## Centrifugation

Starting in the early 1920s with the famous work of Svedberg, centrifugation became probably the most popular method for separation of particles. Based on extensive knowledge and experience of the sedimentation of particles in a natural gravitational field, centrifugation, using more intense inertial forces gen-

where g is the acceleration due to the gravitational or centrifugal field forces and p is the density of the suspending liquid. Force F1 is opposed by the force of friction F2 which is proportional to the velocity of sedimentation U with a constant of proportionality f, called the friction coefficient:

With the exception of the initial short period of time during which the sedimentation velocity of the particle increases until the steady state is reached at which both forces are equal, the velocity of sedimentation in a homogeneous liquid is constant. Stokes calculated the friction coefficient of hard spherical particles and obtained:

f = 6nrjr for a particle of the radius r sedimenting in a liquid of the viscosity r. Einstein derived the relationship between the friction and diffusion coefficients:

It is evident that the sedimentation processes in homogeneous suspending liquids separate the particles according to their effective masses and if the particles are uniform with respect to their densities, the separation proceeds strictly according to the differences in particle size. The analysis of PSD can be realized on the basis of the measurement of the sedimentation velocity during the sedimentation process or from the equilibrium concentration distribution. Nevertheless, it has to be stressed that although cen-trifugation is, in principle, the separation method, the size-based separation of the particles can be rather complicated because various size particles sediment together and form a complex, superposed concentration gradient in which all size particles are always present in various relative proportions. On the other hand, if the separated particles exhibit nonuniformity in both size and density, size separation can be a rather difficult task.

Sedimentation processes can generate the formation of a density gradient in a complex, multicompo-nent suspending liquid. The particles suspended in such a density-gradient forming liquid can undergo focusing phenomena and, as a result, they can be separated according to differences in densities. Recent theoretical and experimental findings demonstrate that the size polydispersity in such cases influences the width of the focused zones. Evidently, therefore, if the particles exhibit polydispersity in size and density, the separation is complicated.

Modern theoretical approaches as well as the experimental results demonstrate that sedimentation and focusing can appear together even in a simple suspending liquid because the size polydispersity of the separated particles is itself able to generate the isoperichoric (from Greek: isos = equal and perichoron = environment) focusing phenomena. It can complicate the use of centrifugation as a simple tool for particle size separation. On the other hand, although not yet fully mastered and understood, these new approaches offer a challenge for fundamental research and development.

## Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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