29 research outputs found
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A catalogue of [100], [110], and [111] symmetric tilt boundaries in face-centered cubic hard sphere crystals.
Engineering and Applied Science
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Grain Growth in Thin Films with a Fibre Texture Studied by Phase-Field Simulations and Mean Field Modelling
The evolution of fiber textured structures is simulated in 2 dimensions using a generalized phase field model assuming two forms for the misorientation, a steady-state regime is reached after a finite amount of grain growth, where the numer and length weighted misorientation distribution functions (MDF) are constant in time, and the mean grain area A as a function of time t follows a power growth law A - A0 = kt^n with n close to 1 and A0 the initial mean grain area. The final shape of the MDF and value of the prefactor k in the power growth law clearly correlate with the misorientation dependence of the grain boundary energy. From a quantitative point of view, the fraction of special boundaries obtained in simulations is quite sensitive to the number of possible discrete orientations. Furthermore, a mean field approach is worked out to predict the growth exponent for systems with nonuniform grain boundary energy. The conclusions from the mean field approach are consistent with the simulation results.Physic
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Local shear transformations in deformed and quiescent hard-sphere colloidal glasses
We performed a series of deformation experiments on a monodisperse, hard-sphere colloidal glass while simultaneously following the 3D trajectories of roughly 50,000 individual particles with a confocal microscope. In each experiment, we deformed the glass in pure shear at a constant strain rate (1 − 5 × 10−5s−1) to maximum macroscopic strains (5 − 10%), then reversed the deformation at the same rate to return to zero macroscopic strain. We also measured 3D particle trajectories in an identically-prepared quiescent glass in which the macroscopic strain was always zero. We find that shear transformation zones exist and are active in both sheared and quiescent colloidal glasses, revealed by a distinctive four-fold signature in spatial autocorrelations of the local shear strain. With increasing shear, the population of local shear transformations develops more quickly than in a quiescent glass, and many of these transformations are irreversible. When the macroscopic strain is reversed, we observe partial elastic recovery, followed by plastic deformation of the opposite sign, required to compensate for the irreversibly transformed regions. The average diameter of the shear transformation zones at maximum strain is 2.3 particle diameters.Engineering and Applied Science
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Stiffness of the Crystal-Liquid Interface in a Hard-Sphere Colloidal System Measured from Capillary Fluctuations
Face-centered cubic single crystals of diameter hard-sphere silica colloidal particles were prepared by sedimentation onto (100) and (110) oriented templates. The crystals had a wide interface with the overlaying liquid that was parallel to the template. The location of the interface was determined by confocal microscopic location of the particles, followed by identification of the crystalline and liquid phases by a bond-orientation order parameter. Fluctuations in the height of the interface about its average position were recorded for several hundred configurations. The interfacial stiffness was determined from the slope of the inverse squared Fourier components of the height profile vs the square of the wave number, according to the continuum capillary fluctuation method. The offset of the fit from the origin could quantitatively be accounted for by gravitational damping of the fluctuations. For the (100) interface, ; for the (110) interface, . The interfacial stiffness of both interfaces was found to be isotropic in the plane. This is surprising for the (110), where crystallography predicts twofold symmetry. Sedimentation onto a (111) template yielded a randomly stacked hexagonal crystal with isotropic . This value, however, is less reliable than the two others due to imperfections in the crystal.Engineering and Applied SciencesPhysic
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Experimental Observation of the Crystallization of Hard-Sphere Colloidal Particles by Sedimentation onto Flat and Patterned Surfaces
We present a confocal microscopy study of 1.55micrometers monodisperse silica hard spheres as they sediment and crystallize at the bottom wall of a container. If the particles sediment onto a featureless flat wall, the two bottom layers crystallize simultaneously and layerwise growth follows. If the wall is replaced by a hexagonal template, only layerwise growth occurs. Our results complement earlier numerical simulations and experiments on other colloidal systems.Physic
Crystallization of undercooled liquid fenofibrate
Formulation of hydrophobic drugs as amorphous materials is highly advantageous as this increases their solubility in water and therefore their bioavailability. However, many drugs have a high propensity to crystallize during production and storage, limiting the usefulness of amorphous drugs. We study the crystallization of undercooled liquid fenofibrate, a model hydrophobic drug. Nucleation is the rate-limiting step; once seeded with a fenofibrate crystal, the crystal rapidly grows by consuming the undercooled liquid fenofibrate. Crystal growth is limited by the incorporation of molecules into its surface. As nucleation and growth both entail incorporation of molecules into the surface, this process likely also limits the formation of nuclei and thus the crystallization of undercooled liquid fenofibrate, contributing to the good stability of undercooled liquid fenofibrate against crystallization
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Comment on “Low-temperature homoepitaxial growth on high-miscut Si(111) mediated by thin overlayers of Pb” [Appl. Phys. Lett. 75, 2954 (1999)]
Physic
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Structural Relaxation of Amorphous : X-Ray Measurements, Electrical-Resistivity Measurements, and a Comparison Using the Ziman Theory
Structural relaxation in amorphous is studied using high-precision x-ray diffraction. The x-ray structure factor S(k) and the density ρ (determined from the x-ray absorption), are measured simultaneously as a function of the annealing conditions. The measured changes in S(k) are compared with those expected from simple densification using a Percus-Yevick model with two hard-sphere diameters. The variation in the electrical resistivity with annealing is also measured and is compared with the resistivity change estimated from the x-ray measurements using the Ziman theory. To allow a direct comparison of the x-ray and electrical measurements, we derive an approximate relationship between the x-ray atomic scattering factor and the pseudopotential as a substitute for a first-principles calculation. The combination of the low scattering rate from the amorphous samples and the high precision (<0.1%) necessary to allow direct comparison requires special techniques to maintain adequate system stability.Engineering and Applied Science