Particle composition shape, standards

A monodisperse aerosol is one with a narrow size distribution, which, for log-normal-distributed particles, usually means a geometric standard deviation of about 1.2 or smaller. Monodisperse particles are expected to have simple shapes and uniform composition with respect to size. A polydisperse aerosol, on the other hand, is one containing a wide range of particle sizes, but which may otherwise be homogeneous in terms of the basic physical and chemical properties that are not related to size. The term heterodisperse is also used occasionally this describes aerosols varying widely in physical and chemical characteristics, as well as size. 

Polarization Fluctuation Spectroscopy is a relatively simple extension to the well established technique of Photon Correlation Spectroscopy. The optical equipment required in addition to that used for PCS is standard, as also is the data processing hardware for determining the correlation functions. The means to analyze the data is now achievable because of the computational techiuques for calculating the scattering properties for particles of complex shape and composition that have been developed in the last decade. The additional functionality afforded by PFS is achieved at modest additional cost, and importantly, by performing measurements that are essentially the same as those for PCS. 

Both the total amount of each component and the microstructure i.e. the sizes, shapes and connectivity of the particles) are important. Together they determine the properties of the composite, i.e. the physical and sensory properties of ice cream. The amounts of the structural components are different for different types of ice cream. Table 7.1 shows typical volume fractions of each component at — 18 °C in standard, premium, low fat and soft scoop ice cream, and water ice. Premium ice cream contains more fat than a standard ice cream, whereas a low fat ice cream (obviously) contains less. Soft scoop ice cream contains less ice than the standard. Water ice is a composite of two materials, ice crystals and matrix, and contains no air or fat. 

Unless a comparison is made with a standard, which is identical in composition to the sample, systematic errors in XRF may be large. Particle size and shape are important and determine the degree to which incident beam is absorbed or scattered. Hence, standards and samples must be powdered to the same mesh size, preferably finer than 200 mesh. Simultaneous determination of several elements is possible by XRF with automated equipment. 

Since nanoparticles in PNC are orders of magnitude smaller than conventional reinforcements, the models developed for composites are not applicable to nanocomposites. However, development of a universal model for PNC is challenging since the shape, size, and dispersion of the nanoparticles vary widely from one system to another. On the one hand, exfoliated clay provides vast surface areas of solid particles (ca. 800 m /g) with a large aspect ratio that adsorb and solidify a substantial amount of the matrix polymer, but on the other hand, the mesoscale intercalated clay stacks have a much smaller specific surface area and small aspect ratio. However, in both these cases the particle-particle and particle-matrix interactions are much more important than in conventional composites, affecting the rheological and mechanical behavior. Thus, the PNC models must include the thermodynamic interactions, often neglected for standard composites. 

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