Dependence of adhesion force on contact time due to water thin-film flow revealed with an AFM cantilever at low humidity

The contact time dependence of adhesion force is not well understood. Adhesion forces between a tipless silica cantilever and some samples with different hydrophilicity were measured to study the dependence at low relative humidity (16 ± 1%). Dependent adhesion forces were observed on hydrophilic samples. The adhesion force has three distinct trends: sharply increasing at first, then followed by moderately increasing and at last remaining almost unchanged (reaching saturation). The saturation time is varied for different samples: from several seconds to tens of seconds. The more hydrophilic the sample surface is, the longer the saturation time is. The increasing tendency is well explained by a model of water thin-film flow. It takes some time for the liquid absorbed on a surface to flow into a growing water bridge due to a large viscosity. The viscosity of the film on a more hydrophilic sample may be larger because of stronger interaction between the film and the sample surface, eventually resulting in a longer saturation time. The reported results may help deepen the understanding of the mechanism of adhesion and explain other experimental results concerning AFM parameters.