Long‐wave instabilities and saturation in thin film equations

Hocherman and Rosenau conjectured that long‐wave unstable Cahn‐Hilliard‐type interface models develop finite‐time singularities when the nonlinearity in the destabilizing term grows faster at large amplitudes than the nonlinearity in the stabilizing term (Phys.˜ D 67, 1993, pp. 113–125). We consider this conjecture for a class of equations, often used to model thin films in a lubrication context, by showing that if the solutions are uniformly bounded above or below (e.g., are nonnegative), then the destabilizing term can be stronger than previously conjectured yet the solution still remains globally bounded. For example, they conjecture that for the long‐wave unstable equation $h_t = - (h^n h_{xxx})_x - (h^m h_x)_x\,$ m > n leads to blowup. Using a conservation‐of‐volume constraint for the case m > n > 0, we conjecture a different critical exponent for possible singularities of nonnegative solutions. We prove that nonlinearities with exponents below the conjectured critical exponent yield globally bounded solutions. Specifically, for the above equation, solutions are bounded if m < n + 2. The bound is proved using energy methods and is then used to prove the existence of nonnegative weak solutions in the sense of distributions. We present preliminary numerical evidence suggesting that m ≥ n + 2 can allow blowup. © 1998 John Wiley & Sons, Inc.