Hydrodynamics of particle–wall interaction in colloidal probe experiments: comparison of vertical and lateral motion

The hydrodynamic interaction between a colloidal particle attached to the tip of an atomic force microscope (AFM) and a wall has been investigated as a function of the angle of inclination of the cantilever with respect to the surface. For high inclination of the cantilever, the motion mostly occurs in the tangential direction. A frequency-dependent drag coefficient is extracted from the cantilever's Brownian motion. In agreement with theoretical predictions, the wall-induced drag for tangential motion is much weaker than that for vertical motion. Mechanical properties of the surface are more easily probed with tangential motion because the hydrodynamic effects do not mask the surface's intrinsic properties as much as for vertical motion.