Microparticle sphere

The peak-to-peak separation of 50 tm and 5 tm microparticle spheres and hemispheres are compared with the case of the planar diffusion. The hemispherical particle exhibits a peak separation of zero at low scan rates, as it behaves as an adsorbed molecule. With increasing scan rates, A( )p increases and finally approaches the value(s) for the planar diffusion model. The larger the diameter of the hemispheres and spheres, the earlier the planar diffusion behavior is reached vdth increasing scan rates, as the curvature of larger particles has a lesser effect on the diffusion. 

One important finding of this study is that, for the case of full-sphere geometry (Figure 6.11b), the planar diffusion model is not an appropriate approximation. Rather, the microparticle sphere exhibits a region, where the reverse scan current decreases with scan rate, as the geometry favors a convergent diffusion of charge -that is, towards the equatorial point of the three-phase contact. This causes a dilution of the product species near this point at scan rates above the thin layer... 

Voltammetric measurements could enable the size determination (see also Section 6.3.2.2.3) of single particles of known shape, such as a sphere on a microelectrode [57]. The respective microparticle is positioned centrally on a microelectrode, after which cyclic voltammetric measurements are recorded over a wider range of scan rates (e.g., from 2 mV s to 1.5 V s ). With slower scan rates, the microparticle sphere increasingly perturbs the voltammetric signal, while the diffusion layer changes simultaneously (in both size and shape) from an almost fiat layer to an approximately hemispherical shape. The sphere affects the voltammetric response, in that there is a relationship between the microparticle size and the peak current (see Figure 6.30). 

The peak current increases (tends towards the value obtained for a bare electrode) with decreasing scan rates, as the mass transport will not be hindered by small sphere sizes. With increasing scan rates, the diffusion layer thickness decreases, and minimizes the effects of mass transport on the voltammetric response. Eventually, numerical simulations of cyclic voltammograms as a function of scan rate will provide information on the microparticle sphere radius. 

Recently, the ability of fi-CyD to form particles without polymers or phosphohpids has been studied. Indeed, by employing a reticulation of -CyD during the preparation process, it is possible to directly obtain microparticles (spheres and capsules) [10, 15]. On the other hand, reticulation of -CyD can also be performed to synthesize a -CD polymer, capable of forming nanospheres mixed with modified polysaccharide spontaneously [46]. 

Recently, the LbL technique has been extended from conventional nonporous substrates to macroporous substrates, such as 3DOM materials [58,59], macroporous membranes [60-63], and porous calcium carbonate microparticles [64,65], to prepare porous PE-based materials. LbL-assembly of polyelectrolytes can also be performed on the surface of MS particles preloaded with enzymes [66,67] or small molecule drugs [68], and, under appropriate solution conditions, within the pores of MS particles to generate polymer-based nanoporous spheres following removal of the silica template [69]. 

The pore size and distribution in the porous particles play essential roles in NPS synthesis. For example, only hollow capsules are obtained when MS spheres with only small mesopores (<3 nm) are used as the templates [69]. This suggests that the PE has difficulty infiltrating mesopores in this size range, and is primarily restricted to the surface of the spheres. The density and homogeneity of the pores in the sacrificial particles is also important to prepare intact NPSs. In a separate study, employing CaC03 microparticles with radial channel-like pore structures (surface area 8.8 m2 g 1) as sacrificial templates resulted in PE microcapsules that collapse when dried, which is in stark contrast to the free-standing NPSs described above [64]. 

A scaled-up version of this central template-concentric sphere surface assembly approach has been demonstrated for the growth of multi-layer core-shell nano- and microparticles, based upon the repeated layer-by-layer deposition of linear polymers and silica nanoparticles onto a colloidal particle template (Figure 6.8) [60]. In this case, the regioselective chemistry occurs via electrostatic interactions, as opposed to the covalent bond formation of most of the examples in this chapter. The central colloidal seed particle dictates the final particle... 

We note that since Q involves the scattering coefficients, the radiation pressure force has resonance or near-resonance behavior. This first was observed and analyzed by Ashkin and Dziedzic (1977) in their study of microparticle levitation by radiation pressure. They made additional measurements (Ashkin and Dziedzic, 1981) of the laser power required to levitate a microdroplet, and Fig. 19 presents their data for a silicone droplet. The morphological resonance spectrum for the 180° backscattered light shows well-defined peaks at wavelengths corresponding to frequencies close to natural frequencies of the sphere. The laser power shows the same resonance structures in reverse, that is, when the scattered intensity is high the laser power required to levitate the droplet is low. 

Silica-based microspheres are typically between 5 and 200 j,m in diameter, have wall thicknesses of 0.5-20 pm and can be filled with up to 100 M Pa of H2. The spheres are formed by melting spray-dried microparticles in free fall and the evolving gases... 

We assume that the adsorbent mass used in the kinetic test consists of a sphere of radius R. It may be composed of several microsize particles (such as zeolite crystals) bonded together as in a commercial zeolite bead or simply an assemblage of the microparticles. It may also be composed of a noncrystalline material such as gels or aluminas or activated carbons. The resistance to mass transfer may occur at the surface of the sphere or at the surface of each microparticle. The heat transfer inside the adsorbent mass is controlled by its effective thermal conductivity. Each microparticle is at a uniform temperature dependent on time and its position in the sphere. 

Figure 5. High-voltage electron micrographs and x-ray spectra of microparticles collected at Whiteface Mountain in 1983. Scale bars equal 0.5 pm. (a) y-Fe203 sphere collected 28 July II. (b) y-Fe203 sphere cluster collected 28 July IV. (c) Mullite sphere collected 28 July IV. (d). Mixed-element sphere collected 28 July III.

Filopodia formation and microparticle formation are results of calcium signaling. Filopodia are little feet, extending from the platelet, which allow the platelet to make better contact with the outside environment. Microparticles are tiny vesicles (phospholipid spheres) that are shed from the platelet, allowing the aforementioned docking proteins easier access to a phospholipid surface, and vastly enhancing the rate of activation of further molecules of thrombin. The generation of free arachidonic acid, and consequent formation and release of thromboxane A2 in the surrounding blood, provokes the activation of nearby platelets. 

A sophisticated enhancement of metal vapor deposition involves silver deposition over an ordered array of polystyrene spheres with submicron diameters (41-43). The Ag atoms deposit both on the spheres and on the flat substrate below the spheres. After deposition, the spheres may be removed to leave an ordered array of microscopic Ag pyramids with regular size and shape. The pyramids have dimensions of a few hundred nanometers, in the range required for EM field enhancement. These arrays of Ag microparticles have provided a direct test of EM field enhancement theory (44), and have been modified with platinum and organic thin films (45). 

Salbutamol (121) was precipitated from DMSO solutions the influence of the initial solution concentration on the dimension of the salbutamol particles was investigated. Interesting results were obtained, since salbutamol formed balloons, hollow spheres, and rodlike microparticles with a tendency to connect together. With increasing the concentration, the organization of the rodlike particles changed. To interpret the formation of the balloons, Reverchon described the dispersion of the liquid phase. The liquid droplets formed at the injector are subject to a very fast expansion owing to antisolvent diffusion, when saturation is reached on the droplet surface, an outer skin of solute is formed. 

Thompson and Compton investigated, from a theoretical standpoint, the case of a spherical microparticle with an electroactive compound on its surface and attached to a solid electrode surface [33, 34]. The movement of charge was assumed to start exclusively from the contact point (or line) between the microparticle and electrode (i.e., at the three-phase boundary, if an electrolyte phase is considered) and to proceed over the particle surface only (see also Section 6.3.1). In Ref. [33], the idealized microparticle geometries of a full sphere, a hemisphere, and an inverted hemisphere have been considered (cf. Figure 6.8). 

Figure 6.29 shows some example linear sweep voltammograms assuming different scan rates (Osrrefers to the dimensionless scan rate Osr = F/RT)(yrl/D)). As the experimental time scale decreases, the diffusional behavior changes from near-steady-state to near-planar diffusion. With respect to the different shapes of microparticles, the mass transport-limiting current was found to be fairly consistent that is, a difference of less than 2% for sphere and hemispheres of equal surface area. 

Lipospheres were first reported by Domb, who described them as water-dispersible solid microparticles of a particle size between 0.2 and 100 pm in diameter, composed of a solid hydrophobic fat core stabilized by a monolayer of phospholipid molecules embedded in the microparticles surface [1], Using this definition, lipo-sphere size is on the nanometer scale. Usually, nanoscale particles consisting of a solid lipid core are termed SLN [16], though sometimes inconsistent nomenclature can be found. Unlike SLN, lipospheres are restricted to the stabilizing material of a phospholipid layer because of their definition [1], This chapter focuses on research results obtained for peptide and protein formulations termed lipospheres, and it does not consider SLN literature at large. 

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