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This analysis deals with new models and computational methods as well as with novel results on the relative importance of "controlling forces" in macromolecular crystal growth. The attention is focused in particular on microgravity fluid-dynamic aspects and on the case of the simultaneous growth of different seeds. A "kinetic-coefficient-based" volume of fraction method is specifically and carefully developed according to the complex properties and mechanisms of macromolecular protein crystal growth. It is shown that the size and the shape of the growing crystals play a "critical role" in the relative importance of surface effects and in determining the intensity of convection. Convective effects, in turn, are found to impact growth rates, macroscopic structures of precipitates, particle size and morphology as well as the mechanisms driving growth. The face growth rates in particular depend on the complex multicellular structure of the convective field and on associated "pluming phenomena." The relative importance of mass transport in liquid phase and surface attachment kinetics is investigated. The simulations show that it does not behave as a "fixed" parameter and that different crystallization conditions may occur in the protein chamber due to mutual interference of the growing seeds, complex convective effects and the "finite size" of the reactor. [ABSTRACT FROM AUTHOR]

This paper reports experimental results and modelling on the crystallisation processes induced by counter diffusion method of a precipitant agent in a lysozyme protein solution. Comparison between experimental observations and numerical simulations in the presence of convection and sedimentation and without them (suppressed using gel) provides a validation of the model. Different values of the initial protein concentration are used, in order to investigate the effects of supersaturation conditions on the process, and in particular on nucleation. The model and the experimental approach may represent a useful methodology for the determination of the parameters and conditions that may lead to protein crystallisation. A Mach-Zehnder interferometer is used to monitor the transport dynamics in situ in the fluid phase by observing the compositional field. The effect of the solute transport gives rise to a "nucleation front" that propagates inside the protein solution. The crystal formation, caused by progressing of the front, results in a modulation in time and in space (similar to Liesegang patterns), due to the non-linear interplay among transport, crystal nucleation and growth. Both experimental observation and numerical modelling show spatial and size distributions of crystals that demonstrate comparable evidences of the phenomena. [ABSTRACT FROM AUTHOR]

Solidification of gallium (Pr=0.02) in liquid bridges in zero-gravity conditions is investigated by numerical solutions of the three-dimensional and time-dependent flow-field equations. A single region (continuum) formulation based on the enthalpy method is adopted to model the phase-change problem. This paper analyzes the influence of the azimuthally asymmetric and steady first bifurcation of the Marangoni flow on the shape of the solid/melt interface during the crystal growth process. The numerical results show that this interface is distorted in the azimuthal direction. The distortion is related to the sinusoidal three-dimensional temperature disturbances due to the instability of the Marangoni flow. The three-dimensional flow field organization, related to the wave number, changes during the solidification process; this behavior is explained according to the variation of the aspect ratio of the solidifying liquid bridge. A correlation law is found for the azimuthal wave number of the instability as function of the melt zone aspect ratio. [Copyright &y& Elsevier]