Confinement of semiflexible polymers

Abstract

A variational framework is developed to examine the equilibrium states of a semi-flexible polymer that is constrained to lie on a fixed surface. As an application the confinement of a closed polymer loop of fixed length $2\pi R$ within a spherical cavity of smaller radius, $R_0$, is considered. It is shown that an infinite number of distinct periodic completely attached equilibrium states exist, labeled by two integers: $n=2,3,4,...$ and $p=1,2,3,...$, the number of periods of the polar and azimuthal angles respectively. Small loops oscillate about a geodesic circle: $n=2$, $p=1$ is the stable ground state; states with higher $n$ exhibit instabilities. If $R\ge 2R_0$ new states appear as oscillations about a doubly covered geodesic circle; the state $n=3, p=2$ replaces the two-fold as the ground state in a finite band of values of $R$. With increasing $R$, loop states alternate between orbital behavior as the poles are crossed and oscillatory behavior upon collapse to a multiple cover of a geodesic circle, (signalled respectively by an increase in $p$ and an increase in $n$). The force transmitted to the surface does not increase monotonically with loop size, but does asymptotically. It behaves discontinuously where $n$ changes. The contribution to energy from geodesic curvature is bounded. In large loops, the energy becomes dominated by a state independent contribution proportional to the loop size; the energy gap between the ground state and excited states disappears.