We consider the thin-film equation Bth`∇¨`h 2 ∇∆h˘" 0 in physical space dimensions (i.e., one dimension in time t and two lateral dimensions with h denoting the height of the film in the third spatial dimension), which corresponds to the lubrication approximation of the Navier-Stokes equations of a three-dimensional viscous thin fluid film with Navier-slip at the substrate. This equation can have a free boundary (the contact line), moving with finite speed, at which we assume a zero contact angle condition (complete-wetting regime). Previous results have focused on the 1`1-dimensional version, where it has been found that solutions are not smooth as a function of the distance to the free boundary. In particular, a well-posedness and regularity theory is more intricate than for the second-order counterpart, the porousmedium equation, or the thin-film equation with linear mobility (corresponding to Darcy dynamics in the Hele-Shaw cell). Here, we prove existence and uniqueness of classical solutions that are perturbations of an asymptotically stable traveling-wave profile. This leads to control on the free boundary and in particular its velocity.1.3. Weak solutions to the thin-film equation. We emphasize that a well-established global existence theory of weak solutions to (1.2) has been developed, starting with Bernis and Friedman [7] and later on upgraded to the stronger entropy-weak solutions by Beretta, Bertsch, and Dal Passo [4], and independently by Bertozzi and Pugh [9], which also exist in higher dimensions (cf. [21,45]). An alternative gradient-flow approach leading to generalized minimizing-movement solutions (that are weak solutions as well) is due to Loibl, Matthes, and Zinsl [61], Matthes, McCann, and Savaré [63], and Otto [69]. Qualitative properties of weak solutions have been the subject of for instance the works of Bernis [5, 6], Grün [43, 44], and Hulshof and Shishkov [48], where finite speed of propagation has been proved, Bertsch, Dal Passo, Garcke, and Grün [10], Dal Passo, Giacomelli, and Grün [22], Fischer [28, 29, 30], Giacomelli and Grün [33], and Grün [46], where waiting-time phenomena have been considered, or Carlen and Ulusoy [19], Carrillo and Toscani [20], and Matthes, McCann, and Savaré [63], where the intermediate asymptotics of (1.2) have been investigated. Partial-wetting boundary conditions have been considered by Bertsch, Giacomelli, and Karali [11], Esselborn [27], Mellet [64], and Otto [69]. We refer to Ansini and Giacomelli [2], Bertozzi [8], and Giacomelli and Shishkov [37] for detailed reviews. Nevertheless, unlike in the porous-medium case (1.3), this theory does neither give uniqueness of solutions nor enough control at the free boundary to give an expression like (1.1d) a classical meaning. Furthermore, the regularity of the free boundary as a sub-manifold of p0, 8qˆR 2 appears to be inaccessible within this theory. This explains the interest in a well-posedness and regularity theory of classical solutions to (1.1).
1.4.Well-posedness and classical solutions. Well-posednes...