Quasi-spherical

Altliough tire majority of studies on model colloids involve (quasi-) spherical particles, tliere is a growing interest in the properties of non-spherical colloids. These tend to be eitlier rod-like or plate-like. 

High-resolution transmission electron microscopy (HREM) is the technique best suited for the structural characterization of nanometer-sized graphitic particles. In-situ processing of fullerene-related structures may be performed, and it has been shown that carbonaceous materials transform themselves into quasi-spherical onion-like graphitic particles under the effect of intense electron irradiation[l 1],... 

Fig. 2. HREM image of a quasi-spherical onion-like graphitic particles generated by electron irradiation (dark lines represent graphitic shells, and distance between layers is 0.34 nm).

The progressive ordering from the surface to the center has been experimentally observed in the case of the electron irradiation-induced formation of the quasi-spherical onion-like particles[25]. In this case, the large inner hollow space is unstable under electron bombardment, and a compact particle (innermost shell C( ) is the final result of the graphitization of the carbon volume (see Fig. 3e-h). 

Fig. 3. Schematic illustration of the growth process of a graphitic particle (a)-(d) polyhedral particle formed on the electric arc (d)-(h) transformation of a polyhedral particle into a quasi-spherical onion-like particle under the effect of high-energy electron irradiation in (f) the particle collapses and eliminates the inner empty space[25j. In both schemes, the formation of graphite layers begins at the surface and progresses towards the center.

We hope that macroscopic samples of quasi-spherical onion-like particles will soon become available, and then we will be able to characterize these systems in detail. Probably a new generation of carbon materi-aks can be generated by the three-dimensional packing of quasi-spherical multi-shell fullerenes. 

Except for Ceo, lack of sufficient quantities of pure material has prevented more detailed structural characterization of the fullerenes by X-ray diffraction analysis, and even for Ceo problems of orientational disorder of the quasi-spherical molecules in the lattice have exacerbated the situation. At room temperature Cgo crystallizes in a face-centred cubic lattice (Fm3) but below 249 K the molecules become orientationally ordered and a simple cubic lattice (Po3) results. A neutron diffraction analysis of the ordered phase at 5K led to the structure shown in Fig. 8.7a this reveals that the ordering results from the fact that... 

The knowledge of turbulent premixed flames has improved from this very simple level by following the progress made in experimental and numerical techniques as well as theoretical methods. Much employed in early research, the laboratory Bunsen burners are characterized by relatively low turbulence levels with flow properties that are not constant everywhere in the flame. To alleviate these restrictions, Karpov et al. [5] pioneered as early as in 1959 the studies of turbulent premixed flames initiated by a spark in a more intense turbulence, produced in a fan-stirred quasi-spherical vessel. Other experiments carried out among others by Talantov and his coworkers allowed to determine the so-called turbulent flame speed in a channel of square cross-section with significant levels of turbulence [6]. 

In Og (Figure 8.1.5b), xmlike in the Ig case, the fuel jet momentum dispersed and the centerline velocity decayed rapidly owing to the lack of buoyancy. As a result, the fuel molecules diffused in every direction and formed a quasi-spherical flame. The slow diffusion processes (1) limited the transport rates of the fuel and oxygen into the flame zone and (2) decreased... 

Further progress depends essentially on the developments in p and C measurements. A new proposal is to apply quasi-spherical resonators for DCGT [40],... 

An elastic continuum model, which takes into account the energy of bending, the dislocation energy, and the surface energy, was used as a first approximation to describe the mechanical properties of multilayer cage structures (94). A first-order phase transition from an evenly curved (quasi-spherical) structure into a... 

The methods described above are appropriate for simple ions, but not for the calculation of the activity coefficients of more complex compounds such as zwitterions, i.e., those which bear more than one functional group, have a low molecular weight, which is arbitrarily put at less than 500, and are approximately spherical in shape so that both the quasi-spherical assumption used in the van der Waals integral and the present definition of cavity area are satisfied. Many substances of interest... 

Equation (22) has been found to be somewhat more useful than Eq. (20) for evaluation of the free energy change related to caSdty formation when more than one solute species is present. In the form given by Eq. (22) the cavity term can be c culated if macroscopic sur K e tension, y, K, and molecular surface area of both the solute and solvent are known. The latter values may be calculated for spherical or quasi-spherical mole- cules as... 

Fig. 9.23 SEM images of monodisperse silver powders obtained by reduction of AgNOj in ethylene glycol in the presence of PVP (a) quasi-spherical particles obtained by spontaneous nucleation (dm = 0.64 pun, cr = 0.13 p.m) (b) rodlike particles obtained by heterogeneous nucleation using H2PtCl6 as nucleating agent (particle dimensions 3 xm long and 0.3 xm thick). (From Ref. 13.)...

This rule works best for apolar, quasi-spherical molecules. Large deviations occur when chemical association is involved (e.g., carboxylic acids), from molecular dipolarity (e.g., dimethyl sulfoxide), and from molecular asphericity (e.g., neopentane/ -pentane). Strongly associating solvents (e.g., HF, H2, NH3, alcohols, carboxylic acids) have Trouton constants which are higher than the average value of 88 J mol K" found for nonassociating solvents such as diethyl ether and benzene. 

The simplest way to classify nanomaterials used in combination with liquid crystal materials or the liquid crystalline state is by using their shape. Three shape families of nanomaterials have emerged as the most popular, and sorted from the highest to the lowest frequency of appearance in published studies these are zero-dimensional (quasi-spherical) nanoparticles, one-dimensional (rod or wirelike) nanomaterials such as nanorods, nanotubes, or nanowires, and two-dimensional (disc-like) nanomaterials such as nanosheets, nanoplatelets, or nanodiscs. 

A complete and satisfactory characterization of quantum dots prepared by any of these methods requires many of the same techniques listed for metal nanoparticles described already (see above). In addition to critical electronic properties, photoluminescence spectroscopy is an extremely valuable tool to obtain preliminary information on size and size distribution of quantum dots, which can in many cases (i.e., for larger sizes and quasi-spherical shapes) be estimated from 2max and the full width at half maximum (fwhm) of the absorption or emission peak using approximations such as Bras model or the hyperbolic band model [113]. 

The nematic phase has unquestionably been the go-to phase to study and understand the major driving forces that govern interactions between suspended, quasi-spherical nanoparticles and liquid crystal molecules or mixtures. We credit this to three important factors (1) early experimental studies [288, 289] based on the foundation of de Gennes very early work on ferronematics [121], (2) the availability of nematic liquid crystals including room temperature and wide temperature... 

In comparison to nematic liquid crystals, examples of smectic liquid crystals doped with quasi-spherical nanoparticles became more elusive over the last few years. This is surprising especially considering recent work by Smalyukh et al., who found that nanoscale dispersion (based on /V-vinyl-2-pyrrolidone-capped gold nanoparticles with 14 nm diameter) in a thermotropic smectic liquid crystal (8CB) are potentially much more stable than dispersions of nanoparticles in nematics [367]. 

It was really only a matter of time until researchers in the field started doping blue phases with quasi-spherical nanoparticles. This area is very much in its infancy, but the few recent reports already show some promising results. Yoshida et al., for example, reported on an expansion of the temperature range of cholesteric blue phases from 0.5 to 5°C by doping blue phases with gold nanoparticles (average diameter of 3.7nm) as well as a decrease in the clearing point of approximately 13°C [427]. A similar effect was also observed by Kutnjak et al. for CdSe quantum dots simultaneously capped with oleyl amine and TOP (diameter of the core 3.5 nm) in CE8 (Merck) and CE6 (BDH). The authors found that particularly blue phase III was stabilized in these mixtures, blue phase II destabilized, and... 

Fig. 16 2D-cartoons of quasi-spherical nanoparticles protected with mesogenic or pro-mesogenic capping agents giving rise to liquid crystalline quasi-spherical nanoparticles. The three major approaches include the decoration with calamitic molecules in an end-on fashion (7), with dendrons featuring calamitic or polycatenar moieties at the termini (8), and with laterally substituted calamitic molecules in a side-on fashion (9). The concept shown for the quasi-spherical nanoparticle 7 was also successfully used for spindle-like nanoparticles [533, 534]... 

---