Browsing by Author "Honig, Christopher D. F."

Now showing 1 - 5 of 5
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    An atomic force microscope tip as a light source
    Lulevich, V.; Honig, Christopher D. F.; Ducker, William A. (AIP Publishing, 2005-12-01)
    We present a simple method for causing the end of a silicon nitride atomic force microscope (AFM) tip to emit light, and we use this emitted light to perform scanning near-field optical microscopy. Illumination of a silicon nitride AFM tip by blue (488 nm) or green (532 nm) laser light causes the sharp part of the tip to emit orange light. Orange light is emitted when the tip is immersed in either air or water; and while under illumination, emission continues for a period of many hours without photobleaching. By careful alignment of the incident beam, we can arrange the scattered light to decay as a function of the tip-substrate separation with a decay length of 100-200 nm. The exponential decay of the intensity means that the emitted light is dominated by contributions from parts of the tip that are near the sample, and therefore the emitted orange light can be used to capture high-resolution near-field optical images in air or water. (c) 2005 American Institute of Physics.
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    A correlation force spectrometer for single molecule measurements under tensile load
    Radiom, Milad; Honig, Christopher D. F.; Walz, John Y.; Paul, Mark R.; Ducker, William A. (American Institute of Physics, 2013-01-07)
    The dynamical-mechanical properties of a small region of fluid can be measured using two closely spaced thermally stimulated micrometer-scale cantilevers. We call this technique correlation force spectroscopy (CFS). We describe an instrument that is designed for characterizing the extensional properties of polymer molecules that straddle the gap between the two cantilevers and use it to measure the stiffness and damping (molecular friction) of a dextran molecule. The device is based on a commercial atomic force microscope, into which we have incorporated a second antiparallel cantilever. The deflection of each cantilever is measured in the frequency range dc-1 MHz and is used to generate the cross-correlation at equilibrium. The main advantage of cross-correlation measurements is the reduction in thermal noise, which sets a fundamental noise limit to force resolution. We show that the thermal noise in our cross-correlation measurements is less than one third of the value for single-cantilever force microscopy. The dynamics of the cantilever pair is modeled using the deterministic motion of a harmonic oscillator initially displaced from equilibrium, which yields the equilibrium auto and cross-correlations in cantilever displacement via the fluctuation-dissipation theorem. Fitted parameters from the model (stiffness and damping) are used to characterize the fluid at equilibrium, including any straddling molecules. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772646]
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    Correlations between the thermal vibrations of two cantilevers: Validation of deterministic analysis via the fluctuation-dissipation theorem
    Honig, Christopher D. F.; Radiom, Milad; Robbins, Brian A.; Walz, John Y.; Paul, Mark R.; Ducker, William A. (AIP Publishing, 2012-02-01)
    We validate a theoretical approach for analyzing correlations in the fluctuations of two cantilevers in terms of a deterministic model, using the fluctuation-dissipation theorem [M. R. Paul and M. C. Cross, Phys. Rev. Lett. 92, 235501 (2004)]. The validation has been made possible through measurement of the correlations between the thermally stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. Validation of the theory enables development of a method for characterizing fluids, which we call correlation force spectrometry. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681141]
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    Lubrication forces in air and accommodation coefficient measured by a thermal damping method using an atomic force microscope
    Honig, Christopher D. F.; Sader, J. E.; Mulvaney, P.; Ducker, William A. (American Physical Society, 2010-05-01)
    By analysis of the thermally driven oscillation of an atomic force microscope (AFM) cantilever, we have measured both the damping and static forces acting on a sphere near a flat plate immersed in gas. By varying the proximity of the sphere to the plate, we can continuously vary the Knudsen number (Kn) at constant pressure, thereby accessing the slip flow, transition, and molecular regimes at a single pressure. We use measurements in the slip-flow regime to determine the combined slip length (on both sphere and plate) and the tangential momentum accommodation coefficient, sigma. For ambient air at 1 atm between two methylated glass solids, the inverse damping is linear with separation and the combined slip length on both surfaces is 250 nm +/- 100 nm, which corresponds to sigma = 0.77 +/- 0.24. At small separations (Kn > 0.4) the measured inverse damping is no longer linear with separation, and is observed to exhibit reasonable agreement with the Vinogradova formula.
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    Rheology of fluids measured by correlation force spectroscopy
    Radiom, Milad; Robbins, Brian A.; Honig, Christopher D. F.; Walz, John Y.; Paul, Mark R.; Ducker, William A. (AIP Publishing, 2012-04-01)
    We describe a method, correlation force spectrometry (CFS), which characterizes fluids through measurement of the correlations between the thermally stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. We discuss a major application: measurement of the rheological properties of fluids at high frequency and high spatial resolution. Use of CFS as a rheometer is validated by comparison between experimental data and finite element modeling of the deterministic ring-down of cantilevers using the known viscosity of fluids. The data can also be accurately fitted using a harmonic oscillator model, which can be used for rapid rheometric measurements after calibration. The method is non-invasive, uses a very small amount of fluid, and has no actively moving parts. It can also be used to analyze the rheology of complex fluids. We use CFS to show that (non-Newtonian) aqueous polyethylene oxide solution can be modeled approximately by incorporating an elastic spring between the cantilevers. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704085]