Wave propagation through a subclass of deterministic nonperiodic media, namely, fractal Cantor multilayer structures are investigated theoretically as well as experimentally. Transmission spectra of Cantor structures are found to have two distinctive properties (scalability and sequential splitting) closely related to the geometrical peculiarities of the multilayers. A systematic correlation between structural self-similarity and spectral regularities of Cantor multilayers is established.
Crystallization represents the prime example of a disorder–order transition. In realistic situations, however, container walls and impurities are frequently present and hence crystallization is heterogeneously seeded. Rarely the seeds are perfectly compatible with the thermodynamically favoured crystal structure and thus induce elastic distortions, which impede further crystal growth. Here we use a colloidal model system, which not only allows us to quantitatively control the induced distortions but also to visualize and follow heterogeneous crystallization with single-particle resolution. We determine the sequence of intermediate structures by confocal microscopy and computer simulations, and develop a theoretical model that describes our findings. The crystallite first grows on the seed but then, on reaching a critical size, detaches from the seed. The detached and relaxed crystallite continues to grow, except close to the seed, which now prevents crystallization. Hence, crystallization seeds facilitate crystallization only during initial growth and then act as impurities.
We report on the preparation and characterization of highly birefringent, monodisperse colloidal particles with sizes between 100 nm and some micrometres made by emulsification of a reactive acrylate monomer in aqueous solution. Photopolymerization of the emulsion droplets in the liquid crystalline state results in particles with frozen orientational order. Particles that had not been polymerized have a higher effective birefringence than the polymerized particles at room temperature, as shown by measurements of the depolarized scattering intensity using quasi-elastic light scattering (QELS). We also present preliminary results showing that larger particles can be made to rotate with optical tweezers with circular polarization.The presence of hydrodynamic interactions (HI) in colloidal suspensions and polymer solutions leads to qualitatively new phenomena that are not found in simple liquids. Examples are the equilibrium dynamics and the flow behaviour of dense colloidal suspensions [1-3]. When a colloidal particle moves due to the random forces imparted by the surrounding fluid molecules, it excites a flow field that transfers linear momentum to surrounding particles. The flow field due to a translating particle is inhomogeneous, which results in neighbouring particles experiencing a torque. Similarly to this coupling of rotation and translation, the flow field excited by the Brownian rotation of a colloidal particle can transfer angular momentum to neighbouring particles, leading to rotation-rotation coupling.While detailed insight on the influence of HI on the translational degrees of freedom in colloidal suspensions has been obtained from quasi-elastic light scattering (QELS) and real-space imaging experiments, much less is known about the effect of HI on the rotational motion. Rotational motion can be observed in depolarized quasi-elastic light scattering experiments on optically anisotropic particles [4,5], using time-resolved phosphorescence anisotropy measurements on specially labelled particles [6], or nuclear magnetic resonance [7]. Measured autocorrelation functions of the scattered electric field measured by depolarized QELS now contain information not only on the particle displacements but also on their
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