Energy levels of steady states for thin-film-type equations

R. S. Laugesen, M. C. Pugh

Research output: Contribution to journalArticlepeer-review


We study the phase space of the evolution equation ht = -(f(h)hxxx)x - (g(h)hx)x by means of a dissipated energy. Here h(x, t) ≥0, and at h = 0 the coefficient functions f > 0 and g can either degenerate to 0, or blow up to ∞, or tend to a nonzero constant. We first show that all positive periodic steady-states are saddles in the energy landscape, with respect to zero-mean perturbations, if (g/f)″ ≥ 0 or if the perturbations are allowed to have period longer than that of the steady-state. For power-law coefficients (f(y) = yn and g(y) = ℬym for some ℬ > 0) we analytically determine the relative energy levels of distinct steady-states. For example, with m - n ∈ [1, 2) and for suitable choices of the period and mean value, we find three fundamentally different steady-states. The first is a constant steady-state that is stable and is a local minimum of the energy. The second is a positive periodic steady-state that is linearly unstable and has higher energy than the constant steady-state; it is a saddle point for the energy. The third is a periodic collection of 'droplet' (compactly supported) steady-states having lower energy than either the positive steady-state or the constant one. Since the energy must decrease along every orbit, these results significantly constrain the dynamics of the evolution equation.

Original languageEnglish (US)
Pages (from-to)377-415
Number of pages39
JournalJournal of Differential Equations
Issue number2
StatePublished - Jul 1 2002

ASJC Scopus subject areas

  • Analysis
  • Applied Mathematics


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