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Equilibrium gels provide physically attractive counterparts of nonequilibrium gels, allowing statistical understanding and design of the equilibrium gel structure. Here, we assemble two-dimensional equilibrium gels from limited-valency "patchy" colloidal particles and follow their evolution at the particle scale to elucidate cluster-size distributions and free energies. By finely adjusting the patch attraction with critical Casimir forces, we let a mixture of two-valent and pseudo-three-valent patchy particles approach the percolated network state through a set of equilibrium states. Comparing this equilibrium route with a deep quench, we find that both routes approach the percolated state via the same equilibrium states, revealing that the network topology is uniquely set by the particle bond angles, independent of the formation history. The limited-valency system follows percolation theory remarkably well, approaching the percolation point with the expected universal exponents.
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We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.
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This feature issue of Applied Optics contains 31 research papers on photon correlation and scattering, many of which were presented at an OSA Topical Meeting that was held 21-23 August 2000 in Whistler, British Columbia, Canada. These papers focus on research in dynamic light scattering, surface light scattering, photon correlation, and laser velocimetry and their applications to physical, chemical, and biological processes.
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Bounding the errors of measurements derived from correlation functions of light scattered from some physical systems is typically complicated by the ill conditioning of the data inversion. Parameter values are estimated from fitting well-chosen models to measurements taken for long enough to look acceptable, or at least to yield convergence to some reasonable result. We show some simple numerical simulations that indicate the possibility of substantial and unanticipated errors even in comparatively simple experiments. We further show quantitative evidence for the effectiveness of a number of ad hoc aspects of the art of performing good light-scattering experiments and recovering useful measurements from them. Separating data-inversion properties from experimental inconsistencies may lead to a better understanding and better bounding of some errors, giving new ways to improve overall experimental accuracy.
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In recent years surface-light-scattering spectroscopy has been transformed from a complex optical experiment requiring substantial effort to operate effectively to a simpler instrument for which an accurate theory of operation has been developed. The accuracy and precision are sufficiently enhanced that refinement of the theory of spectral band shapes is justified to include more subtle effects such as bending moduli and thin-film forces associated with the van der Waals and the Casimir effects. We show how to develop extensions of the theory of interfacial fluctuations through the mass and the momentum balances of interfacial transport. We also show that free-energy functionals can be used to express curvature effects crisply. The results are detailed formulas that can be used to fit experimental spectra.
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This paper demonstrates the utility of a noninvasive backscatter fiber optic probe for dynamic light-scattering characterization of a microemulsion comprising sodium dodecyl sulfate/1-butanol/ brine/heptane. The fiber probe, comprising two optical fibers precisely positioned in a stainless steel body, is a miniaturized and efficient self-beating dynamic light-scattering system. Accuracy of particle size estimation is better than ±2%.
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The hard sphere disorder-order transition serves as the paradigm for crystallization. However, measurements of the crystallization kinetics for colloidal hard spheres in the coexistence regime are incomplete for early times and are affected by sedimentation. We use time resolved Bragg light scattering to characterize crystal nucleation and growth in a microgravity environment on the space shuttle. In contrast to the classical picture of the nucleation and growth of isolated crystallites, we find substantial coarsening of growing crystallites. We also observe dendritic growth and face-centered cubic as the stable structure.
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This feature issue of Applied Optics contains 25 research papers on photon correlation and scattering. Many of the papers in this volume were presented at an OSA Topical Meeting that was held 21-24 August 1996 in Capri, Italy. The focus of these papers is research in dynamic light scattering, surface light scattering, photon correlation, laser velocimetry, and their applications to biological, chemical, and physical processes.
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We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented.
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We have developed a fiber-optics surface-light-scattering spectrometer completely designed with optical fiber components. To the best of our knowledge, this is the first demonstration of a noninvasive measurement of the surface tension and the viscosity of simple liquid-vapor interfaces with a fiber-optics-based sensor system. With this approach we obtain a compact size, a significant increase in the signal-to-noise ratio, and the ability to select from a continuum of wave vectors.
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A new generation of vibration-mitigating surface-light-scattering instrumentation has been designed and built. The computational application of an instrument function derived by use of Fourier optics is presented. This instrument and its accompanying suite of analysis software allow us to easily make accurate and noninvasive measurements of the interfacial tension, volume viscosity, and other interfacial parameters of fluids. We derived the necessary surface response function algorithms to study both simple fluids and binary fluids at their wetting transition and near their critical points. These developments can be applied to study systems with liquid-vapor and liquid-liquid interfaces, including spread monolayers, whenever optical access for a laser beam is available.
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Surface light-scattering measurements have been carried out on thin asymmetric films of pentane on water. We vary the film thickness ? over a wide range (10(-9) m < ? < 10(-5) m). Compared with the ripplons wavelength 1/q, thick films of pentane (? ? 1/q) display the same power spectrum as a pure pentane-vapor interface: a single peak. When thinning the film down to ? approximately 1/q, hydrodynamic coupling between the layers of the interface is revealed by the appearance of a second peak beside the first one. We describe the thickness dependence of the two coupled modes through variations of the positions, the widths, and the amplitudes of both peaks.
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The hard-sphere disorder-order transition serves as the paradigm for crystallization. We used time-resolved Bragg light scattering from the close-packed planes to measure the kinetics of nucleation and growth of colloidal hard-sphere crystals. The effects of gravity are revealed by comparison of the experiments in microgravity and normal gravity. Crystallites grow faster and larger in microgravity, and the coarsening between crystallites is suppressed by gravity. The face-centered-cubic structure was strongly indicated as being the stable structure for hard-sphere crystals. For a sample with a volume fraction of 0.552, the classic nucleation and growth picture is followed.
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A general-purpose, multifunction light-scattering instrument has been developed at the NASA Lewis Research Center for Space Shuttle and Space Station colloid crystallization and other microgravity experiments. For a single sample, the instrument can measure two-dimensional Bragg scattering from 0.5 degrees to 60 degrees , dynamic and static light scattering from 10 degrees to 170 degrees , the shear modulus of samples before and after crystallization, and digital color images of the sample. A carousel positions any one of eight 3-ml samples into the test position for separate experiments. Program challenges and flight results from the STS-83 Space Shuttle mission are discussed.
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A hybrid reflection-transmission surface light-scattering instrumental design is presented, examined theoretically, and tested experimentally. The purpose of the design is to reduce the sensitivity of the instrument to vibration in general and surface sloshing in particular while sacrificing little performance. Traditional optical arrangements and two new optical configurations with varying trade-offs between slosh resistance and instrumental simplicity and accuracy are examined by use of Fourier optics methods. The most promising design was constructed and tested with acetone, ethanol, and water as subject fluids. The test involved backcalculation of the wave number of the capillary wave examined with the known physical parameters for the test fluids. The agreement of the computed wave number was +/-1.4%.
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The utility of the one-beam cross-correlation dynamic light-scattering system for sizing small particles in suspension was previously limited by its small-intensity signal-to-baseline ratio for strongly turbid suspensions. We describe three improvements in the optical system and sample cell that raise the ratio to a value comparable with that of other cross-correlation dynamic light-scattering systems. These improvements are (i) using a square cross-sectional sample cell to minimize the attenuation of the incident beam and singly scattered light, (ii) placing a 200-microm-wide slit between the sample cell and the detector fibers to mask off the region of weak single scattering and strong multiple scattering from the detectors' field of view, and (iii) aligning the center of the detectors' field of view with the region of strongest single scattering. We analyze a number of suspensions of polystyrene latex spheres with a diameter between 65 and 562 nm in water using this improved one-beam instrument and find that the measured radius is determined in a 2-min data collection time to better than +/-10% for volume fractions of the suspended polystyrene latex spheres up to a few percent.
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NASA has developed a compact laser light-scattering instrument that employs both static and dynamic light-scattering techniques for microgravity research. The first use of this instrument was to study the behavior of colloidal hard spheres in a reduced gravity environment during the Second United States Microgravity Laboratory space shuttle mission. We discuss the instrument design and possible improvements based on our observations of significant differences between hard-sphere behavior in Earth's gravity and microgravity.