different radii (R = 1.80-4.38 mum) were attached to atomic force microscope (AFM) cantilevers. Polystyrene pa rticles were sintered onto the AFM cantilevers to provide stability in orga nic solvent. Then the equilibrium position of the microsphere at the air-wa ter interface of a drop or an air bubble was measured with an AFM-related s et-up. From the equilibrium position the contact angles were calculated. Ad vancing and receding contact angles determined with silanated silica partic les (Theta (a) = 97 degrees and Theta (r) = 81 degrees) agreed roughly with contact angles measured on similarly prepared planar surfaces (Theta (a) = 95 degrees and Theta (r) = 81 degrees). The apparent contact angles measur ed on polystyrene particles decreased with increasing particle size. This c an be interpreted by assuming a negative line tension of the order of -0.3 muN.