Viscosity Effect On The AFM Force Measurement

Abstract

Atomic force microscopy based techniques have been used in the investigation of protein folding/unfolding in order to study the protein properties at the single molecule level. These experiments are performed in buffer solution, where the AFM cantilever is used to exert and measure the mechanical forces on the protein molecules under study. Due to the motion of the cantilever relative to the liquid, a viscous drag force, which can be significant when the liquid is viscous or the cantilever's speed is high, acts on the cantilever. The irregular shape of the liquid chamber and cantilever, and the special boundary conditions of the fluid make this problem complicated and difficult to solve analytically. We measured the viscous drag forces on different cantilevers for several different cantilever speeds in solutions of different viscosities. The results show that the viscous drag on a cantilever is determined by its geometry, its relative speed, the viscosity of the solution, and the separation between the cantilever and the sample surface. This method will be useful to make corrections to the unfolding force data of proteins as well as that of AFM based force measurements.