Sedimenting particles, dynamic interaction

It is now time to discuss yet another mechanism of particle motion in a suspension in view of influence of ambient particles. Up till now, slow motion of a test particle was usually considered with the proper account taken of interactions with the neighboring particles, but with the assumption of sufficiently small volume concentration of particles W. In spite of this restriction, the formula (8.150) can be used to And the velocity of constrained motion of a particle at rather high values of j, i.e. in highly concentrated suspensions. In such suspensions, particle motion (gravitational sedimentation or motion in a shear flow) has a random character, which is different, however, from the character of Brownian motion, because particles sizes are much greater. This type of motion is caused by hydro-dynamic interactions with the neighboring particles. 

Soil plays the central role as organizer of the terrestrial ecosystem (Coleman et al., 1998). It may be perceived as the center of tire ecosystem, which evolves because of interactions of the lithosphere, hydrosphere, atmosphere, and biosphere. A factor of central importance of soil to ecological studies is that soils on a global scale have a range of characteristics, which enable an enormous array of microorganisms, plants, animals, and humans to coexist and thrive. Among the environmental compartments, about 90% of environmental pollutants are bound with soil particles and 9% of the pollutants are bound with aquatic sediments (Table 1.7). These soil- and sediment-bound pollutants are in dynamic equilibrium with the hydrosphere, atmosphere, and biosphere. Soil physicochemical and... 

Sedimentation Field Flow Fractionator. The chromatography-related principle of this particle size and size distribution analyzer is based upon the interaction of the particle suspension under centrifugal field motion in a thin channel. The elution time of the particles is a function of particle size, particle density, flow rate of mobile phase, density of mobile phase, and the centrifugal force applied. After the size separation has occurred, the particles are detected in the mobile phase using a turbidity detection system. The dynamic range of the instrument is dependent on particle density and operating conditions and is typically within 0.03 /rm— 1 /rm range. 

Stokesian dynamics is a numerical technique for simulating the dynamic hehaviour of colloidal suspensions (sedimentation, rheology), where the motions of the individual particles is driven hy Brownian and volume forces (including particle interactions) and coupled by hydrodynamic interaction. In a more general approach than in Eq. (4.69), the hydrodynamic forces are traced back to the generalised particle velocities Vp and the velocity gradients E ... 

For a very dilute suspension of rigid non-interacting particles, the rate of sedimentation Vo can be calculated by application of Stokes law, whereby the hydro-dynamic force is balanced by the gravitational force,... 

The term hydrodynamic interactions describes the dynamic correlations between the particles, induced by diffusive momentum transport through the solvent. The physical picture is the same, whether the particle motion is Brownian (i.e., driven by thermal noise) or the result of an external force (e.g., sedimentation or electrophoresis). The motion of particle i perturbs the surrounding solvent, and generates a flow. This signal spreads out diffusively, at a rate governed by the kinematic viscosity of the fluid J]kin = tl/p (t] is the solvent shear viscosity and p is its mass density). On interesting (long) time scales, only the transverse hydrodynamic modes [14] remain, and the fluid may be considered as incompressible. The viscous momentum field around a particle diffuses much faster than the particle itself, so that the Schmidt number... 

This chapter discusses the dynamics of particle interaction, that is, the rate dependence of the various subprocesses taking place when two particles meet. For the overall outcome, the extent to which subprocesses can relax during such an encounter is important. These subprocesses must be identified and their rates established relative to the rate of particle interaction. As an exercise, these ideas are elaborated for encounters between surfactant-covered particles. The dynamic differences between particle encounters in a sol and shelf stability in a sediment are briefly discussed. New insights into lateral transport by surface conduction are presented. 

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