Non-linear behaviour of Lysozyme crystallisation

A crystallisation process of biological macromolecules in gel-matrix was studied by Mach-Zehnder interferometer and the results were compared with a numerical model of periodic precipitation. This investigation is focused on the coupling between transport phenomena, nucleation and crystal growth, which may lead to non-linear behaviour. In general, transport phenomena involve double diffusion of the two main solutes (protein and precipitant agent) and buoyant convection that may arise due to density gradients in the liquid. Both transport conditions were addressed. In absence of convection, in pure diffusive regime, non-uniform crystal distributions have been observed. The results can be described in terms of a modulation in space and in time of supersaturation that leads to a nucleation front that propagates inside the protein solution. The experimental data are in good agreement with numerical simulations. When convection is not negligible, it leads to a deformation of the diffusive solutal distribution, characterised by a periodic wave-like pattern observed by interferometry. The observed instabilities are driven by double diffusion and agree with a stability model of double diffusive systems.