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Diffusion in confined systems

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At small scales, the evolution of the mesoscopic degrees of freedom of a system is frequently influenced by the presence of constraints. The case of transport of particles in narrow and tortuous channels where the presence of obstacles and irregularities of the boundaries alter their trajectories is a typical example. Motion of particles in the interior of a living cell or through an ion channel, diffusion in zeolites and in microfluidic devices, and folding of proteins modeled as motion of the state of the protein through a phase space funnel-like region are cases in which the system proceeds in a bounded region. Confinement is a source of continuous dynamic changes and consequently of modifications of the transport properties of a system.

In many situations, the geometry of the system is such that transport takes place along a preferred direction of motion. Changes in the position and momentum of the particles thus occur mainly along this direction whereas local equilibration is rapidly reached in the transverse directions. Under this circumstance, transport becomes practically one-dimensional and one can consider the effect of the tortuosity of the boundaries by introducing entropic barriers. To analyze those situations, an entropy-driven diffusion model has been proposed. The model has revealed the many peculiarities of transport through entropic barriers, or entropic transport.

This talk is part of the Theory - Chemistry Research Interest Group series.

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