Pattern formation in Laplacian growth: Theory

A first-principles statistical theory is constructed for the evolution of two dimensional interfaces in Laplacian fields. The aim is to predict the pattern that the growth evolves into, whether it becomes fractal and if so the characteristics of the fractal pattern. Using a time dependent map the growing region is conformally mapped onto the unit disk and the problem is converted to the dynamics of a many-body system. The evolution is argued to be Hamiltonian, and the Hamiltonian is shown to be the conjugate function of the real potential field. Without surface effects the problem is ill-posed, but the Hamiltonian structure of the dynamics allows introduction of surface effects as a repulsive potential between the particles and the interface. This further leads to a field representation of the problem, where the field’s vacuum harbours the zeros and the poles of the conformal map as particles and antiparticles. These can be excited from the vacuum either by fluctuations or by surface effects. Creation and annihilation of particles is shown to be consistent with the formalism and lead to tip-splitting and side-branching. The Hamiltonian further allows to make use of statistical mechanical tools to analyse the statistics of the many-body system. I outline the way to convert the distribution of the particles into the morphology of the interface. In particular, I relate the particles statistics to both the distributions of the curvature and the growth probability along the physical interface. If the pattern turns fractal the latter distribution gives rise to a multifractal spectrum, which can be explicitly calculated for a given particles distribution. A ‘dilute boundary layer approximation’ is discussed, which allows explicit calculations and shows emergence of an algebraically long tail in the curvature distribution which points to the onset of fractality in the evoloving pattern. CNLS Newsletter 112, LALP-95-012, April 1995