EXISTENCE AND POSITIVITY OF SOLUTIONS OF A 4TH-ORDER NONLINEAR PDE DESCRIBING INTERFACE FLUCTUATIONS

We study the partial differential equation w(t) = -w(xxxx) + (w(x)2/w) xx which arose originally as a scaling limit in the study of interface fluctuations in a certain spin system. In that application x lies in R, but here we study primarily the periodic case x is-an-element-of S1 . We establish existence, uniqueness, and regularity of solutions, loc ally in time, for positive initial data in H1(S1), and prove the exist ence of several families of Lyapunov functions for the evolution. From the latter we establish a sharp connection between existence globally in time and positivity preservation: if [0, T) is a maximal half ope n interval of existence for a positive solution of the equation, with T < infinity, then lim(t --> T*) w(t, .) exists in C1(S1) but vanishe s at some point. We show further that if T > (1 + cube-root 3)/16pi2 cube-root 3 then T = infinity and lim(t --> infinity) w(t, .) exists and is constant. We discuss also some explicit solutions and propose a generalization to higher dimensions. (C) 1994 John Wiley & Sons, Inc.