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

Advancing and receding contact angles on individual hydrophobic microspheres at the air-water and liquid-liquid, namely hexadecane-water, interfaces were determined. For this purpose, spherical silanated silica particles (R = 2.35 μm) and polystyrene particles of different radii (R = 1.80-4.38 μm) were attached to atomic force microscope (AFM) cantilevers. Polystyrene particles were sintered onto the AFM cantilevers to provide stability in organic solvent. Then the equilibrium position of the microsphere at the air-water interface of a drop or an air bubble was measured with an AFM-related set-up. From the equilibrium position the contact angles were calculated. Advancing and receding contact angles determined with silanated silica particles (Θa = 97° and Θr = 81°) agreed roughly with contact angles measured on similarly prepared planar surfaces (Θa = 95° and Θr = 83°). The apparent contact angles measured on polystyrene particles decreased with increasing particle size. This can be interpreted by assuming a negative line tension of the order of-0.3 μN.