Dispersed particle behaviour

Investigations of the rheological properties of disperse systems are very important both from the fundamental and applied points of view (1-5). For example, the non-Newtonian and viscoelastic behaviour of concentrated dispersions may be related to the interaction forces between the dispersed particles (6-9). On the other hand, such studies are of vital practical importance, as, for example, in the assessment and prediction of the longterm physical stability of suspensions (5). 

A most surprising experimental result is the increase in density of carbon-black-filled or pigment systems, which show a non-linear behaviour as the volume concentration increases (when the dispersed particle size is much less than 1 pm and the dispersion is optimal). We have recently shown [72e] that this density non-linearity is connected with the sudden conductivity increase observed in carbon-black and polyaniline-filled systems. 

The question of whether the same term is warranted for chains irreversibly attached to dispersed particles, which are capable of translational motion, is not so easily resolved. The centres of mass of the attached chains possess translational movement by virtue of the motion of the particles. However, the chains attached to one particle undergo correlated movement, quite different from their uncorrelated behaviour in free solution. Hence, even for dispersed particles, this term is unlikely to be correct but it is not immediately obvious how the configurational entropy of the sterically stabilized particles can be properly introduced. Finally, we note parenthetically that the presence of a term associated with the osmotic pressure of the dispersion medium may be appropriate at high compressions. 

The impact behaviour of HDPE/iPP (75/25) binary blend is of brittle nature with no stress whitening phenomenon. As shown in the SEM micrograph of Fig. 7a, the fractured surface of this binary mixture exhibits a "honeycomb morphology with evidence of dispersed particles (iPP) enclosed in the cells. Furthermore, there seems to be very negligible adhesion between the matrix and the dispersed phase. This observation accounts for the lower impact strength values compared with those obtained for the pure homopolymers. 

Field Emission Gun with Energy Dispersive X-ray detection (FEG/EDX) interfaced with MRS is illustrated in Fig. 8, where it is observed that the particle has undergone changes after exposure to the electron beam. In addition, Raman spectra obtained after exposure to the electron beam showed band broadening, higher background and lower spectral quality. To this end, the importance of a robust protocol to be used for the analysis of environmental particles (especially of the finer fraction) with an interfaced two tier approach has been illustrated. It also seems from these preliminary observations that the protocol may have to be particle type specific. In addition, particle behaviour when exposed to the interfaced instrument may not mirror its behaviour when analysed under stand-alone conditions. 

Most slurries are shear-thinning. It is h5q>othesized that this shear-thinning behaviour is due to the formation of particulate aggregates which provide a lower resistance to flow than fully dispersed particles. 

In dispersed systems with a dispersed phase or dispersion medium concentration gradient (chemical potential gradient), a spontaneous process occurs to balance the concentration. This kinetic behaviour, the abiUty of a substance to undergo diffusion, is called diffusion. The speed of diffusion is characterised by the diffusion coefficient (D). Its value increases with increasing temperature and decreases with increasing viscosity of the dispersion medium and radius of dispersed particles. The diffusion rate of the dispersed phase is therefore highest in molecular dispersed... 

During the relative motion of dispersed particles with the electric double layer against the dispersion medium, the Stern layer and part of the diffusion layer move with the particle while the rest of the diffusion layer moves with the fluid. A potential thus arises in the interface with the liquid, which is called the electrokinetic potential or ((-potential (zeta-potential). Its size depends on the type of electrolyte and the ability to adsorb ions. The existence of electrical charge on the dispersed phase particles (the existence of electric double layer) significantly affects the stability of many dispersed systems. It is also associated with the phenomena that occur when one phase moves relative to another (in liquid-gas, liquid-liquid and liquid solid systems) or with the behaviour of dispersed systems under an external electric potential gradient. 

In practice, e.g., in nature or in fonnulated products, colloidal suspensions (also denoted sols or dispersions) tend to be complex systems, consisting of many components that are often not very well defined, in tenns of particle size for instance. Much progress has been made in the understanding of colloidal suspensions by studying well defined model systems, which allow for a quantitative modelling of their behaviour. Such systems will be discussed here. 

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

By the term particulate composites we are referring to composites reinforced with particles having dimensions of the same order of magnitude. Particulate composites are produced from a polymeric matrix, into which a suitable metal powder has been dispersed, and exhibit highly improved mechanical properties, better electrical and thermal conductivity than either phase, lower thermal expansivity, and improved dimensional stability and behaviour at elevated temperatures. 

Airborne particulate matter may comprise liquid (aerosols, mists or fogs) or solids (dust, fumes). Refer to Figure 5.2. Some causes of dust and aerosol formation are listed in Table 4.3. In either case dispersion, by spraying or fragmentation, will result in a considerable increase in the surface area of the chemical. This increases the reactivity, e.g. to render some chemicals pyrophoric, explosive or prone to spontaneous combustion it also increases the ease of entry into the body. The behaviour of an airborne particle depends upon its size (e.g. equivalent diameter), shape and density. The effect of particle diameter on terminal settling velocity is shown in Table 4.4. As a result ... 

The stability of these dispersions has been investigated. A strong dependence of critical flocculation conditions (temperature or volume fraction of added non-solvent) on particle concentration was found. Moreover, there seems to be little or no correlation between the critical flocculation conditions and the corresponding theta-conditions for the stabilising polymer chains, as proposed by Napper. Although a detailed explanation is difficult to give a tentative explanation for this unexpected behaviour is suggested in terms of the weak flocculation theory of Vincent et al. 

The effect of an applied pressure on the UCFT has been investigated for polymer particles that are sterically stabilized by polyisobutylene and dispersed in 2-methy1-butane. It was observed that the UCFT was shifted to a higher temperature as the hydrostatic pressure applied to the system increased. There was also a qualitative correlation between the UCFT as a function of applied pressure and the 6 conditions of PIB + 2-methylbutane in (P,T) space. These results can be rationalized by considering the effect of pressure on the free volume dissimilarity contribution to the free energy of close approach of interacting particles. Application of corresponding states concepts to the theory of steric stabilization enables a qualitative prediction of the observed stability behaviour as a function of temperature and pressure. 

---