School of Chemistry - Theses
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ItemFrequency response of atomic force microscope cantilevers immersed in fluids and applications( 2003)In the last decade, atomic force microscopy (AFM) has emerged as a fundamentally important microscopy technique, catering for many fields of science and technology. The fact that the instrument "feels" rather than "sees" the surface has revolutionized our perception of microscopy, and this advantage has only been possible with the cantilever probe, the heart of the AFM. Because it is fundamental to the operation of the AFM, the cantilever probe has been in the centre of intense research, and we are yet to fully comprehend and exploit its unique vibrational characteristics and dynamic interactions with the surrounding medium. Recently, Sader presented a new comprehensive treatment of cantilever beam resonance, immersed in arbitrary fluids. This so-called viscous model, unlike its predecessors, correctly accounts for the true geometry of the cantilever beam, viscous and inertial loading on the cantilever, and is able to predict the full frequency response of the cantilever beam. The focus of this thesis is to experimentally validate the viscous model for various physical conditions of cantilever beam and immersion fluid. The experimental validation of the theory is conducted with 5 different fluids of varying physical properties and 7 different geometries of cantilever beam, and the viscous model is demonstrated to be exceptionally accurate in predicting the cantilever beam resonance. Equipped with a better understanding of cantilever beam resonance in fluids, new applications of a cantilever beam as local probe sensors are then proposed and demonstrated in this thesis. The first application is a new spring constant calibration method, where the spring constant of a cantilever beam is measured from the cantilever fundamental resonance frequency and its quality factor in air. The new method is shown to be a non-destructive, accurate, and fast method of spring constant calibration. Furthermore, the new method is used to measure the thickness and Young's modulus of the beam under test, and as a result, provides material characterization of small structures. This in turn can be used to calibrate cantilevers of wide ranging geometries. The third application is the potential use of the cantilever beam as a micro-rheometer. Utilizing the viscous model, a new method of determining the fluid viscosity and density around a cantilever beam is proposed and demonstrated experimentally. The new method is capable of measuring viscosities and densities of both gases and liquids, which could range over several orders of magnitude, with a single cantilever beam. The method only requires minute volumes of fluid, and enables rapid and in situ rheological measurements. The work conducted in this thesis elucidates fundamentals of the dynamics of AFM microcantilevers immersed in fluids, and explores new applications and methodologies of these highly versatile local probes.