Methods in force microscopy

Abstract

This thesis describes the direct measurement of surface forces in aqueous systems using an Atomic Force Microscope (AFM). The imaging of surfaces is usually regarded to be the primary function of the AFM, however in this study, imaging is regarded as secondary to force measurement. Experimental procedures have been developed for the measurement and assessment of surface forces. These procedures include control over geometry, scaling of force and avoidance of contamination. Three distinct aspects of this work build upon those procedures. The first step in quantifying the surface force is the determination of the geometry of interaction. A novel method of determining the effective radius of curvature of an irregular, or colloidal particle is demonstrated with a microfabricated silicon nitride tip. Both the long and short range structure of the tip is investigated. Long range radii were found to range from about 100 to over 400 nm. Short range radii were found to be affected by the polishing action of imaging. The commonly encountered interaction of a tip with mica is presented as a function of NaCl concentration and pH. This asymmetric interaction is studied in parallel with the symmetric interaction of two silicon nitride surfaces. The high affinity of chloride for silicon nitride is notable, as is the displacement of the hydration wall at concentrations >10'2 M. This latter point has implications for the electrical double layer structure. Both types of interactions studied prove the utility of the AFM in force measurement, and also provide a connection to surface imaging. This final aspect of the work includes the demonstration of a new approach to non-destructive imaging, and suggests the possibility of extracting surface assembly information with Fourier analysis.