Dynamic Atomic Force Microscopy in liquids and applications to biological samples

Xin Xu, Purdue University

Abstract

This thesis investigates four different problems relevant to the dynamics of microcantilevers in liquids with applications to the imaging of fragile biological samples in Atomic Force Microscopy (AFM). Magnetic, acoustic, and thermal excitations are commonly used for dynamic AFM in liquids. The major differences between the amplitude and phase response of these three excitation modes in liquids are fully discussed and a method to estimate quantitatively the unsteady structure-borne and fluid-borne excitation forces acting on the acoustically excited AFM cantilever is proposed. Then the peak tip-sample interaction force during imaging is investigated. Both theory and experiments in liquids on virus capsids have been presented to propose scaling laws for peak interaction forces exerted on soft samples in liquid environments. It is also demonstrated how these laws can be used to choose probes and operating conditions to minimize interaction forces and thereby robustly image fragile biological samples. Next we investigate the momentary excitation of higher cantilever eigenmodes in dynamic AFM in liquids and its close link to the local elasticity of the sample. We demonstrate how the momentary excitation can be used to improve compositional contrast of soft biological materials in liquid dynamic AFM. Finally the momentary excitation can also distort significantly the oscillation waveform making the existing scanning probe acceleration microscopy (SPAM) method for reconstruction of tip-sample interaction forces inaccurate. We develop a simple experimental method to decouple the higher eigenmode dynamics from that of the fundamental and propose an important correction to SPAM method for soft cantilevers tapping on samples in liquids.

Degree

Ph.D.

Advisors

Raman, Purdue University.

Subject Area

Mechanical engineering

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