A continuum model for non-dense growth

We present a continuum model for diffusion-limited non-dense growth. Our ap proach leads to a set of two coupled partial differential equations which d escribe the time evolution of the (spherically) averaged aggregation densit y and concentration of growth units in the liquid phase. For time-independe nt parameters the solution of the equations yields a constant (non-fractal) aggregation density. The model gives a phenomenological description of non -fractal unstable growth, e.g. non-fractal spherulitic growth on a macrosco pic scale in terms of a minimal number of parameters and can be used in com bination with experimental data, such as the front velocity and the width o f the growth front, for both a qualitative and quantitative interpretation of the growth process. The analytical solution of the equations in the diff usion-limited regime leads to simple relations involving the aggregation de nsity and the velocity and width of the growth front. This allows for an ea sy quantitative analysis of experimental data.