225 research outputs found
Characterization of anisotropic nano-particles by using depolarized dynamic light scattering in the near field
Light scattering techniques are widely used in many fields of condensed and
sof t matter physics. Usually these methods are based on the study of the
scattered light in the far field. Recently, a new family of near field
detection schemes has been developed, mainly for the study of small angle light
scattering. These techniques are based on the detection of the light intensity
near to the sample, where light scattered at different directions overlaps but
can be distinguished by Fourier transform analysis. Here we report for the
first time data obtained with a dynamic near field scattering instrument,
measuring both polarized and depolarized scattered light. Advantages of this
procedure over the traditional far field detection include the immunity to
stray light problems and the possibility to obtain a large number of
statistical samples for many different wave vectors in a single instantaneous
measurement. By using the proposed technique we have measured the translational
and rotational diffusion coefficients of rod-like colloidal particles. The
obtained data are in very good agreement with the data acquired with a
traditional light scattering apparatus.Comment: Published in Optics Express. This version has changes in bibliograph
Tunable anisotropy in inverse opals and emerging optical properties
Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the realizable structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method to introduce anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol-gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure, while completely avoiding the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy studies clearly identify increasing degrees of sol-gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature
Excitations of Ellipsoid Packings near Jamming
We study the vibrational modes of three-dimensional jammed packings of soft
ellipsoids of revolution as a function of particle aspect ratio and
packing fraction. At the jamming transition for ellipsoids, as distinct from
the idealized case using spheres where , there are many
unconstrained and non-trivial rotational degrees of freedom. These constitute a
set of zero-frequency modes that are gradually mobilized into a new rotational
band as increases. Quite surprisingly, as this new band is
separated from zero frequency by a gap, and lies below the onset frequency for
translational vibrations, , the presence of these new degrees of
freedom leaves unaltered the basic scenario that the translational spectrum is
determined only by the average contact number. Indeed, depends
solely on coordination as it does for compressed packings of spheres. We also
discuss the regime of large , where the two bands merge.Comment: 6 pages, 4 figure
Crystalline Assemblies and Densest Packings of a Family of Truncated Tetrahedra and the Role of Directional Entropic Forces
Polyhedra and their arrangements have intrigued humankind since the ancient
Greeks and are today important motifs in condensed matter, with application to
many classes of liquids and solids. Yet, little is known about the
thermodynamically stable phases of polyhedrally-shaped building blocks, such as
faceted nanoparticles and colloids. Although hard particles are known to
organize due to entropy alone, and some unusual phases are reported in the
literature, the role of entropic forces in connection with polyhedral shape is
not well understood. Here, we study thermodynamic self-assembly of a family of
truncated tetrahedra and report several atomic crystal isostructures, including
diamond, {\beta}-tin, and high- pressure lithium, as the polyhedron shape
varies from tetrahedral to octahedral. We compare our findings with the densest
packings of the truncated tetrahedron family obtained by numerical compression
and report a new space filling polyhedron, which has been overlooked in
previous searches. Interestingly, the self-assembled structures differ from the
densest packings. We show that the self-assembled crystal structures can be
understood as a tendency for polyhedra to maximize face-to-face alignment,
which can be generalized as directional entropic forces.Comment: Article + supplementary information. 23 pages, 10 figures, 2 table
Electric-field-induced alignment of electrically neutral disk-like particles: modelling and calculation
This work reveals a torque from electric field to electrically neutral flakes that are suspended in a higher electrical conductive matrix. The torque tends to rotate the particles toward an orientation with its long axis parallel to the electric current flow. The alignment enables the anisotropic properties of tiny particles to integrate together and generate desirable macroscale anisotropic properties. The torque was obtained from thermodynamic calculation of electric current free energy at various microstructure configurations. It is significant even when the electrical potential gradient becomes as low as 100 v/m. The changes of electrical, electroplastic and thermal properties during particles alignment were discussed
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