Existence of Solutions to a Viscous Thin Film Equation

A fourth-order degenerate parabolic equation with a viscous term: ( ) ( ) ( ) ( ) ( ) ( ) 0 in 1,1 0, , in 1,1 ) 0, t x x xx t u m u w T w u u T ν  − = − ×   = − + − ×  is studied with the initial-boundary conditions 0 x x u w = = on { } ( ) 1,1 0,T − × , ( ) ( ) 0 ,0 u x u x = in ( ) 1,1 − . It can be taken as a thin film equation or a Cahn-Hilliard equation with a degenerate mobility. The entropy functional method is introduced to overcome the difficulties that arise from the degenerate mobility ( ) m u and the viscosity term. The existence of nonnegative weak solution is obtained.

separation.It (see [1]) has the form ( ) ( ) ( ) = where the constants k, A, 2 ε denote the atomic mobility, the free energy, the parameter proportional to the interface energy respectively and ( ) ( ) is a kind of chemical potential.For the existence and the properties of solutions, Elliott, Zheng and Garcke (see [2] [3]) have studied this equation with a linear and a degenerate mobility respectively.Xu, Zhou, Liang and Zheng (see [4] [5] [6]) have applied the semi-discrete method to obtain the existence and stability results to this model with a gradient mobility.
The thin film equation can analyze the motion of a very thin layer of viscous incompressible fluids along an inclined plane or model the fluid flows such as draining of foams and the movement of contact lenses.The thin film equation belongs to a class of fourth order degenerate parabolic equations (see [7]) and the first mathematic result, the existence and nonnegativity of weak solutions, are given by Bernis and Friedman [8] to the equation ( ) The thin film equation with a second-order diffusion term was studied by Bertozzi and Pugh [9].Moreover, for a generalized thin-film equation with period boundary in multidimensional space, Boutat et al. [10] obtained its existence.
For other results, the readers may refer to the papers [11] [12].
In this paper, we study the following initial and boundary value problems for the viscous thin film equation: Formally, if we substitute the second equation into the first one, we can get another form for this question: Our main result is the following theorems.

u L
∈ Ω and 0 ν > .Then there exists at least one pair ( ) 2) For any test function Theorem 2. Let ( ) ∈ Ω and 0 ν > .Then there exists at least one pair ( ) 2) For any test function The following lemmas are needed in the paper: Lemma 1. (Aubin-Lions, see [13]) Let X, B and Y be Banach spaces and assume Lemma 2. (see [14] or [15]) Let V be a real, separable, reflexive Banach space and H is a real, separable, Hilbert space.
In this paper, C is denoted as a positive constant and may change from line to line.The paper is arranged as follows.The existence of solutions to the approximate problem will be proved in Section 2. In Section 3, we will take the limit for small parameters 0 δ → .

Approximate Problem
For any 0 1 δ < < , we consider the following approximate problem.In order to apply existence theory better, we transform (1) into a system: There exists at least one solution u δ to (3) satisfying 1) 2) For any test function Proof.We apply the Galerkin method to prove this Lemma and so we choose that the eigenfunctions are orthogonal in the H 1 and L 2 spaces.We use ( ) , ⋅ ⋅ to denote the scalar product in L 2 space and we can normalize i φ such that ( ) Besides, we can choose 1 0 λ = and 1 1 φ = .
For any positive integer M, we define . Now we consider the following ordinary differential equations system: , which yields an initial value problem for the ordinary differential equations: ∑ is positive definite.In order to get the global solvability, we need establish more energy estimates.Multiply (4) by M w δ to get ( ) Taking M t u δ as the test function in (5), we have Thus, we have ( ) Therefore, for any 0 t T < ≤ , it has ( ) ( ) 4) with 1 j = , we can apply Poincaré's inequality to obtain the following estimates: .
where the last estimate is from Lemma 1.By ( 13)-( 17), we can perform the limit M → ∞ in a standard fashion and the strong convergence in

The Limit 0 δ →
In the section, we will perform the limit 0 δ → to the solutions from Lemma 3.
For the purpose of the existence, we need establish some uniform estimates independent of δ .Thus, we define a convex function By applying this function, we can get the following estimates.
Lemma 4.There exist some constants C independent of δ such that 1) Thus, it yields the results 1 -3.We can prove 4 and 5 from (8).By choosing w δ as a test function in the second equation of (3), we get We have completed the proof of this lemma.
On the other hand, by integrating by parts, it implies give the existence of strong solutions.On the another hand, the viscous term affects the regularity of solutions and we have shown that ( ) We can expect that we can show that the existence results would be true with some conditions in high-dimensional space.
In recent years, the research of nonlinear fourth-order degenerate parabolic equations has become an interesting topic.The typical examples include the Cahn-Hilliard equation and the thin film equation.The Cahn-Hilliard equation can describe the evolution of a conserved concentration field during phase

3
as the eigenfunctions of the Laplace operator with Neumann boundary value conditions such that ixx i i φ λ φ − = .Moreover, we can suppose Through this paper, two forms of a viscous thin film equation are studied (see Y. Qiu, B. Liang DOI: 10.4236/jamp.2018.6101782125 Journal of Applied Mathematics and Physics the Equations (1) and (2)) and we give the corresponding existence theorems of weak solutions (see Theorem 1 and Theorem 2).For any test function φ , we have proved that the weak solutions satisfy the equalities: Since the thin film equation is a degenerate parabolic equation, it is hard to