Contact time dependence of adhesion force studied at low, moderate, and high relative humidities on AFM: Influence of surface hydrophilicity

Abstract

Contact time dependence of adhesion force remains insufficiently comprehended and demands further clarification. Adhesion forces between cantilevers and various substrates were measured with atomic force microscopy to study the dependence at low, moderate, and high relative humidities (RH). Results indicate a strong correlation between the dependence and surface hydrophilicity. In some situations, the adhesion force exhibits a dependence on contact time: it experiences a rapid increase, followed by a gradual rise and eventual saturation with a saturation time of 20 ∼ 200 s. On hydrophilic surfaces, the force is dependent only at some RHs. On most partially hydrophobic surfaces, the dependent behaviors are observed under all levels of RH. However, the force on hydrophobic surfaces is independent under all levels of RH. With the dominant capillary force, the dependent behavior was attributed to the slow flow of thin films caused by the high viscosity of interfacial water. The high viscosity at low and moderate RHs was due to its configuration and film thickness, while that at high RH was due to contaminants and the formation of silica sols and gels. The findings have the potential to enrich the comprehension of adhesion and provide remedies for stiction-related problems in miniature devices.