Abstract.The paper is concerned with the free boundary problem of a semi-infinite body with an arbitrarily prescribed initial condition and an arbitrarily prescribed boundary condition at its face. An analytically exact solution of the problem is established, which is expressed in terms of some functions and polynomials of the similarity variable x/tin and time t. Convergence of the series solution is considered and proved. Hence the solution also serves as an existence proof. Some special initial and boundary conditions are discussed, which include the Neumann problem and the one-phase problem as special cases.
Introduction.Diffusive processes with a change of phase of the materials occur in many scientific and engineering problems. They are known as Stefan problems or free boundary problems. Many examples of these problems can be given, e.g. the melting or freezing of an ice-water combination, the crystallization of a binary alloy or dissolution of a gas bubble in a liquid. To find the solutions to this class of problems has been the subject of investigations by many researchers. Because of the presence of a moving boundary between the two phases, the problem is nonlinear. Various mathematical methods and techniques have been used to study free boundary problems. They have been discussed and summarized in several books [1][2][3][4][5] and many survey papers [6][7][8][9].Free boundary problems have been studied since the nineteenth century by Lame and Clapeyron in 1831, Neumann in the 1860s and Stefan in 1889. However, the only known exact solutions are those of Neumann and some of their extensions and variations. These solutions are all expressible in a single similarity variable x/t1'2. This is to say that the partial differential equations of the problem are reducible to a set of ordinary differential equations. In the case of classical free boundary problems, the reductions are possible only when the body is semi-infinite and the boundary and initial conditions are of certain special forms. Neumann considered a semi-infinite body of a constant initial temperature which is suddenly in contact with a different temperature at its face. The exact solutions of the temperature of both phases are then found in the form of the similarity variable x/t1/2. And the solution of the interface location is proportional to t1/2. A generalization of the problem to cover arbitrary initial and boundary conditions is nontrivial, since the reduction to ordinary differential equations is no longer possible. No exact solutions have yet been found for the Neumann problem with arbitrary initial and boundary conditions.It is the purpose of this paper to study the Neumann problem with arbitrary initial and boundary conditions, i.e., the free boundary problem of a semi-infinite body with an