Toward an Improved Method for Determining the Hamaker Constant of Solid Materials Using Atomic Force Microscopy. III: Experimental Validation of a New Approach-to-Contact Method

A robust experimental validation of an existing approach-to-contact atomic force microscopy (AFM) method for determining the self-Hamaker constant, A11, of a solid, nondeformable material is presented. By explicitly accounting for the underlying surface roughness of a given substrate, this method allows for an estimate of A11 to be obtained from the resulting distribution of AFM cantilever deflections at first contact with the surface, dc. Using this method, the self-Hamaker constants of amorphous silica, stainless steel, and sapphire were determined. Moreover, several plates comprised of the same material, but with different topographies (or values of their root-mean squared surface roughness), were also considered. The values of A11 obtained for these same material plates were found to be in excellent agreement, verifying that this approach-to-contact method properly accounts for the underlying roughness of a given surface. Furthermore, the average value of A11 across all plates for a particular material was in very good agreement with those results previously derived from Lifshitz theory. Finally, the uncertainties in these experimental estimates of A11 were found to be significantly reduced, compared to the uncertainties that arose in some previous approach-to contact methods.