Rationalized approach to the determination of contact point in force-distance curves: application to polymer brushes in salt solutions and in water.

In this work, we present two methods to determine the contact point in force-distance curves obtained with the atomic force microscope. These procedures are compared with the typical determination of contact point by a visual assessment of the data. One method, based on the assumption that the sample shows linear elastic behavior, provides results similar to those obtained by a visual assessment of the data, and will be suitable for determining the contact point in cases where ionic repulsion is not significant. The second method is based on a series of measurements in which the sample deformation is measured at increasing values of applied load; the contact point is determined by extrapolation to zero load. Because this method is based on extrapolation of measurements made in the contact regime, it is not subject to long-range repulsion. It is thus suitable for the analyses of the contact point even in cases where ionic repulsions will affect the point at which individual force curves deviate from the baseline or zero-force regime. The methods described here are demonstrated with a glycopolymer brush compressed with a colloidal silica particle on the tip of the AFM cantilever.