Browsing by Author "McKee, Clayton T."

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    Investigation of Non-DLVO Forces using an Evanescent Wave Atomic Force Microscope
    McKee, Clayton T. (Virginia Tech, 2006-07-26)
    This dissertation describes new methods for measuring surface forces using evanescent waves, and applications to non-DLVO forces. An evanescent wave, generated at a solid-liquid interface, is scattered by AFM tips or particles attached to AFM cantilevers. The scattering of this wave is used to determine absolute separation between surfaces and/or the refractive index as a function of separation in AFM measurements. This technique is known as evanescent wave atomic force microscopy (EW-AFM). The scattering of an evanescent wave by Si3N4 AFM tips is large and decays exponentially with separation from a refractive index boundary. Thus, scattering is a useful method for measuring the separation between a Si3N4 tip and sample. This method has been used to measure the absolute separation between a tip and sample in the presence of an irreversibly adsorbed polymer film. Measurement of the film thickness and time response of the polymer to applied loads has also been studied. These measurements are not possible using current AFM techniques. In addition to measurements in polymer systems, the simple scattering profile from Si3N4 tips was used to re-examine short range hydration forces between hydrophilic surfaces. Results presented in this thesis suggest this force does not depend on the hydrated radius of the ion between glass and silicon nitride. The scattering generated by a Si3N4 tip has also been used to measure the refractive index of bulk fluids and thin films between hydrophobic surfaces. Based on these results, I have shown that a long-range attraction between hydrophobic surfaces is accompanied by an increase in the refractive index between the tip and surface. From this I have concluded that the attractive force, measured in this study, is the result of an increase in the concentration of organic material between surfaces. Finally, I have shown that the scattering profile depends on the material and size of the scattering object. Scattering from silicon nitride tips is exponential with separation. In contrast, the scattering profile from silicon tips, which are similar in size and geometry, is not a simple exponential. The scattering profile of larger spherical particles attached to cantilevers is also not exponential. It is approximately the sum of two exponentials. The functional form of the scattering profile with separation is consistent with the transmission of evanescent light through flat planar films. This result would suggest that a re-examination of the separation-dependence of scattering in TIRM measurements is necessary.
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    Measurement of the absolute separation for atomic force microscopy measurements in the presence of adsorbed polymer
    McKee, Clayton T.; Mosse, Wade K. J.; Ducker, William A. (AIP Publishing, 2006-05)
    We demonstrate that the absolute separation between an atomic force microscope (AFM) tip and a solid substrate can be measured in the presence of an irreversibly adsorbed polymer film. The separation is obtained from the analysis of a scattered evanescent wave that is generated at the surface of the solid. By comparing our scattering measurements to conventional AFM measurements, we also show an example where a conventional AFM measurement gives the incorrect force-distance profile. We validate the measurement of separation from scattering by examining the force-separation profile in the presence of surfactant solution. This validation is possible because the tip can be used to desorb the surfactant film that forms in surfactant solution, so we are able to measure both the scattering and the cantilever deflection when the tip is in contact with the solid substrate. The main limitation of our technique now is the lack of a rigorous method for predicting the intensity of scattering from the tip in contact with a solid that is coated with a film. (c) 2006 American Institute of Physics.