Dynamic Effects in Microparticle Pull-Off Using an AFM

We report a series of measurements aimed at understanding the dynamics of microparticle detachment from surfaces. Microparticles were adhered to AFM cantilever tips and load/displacement curves were obtained while the particles were repeatedly attached and detached from the surface with frequencies ranging from 1 to 30 Hz. Particles ranging in size from 1 to 30 micrometers and composed of alumina and polystyrene were studied. For each particle studied, a decrease in the pull-off force was consistently observed with increasing measurement frequency, indicating a dynamic effect that is not accounted for by equilibrium adhesion models. We present a lumped mass model of the canitilever/particle/surface system to definitively rule out the possibility that bulk inertial effects within the particle or cantilever may be responsible at these time scales. Furthermore, we ensured dry conditions for each experiment making it unlikely that humidity effects, such as water bridge formation, could have been responsible. Although we have not pinpointed the physical nature of the dynamics, we offer the possibility that energy dissipation mechanisms at the particle/surface interface may cause dynamic effects during microparticle attachment or detachment on time scales similar to those of the present experiments.