Influence of lateral velocity on adhesion force of surfaces with different hydrophilicity revealed by an AFM colloidal probe at humid environments

Adhesion forces between a polystyrene microsphere and some samples with different hydrophilicity were measured to investigate the effect of lateral velocity by using an atomic force microscope (AFM) at 30% ± 1% and 70% ± 1% relative humidities. The sample at test was driven to move laterally (forth and back) with a scan distance 80 μm and lateral velocities between 0.16 and 16,000 μm/s. For hydrophilic samples, the mean adhesion force follows two regimes with lateral velocity: At first it either increases or decreases or remains stable, and then it decreases logarithmically after a critical velocity. However, for hydrophobic samples, the mean adhesion force not only follows these two regimes but also has a third regime, i.e., remains stable at large lateral velocities. The logarithmical decrease was ascribed to the contact time dependence of water bridges. The stable trend at high lateral velocities on hydrophobic samples was attributed to the fact that the capillary force vanishes and the microsphere can pull off a sample surface at a contact region with the least number of adhesive asperity junctions on the scan path. The water contact angle (CA) has some effect on the critical velocity—the smaller the CA, the larger the critical velocity.