Sedimentation driving force

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

In this chapter the thermal motion of dissolved macromolecules and dispersed colloidal particles will be considered, as will their motion under the influence of gravitational and centrifugal fields. Thermal motion manifests itself on the microscopic scale in the form of Brownian motion, and on the macroscopic scale in the forms of diffusion and osmosis. Gravity (or a centrifugal field) provides the driving force in sedimentation. Among the techniques for determining molecular or particle size and shape are those which involve the measurement of these simple properties. 

Solid-liquid separation by flotation may be achieved by gravity alone or induced by dissolved-air or vacuum techniques. The mechanisms and driving forces are similar to those found in sedimentation, but the separation rate and solids concentration can be greater in some cases. 

A number of different driving forces have been used to implement CSF including gravitational sedimentation [113] diffusive transport [114,115] electrically driven transport [116] and hydrodynamic lift forces [117]. 

Before beginning a size determination, it is customary to look at the material, preferably under a microscope. This examination reveals the approx size range and distribution of the particles, and especially the shapes of the particles and the degree of aggregation. If microscopic examinatiori reveals that the ratios between max and min diameters of individual particles do not exceed 4, and indirect technique for particle size distribution based on sedimentation or elutria-tion may be used. Sedimentation techniques for particle size determination were first used by Hall (Ref 2) in 1904. He showed that the rate of fall of individual particles in a fluid was directly related to the particle size by the hydrodynamic law derived by Stokes from Newton s law of fluids in 1849 (Ref 1). This basic equation of the motion of a particle suspended in a fluid assumes that when subjected to constant driving force the particle acceleration is opposed by the... 

Creaming. Droplets in an emulsion will have some tendency to rise or settle according to Stokes law. An uncharged spherical droplet in a fluid will sediment if its density is greater than that of the fluid. The driving force is that of gravity the resisting force is viscous and is approximately proportional to the droplet velocity. After a short period of time the particle reaches terminal (constant) velocity, dx/dt, when the two forces are matched. Thus... 

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