Disc-like particles

Disc-like particles can also undergo an Onsager transition—here tire particles fonn a discotic nematic, where tire short particle axes tend to be oriented parallel to each other. In practice, clay suspensions tend to display sol-gel transitions, witliout a clear tendency towards nematic ordering (for instance, [22]). Using sterically stabilized platelets, an isotropic-nematic transition could be observed [119]. 

Membrane Formation. In earlier work. 2.) it was found that fumed silica particles could be dispersed in aqueous suspension with the aid of ultrasonic sound. Observations under the electron microscope showed that the dispersion contained disc-like particles, approximately 150-200 1 in diameter and 70-80 1 in height. Filtration experiments carried out in the "dead-end" mode (i.e., zero crossflow velocity) on 0.2 urn membrane support showed typical Class II cake formation kinetics, i.e., the permeation rate decreased according to equation (12). However, as may be seen from Figure 7, the decrease in the permeation rate observed during formation in the crossflow module is only t 1, considerably slower than the t 5 dependence predicted and observed earlier. This difference may be expected due to the presence of lift forces created by turbulence in the crossflow device, and models for the hydrodynamics in such cases have been proposed. 

The approximate symmetry between the phase diagrams for rod-like and disc-like particles was also noted by K. L. Savithramma and N. V. Madhusudana, Mol. Cryst. Liquid Cryst., 74, 243 (1981) 90, 35 (1982), who used the scaled particle theory. 

Abstract Self-assembly is an important phenomenon that leads to formation of interesting and novel structures in colloidal dispersions. We present experimental evidence for the existence of a cubatic phase in a colloidal dispersion of disc-like particles of nickel hydroxide colloidal dispersions. In this structure, disc-like particles self-assemble as domains of a few parallel discs and the orientation tends to be orthogonal in adjacent domains. This phase has been predicted previously by computer simulations. The domains are approximately equiaxial and are predicted to exist only within a limited range of aspect ratios and volume fractions. We have used the real space technique of cryo-transmission electron microscopy in our studies as this locally ordered structure could not be identified readily using scattering techniques, since the patterns are expected to be similar to those of isotropic liquid phases. 

It is very important to understand the effect of size shape and polydispersity on the ordered mesostructures in the design of materials with organization at nanometre length scale. This study provides a further indication of how polydispersity might alter the packing of discs and complements the observations of columnar phases. Previous studies of dispersions of disc-like particles using X-ray and neutron... 

Walpole [10] for the overall moduli of composite materials reinforced by disc-like particles, i.e. transversely isotropic spheroids with zero aspect ratio. Note that the developments drawn in this paper give a general formulation, function of an arbitrary aspect ratio, which lead to the Walpole solution [10] if the aspect ratio is zero. 

Palladiiun clusters were prepared in KL zeolite by ion exchange in Pd(NH3)4(N03)2 solutions, calcination at 573 K, and H2-reduction at 573 K [247]. X-ray absorption spectroscopy measurements detected the formation of ca. 10-and 16-atom disc-like particles for 2 and 3 wt.% Pd/KL-zeoUte, respectively. 

Fig. 27. Negative double refraction for disc-like particles.

When reaction is absent from certain crystallographic surfaces, the formulation of rate equations based on geometric considerations proceeds exactly as outlined above, but includes only the advance of interfaces into the bulk of the reactant particle from those crystallographic surfaces upon which the coherent reactant/product contact is initially established. When reaction occurs only at the edges of a disc or plate-like particle... 

In the quest for a universal feature in the short-to-intermediate time orientational dynamics of thermotropic liquid crystals across the I-N transition, Chakrabarti et al. [115] investigated a model discotic system as well as a lattice system. As a representative discotic system, a system of oblate ellipsoids of revolution was chosen. These ellipsoids interact with each other via a modified form of the GB pair potential, GBDII, which was suggested for disc-like molecules by Bates and Luckhurst [116]. The parameterization, which was employed for the model discotic system, was k = 0.345, Kf = 0.2, /jl= 1, and v = 2. For the lattice system, the well-known Lebwohl-Lasher (LL) model was chosen [117]. In this model, the particles are assumed to have uniaxial symmetry and represented by three-dimensional spins, located at the sites of a simple cubic lattice, interacting through a pair potential of the form... 

Along this line of using amphiphilic features of particles to drive assembly using a hydrophobic effect, there has been a recent surge of interest in the fabrication and behavior of anisotropic patchy or Janus-type colloidal particles as a promising route to innovative nanocomposite materials [40, 41]. Whereas a thorough review lies outside our scope, we would like to highlight a few examples. Muller and coworkers prepared disc-like polymer Janus particles from assem-... 

Muller and coworkers prepared disc-like polymer Janus particles from assembled films of the triblock copolymer SBM and, after hydrolysis of the ester groups into methacrylic acid units, used these as Pickering stabilizer in the soap-free emulsion polymerization of styrene and butyl acrylate [111]. Armes and coworkers described the synthesis of PMMA/siUca nanocomposite particles in aqueous alcoholic media using silica nanoparticles as stabilizer [112], extending this method to operate in water with a glycerol-modified silica sol [113, 114]. Sacanna showed that methacryloxypropyltrimethoxysilane [115] in the presence of nanosized silica led to spontaneous emulsification in water, which upon a two-step polymerization procedure afforded armored particles with an outer shell of PMMA [116]. Bon and coworkers demonstrated the preparation of armored hybrid polymer latex particles via emulsion polymerization of methyl methacrylate and ethyl methacrylate stabilized by unmodified silica nanoparticles (Ludox TM O) [117]. Performance of an additional conventional seeded emulsion polymerization step provided a straightforward route to more complex multilayered nanocomposite polymer colloids (see Fig. 14). 

Like the liquid crystalline ordering of collection of rod-shaped particles in concentrated dispersions, collections of disc-shaped particles should also exhibit orientational order with sufficient concentration and narrow size and shape distributions [113, 114]. Moreover, disc-like objects are experimentally observed to exhibit nematic, smectic and columnar type mesophases in consistent with theoretical predictions. Since graphene can be considered as disc-like nano-object with very high aspect ratio, it has been shown that upon dispersion in sufficiently high concentrations it exhibits liquid crystalline phase behavior. The following are the different methods to fabricate and stabilize graphene based liquid crystalline phases. 

The nematic phase was described in Chapters 1, 3 and 4 and is exhibited by certain rod-like molecules and certain disc-like molecules. The nematic phase is the least ordered liquid crystal phase and it is usually very easy to identify a nematic phase by optical polarising microscopy. The high degree of disorder of the phase stmcture means that the nematic phase is very fluid and dust particles within the sample are seen to undergo intense Brownian motion. When the coverslip is displaced (a vital operation when identifying liquid crystal phases by optical polarising microscopy), it is easy to detect how fluid the phase actually is and the sample shirmners quite intensely at the time of impact. 

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