Forces in Motzkin paths in a wedge

Abstract

Entropic forces in models of Motzkin paths in a wedge geometry are considered as models of forces in polymers in confined geometries. A Motzkin path in the square lattice is a path from the origin to a point in the line Y = X while it never visits sites below this line, and it is constrained to give unit length steps only in the north and east directions and steps of length in the north-east direction. Motzkin path models may be generalized to ensembles of NE-oriented paths above the line Y = rX, where r > 0 is an irrational number. These are paths giving east, north and north-east steps from the origin in the square lattice, and confined to the r-wedge formed by the Y-axis and the line Y = rX. The generating function gr of these paths is not known, but if r > 1, then I determine its radius of convergence to be and if r (0, 1), then tr = 1/3. The entropic force the path exerts on the line Y = rX may be computed from this. An asymptotic expression for the force for large values of r is given by In terms of the vertex angle α of the r-wedge, the moment of the force about the origin has leading terms as α → 0+ and F(α) = 0 if α [π/4, π/2]. Moreover, numerical integration of the force shows that the total work done by closing the wedge is 1.085 07... lattice units. An alternative ensemble of NE-oriented paths may be defined by slightly changing the generating function gr. In this model, it is possible to determine closed-form expressions for the limiting free energy and the force. The leading term in an asymptotic expansions for this force agrees with the leading term in the asymptotic expansion of the above model, and the subleading term only differs by a factor of 2.