Atomic force and optical force microscopy: applications to interfacial microhydrodynamics

Atomic force microscopes (AFM) and related scanning probe microscopes are most commonly used for imaging interfaces and measuring surfaces and colloidal forces. More recently, their use in probing mechanical and viscoelastic properties of macromolecules and fluids near interfaces has begun to receive attention. Here, we examine the dissipative responses of an AFM cantilever and of a spherical particle used as a probe in an optical force microscope (OFM), as a first step in the eventual development of AFMs and OFMs for microrheological measurements near interfaces. We focus specifically on measurements of probe motions normal to the substrate in viscous fluids in order to examine the accuracy of simple models for the oscillations of the cantilever in the case of AFMs and for testing the accuracy of measurements in the case of OFMs. The advantages and disadvantages of the two techniques and some of the outstanding issues are also discussed.