# The Soap Bubble Problem

A soap film bounded by a wire frame has the shape determined by the minimization of its internal energy. The shape of the film is described by the height u(x,y) over a given domain. The height is fixed at the domain boundary, by the height of the wire frame g If the film is homogeneous, minimizing the internal energy is equivalent to minimizing its area, the integral of s(u) = sqrt(1+|grad(u)|^2)
This in its turn is equivalent to finding u such that

• -div( grad(u)/s(u) ) = 0, inside the domain
• u(x,y) = g(x,y), on the domain boundary

In this example you look at a wire frame with two circular components. The inner frame is at height 0 while the outer frame is given by g(x,y) = x^2.

The wire frame for a soap bubble. The computational domain

### Geometry

First you draw the domain which is an annulus (a two-dimensional doughnut), i.e. the region between two concentric circles:

An annulus obtained by subtracting the small disk from the large one. This is expressed by the set formula C1-C2

### Boundary conditions

Next you set the boundary conditions, u = 0 on the inner frame, and u(x,y) = x^2 on the outer frame:

Boundary conditions dialog box.

### PDE Coefficients

Setting the equation coefficients amounts to typing in MATLAB statements. These can be constants, expressions containing the space variables x and , y , the unknown u , its partial derivatives ux and uy , or even user-defined functions. The general form of the time-independent, elliptic, scalar PDE is
-div( c*grad(u) ) + a*u = f

In the soap bubble problem, you have a = f = 0 and c = 1./sqrt(1+ux.^2+uy.^2).

### Mesh generation

The mesh generator initializes a coarse mesh. Further refinements of the mesh are necessary to obtain a reasonable solution.

A triangularization of the domain

Mesh generation is done automatically, but it you can also steer it by setting parameters. In this example the default mesh is initialized and then uniformly refined. The fine mesh contains 590 nodes and 1096 triangles.

### Solvers

The Finite Element Method discretization is performed. Linear problems are solved by MATLAB's internal solver. However, the soap bubble equation is nonlinear, and you have to tell that to the solver. After setting the nonlinear option among the Solve parameters, the equations are discretized and solved. A damped Gauss-Newton iterative is employed. Set the Jacobian option to Full. While the solver works, you can see the reduction of the residual in your workspace window.

The shape of the soap film