Direct force and energy landscape reconstruction of interfacial and intermolecular interactions with excitation enhanced force spectroscopy

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Liu, Alan Y.
Sulchek, Todd
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Energy landscape theory provides the most comprehensive framework to understand intermolecular and interfacial interactions. Researchers have developed many force spec- troscopy techniques through sensitive force probes, such as Atomic Force Microscopy (AFM), to experimentally probe the energy landscape of interaction of interests. How- ever, few methods were able to reconstruct the full view of energy landscape, and these methods are limited by the assumptions made before the experiments, such as the shape of the energy landscape or reversible interactive process. Particularly for reconstruction meth- ods based on Boltzmann equations, which are the only methods capable of reconstructing the continuous energy landscape, the reconstruction results relied on perfect matching of the stiffness of force probes and strength of the interaction, and therefore difficult to apply to probe strong, multi-step interactions. In order to probe the true energy landscape of in- teraction of interests, an AFM based framework was developed to interconnect the energy landscape perspective of physical interaction and measuring coordinate of the force probes. The newly developed framework enabled a direct comparison of force measurement results and intrinsic reaction coordinate, and therefore integrated an unprecedented spatial resolu- tion into Boltzmann based reconstruction methods. A continuous energy landscape up to 100 nm was reconstructed using the point of equilibrium of the interaction defined by the framework. Based upon the same framework, I investigated the force-distance curves of AFM measurements, and revealed a characteristic fluctuation occurring during the so- call ”snaps” during the measurements. The fluctuation feature was then used to develop a point-density method to reconstruct the key undersampled region of the force landscape as- sociated with the underlying energy landscape of the interactions without using Boltzmann- based methods. The point-density method was used to characterize the critical nucleation distance of capillary bridge due to water condensation together with velocity-based force spectroscopy. The nucleation of water bridge is notoriously difficult to measure because of its strong adhesive nature over a short distance within a few nanometer. The method developed in this thesis is the first direct measurement to probe the critical nucleation dis- tance of capillary bridge. Furthermore, to probe the rate-dependent interaction of rupture of capillary bridge, I developed a bandwidth excitation method to probe the rupture dis- tance of capillary bridge without changing the probe velocity. The method used excitation at specific bandwidth to enhance only the unbinding reaction without changing binding in- teraction. The method was applied to characterize the critical rupture distance of capillary bridge, which is more than 10-fold longer than the nucleation distance.
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