= Reptation
<Reptation> is the snake-like motion of a long <polymer> through the constraints imposed by neighboring chains or a fixed porous matrix. A tube model restricts large transverse motion while allowing curvilinear motion along the chain contour. For a flexible chain with additive segment friction, contour diffusion scales as $N^{-1}$ and the time to renew a contour of length proportional to $N$ scales as $N^3$. With coil size squared proportional to $N$, this gives a centre-of-mass <diffusion coefficient> proportional to $N^{-2}$. Applying this tube picture to weak-field <DNA> <gel electrophoresis> gives an inverse-length mobility over its appropriate regime; field-induced orientation and trapping can invalidate that simple law.
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