Latex spheres, application

A simple calibration has been carried out with latex spheres and a practical application on time dependent growth of calcium carbonate [88]. A further experiment was carried out to monitor the wet grinding of submicron color pigment using diluted, extracted samples. Further work was proposed to investigate the effect of particle shape. 

Recently, inqjortant efforts have been made to fabricate photonic crystals by colloidal crystal templating [1-5], Colloidal crystals are self-organized arrays of silica or latex spheres, having the periodicity required for photonic band gaps. The basic idea is to use a colloidal crystal as a template, infiltrate the interstitial spaces between the spheres with another material and, then, remove the spheres by chemical etching or combustion. Since the resulting macroporous structures have a complete photonic bandgap [6,7], these materials have many applications in photonics. 

An example of an application of homodyne spectroscopy is the measurement of the size distribution of small particles in the nanometer range that are dispersed in liquids or gases and fly through a laser beam. The intensity 4 of the scattered light depends in a nonlinear way on the size and the refractive index of the particles. For small homogeneous spheres with diameters d, which are small compared to the wavelength (d A), the relation oc d holds. In Fig. 12.25 the measured intensity distribution of laser light scattered by a mixture of latex spheres with d = 22.8 nm and d = 5.7 nm (small squares) is compared with the size distribution obtained from electron microscopy, which can be used for calibration [12.91]. 

Sihcate composites can provide an efficient size-exclnsion membrane. The pore size distribntion can be controlled, at least to some extent, by incorporation of water-solnble polymers such as poly(ethylene glycol) and their leaching after sol-gel formation or by pH control. On the low-pore-size side, low-pH silicate processing may yield silicate membranes that exhibit Knudsen flow and can separate nitrogen from air [245]. The npper limit for porous silicates probably lies in living cell templated materials [246,247] or latex sphere templates [248], and in sol-gel-derived ruthenium and vanadinm oxide aerogels for supercapacitor applications [249,250]. 

Steric hyperlayer-FFF is well established as a fast separation technique for mi-cron-sized particles, although the hydrodynamic lift forces are not yet well understood. This is worse for the steric elution of non-spherical particles. Despite over thirty years of application of FFF techniques, only very little has been reported about the fractionation of non-spherical particles by any FFF mode. The few available studies so far reported are the investigation of coal particles [423,424], inorganic colloids [462], metal particles [69] and doublets of polystyrene beads, rod-shaped glass fibers, compressed latex discs and quartz particles with complex shape [427]. In the latter paper, systematic studies of particle shape on the retention behavior of non-spherical particles are reported with the result that the qualitative major retention behavior of spheres and other shapes is equal (e.g. response to increase in the field strength, etc.). However, the quantitative differences in the retention behavior were found to depend on numerous factors in a complex way so that no quantitative relation between the hydrodynamic radius and the retention ratio could be established. 

Practicability. Both hemagglutination-inhibition and latex inhibition tests are easy to perform and are inexpensive. In addition, the results of the tests can be read in a few hours. As mentioned previously, their main disadvantage is the need to use large quantities of purified hormones in order to coat the red cells. Stockell Hartree (S25) has shown that these assays are extremely useful in order to monitor hormone extraction procedures, but in the absence of a supply of purified hormones their sphere of applicability to clinical problems is limited. When crude hormones and nonspecific antisera are used the quantitative significance of assays based on hemagglutination-inhibition reactions is doubtful. 

The application of widespread standard polymer processing techniques to the formation of colloidal crystals was introduced by Ruhl et al. [60]. They demonstrated the fabrication of a large colloidal crystal film, which comprises core-shell latex particles. Upon coagulation the soft shell of the particles causes the formation of a rubbery mass, which can be uniaxially compressed. The radial horizontal flow during the compression induces crystallization of the particles from the surface of the plates inward. The soft shell constitutes the matrix in which the hard spheres are embedded. This technique is promising for efficient application to other polymer processing methods like extrusion or injection molding. 

In so far as equation (14.31) is valid for flat plates, the Deijaguin integration procedure implies that it can also be applied to spheres, at least for particles whose barrier layer thicknesses are small compared with the radii of the particles. Inspection of the form of equation (14.31) reveals that, e.g. polyacrylonitrile latex particles stabilized by a mixture of polystyrene and polyisobutylene may exhibit stability in cyclohexane at temperatures well below the 0-temperature for polystyrene. Indeed, using the numerical data set forth earlier, it is readily shown, by application of equation (14.31) that the addition of small amounts (e.g. less than 1 %) of polyisobutylene to the barrier would be all that is required to impart stability at 5 K below the 0-temperature for polystyrene in cyclohexane. This surprising result arises from the strong repulsions that occur between the polystyrene and polyisobutylene chains. Increasing the proportion of polyisobutylene in the barriers would increase dramatically the penetration into the worse than 0-solvent domain for polystyrene that could be tolerated before the onset of flocculation. 

For a homogeneous sphere with radius R, Rg=Rg, oo which may be calculated by the relation R =3I5 R. Thus, a homogenous sphere of 100 nm diameter is characterized by Rg=38.73 nm. The validity of the Guinier-law Equation (17) requires that Rgqlatex systems with sufficient accuracy. 

Organic spheres are predominantly polymeric, consisting of synthetic or natural polymers. The field of polymeric nano- and microparticles is vast, comprising, for instance, latex particles for coatings, hollow particles for syntactic foams, and microcapsules for foaming and additive release. In addition, there are core-shell microbeads and coated polymeric particles, where the particles can exhibit multiple functionalities, thanks to the individual features of their different layers 1]. As fillers in thermosets and thermoplastics, hollow microspheres and expandable microcapsules are among the most frequently used in commercial applications. 

The influence of particle size distribution in the use of latex dispersions is shown to be of great practical importance. The practical consequences are examined of bimodal particle size distribution with respect to coatings applications. The introduction of polydispersity in acrylic dispersions is examined as a way of obtaining a lower dispersion volume loading at an equivalent viscosity as for monodisperse spheres. Aspects such as film formation, rheology, and drying behaviour are discussed. 39 refs. 

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