A series of microgravity experiments was carried out over the last several decades on the International Space Station (ISS) to study phase transitions in colloids and colloid-polymer mixtures without masking gravity effects, such as buoyancy-driven convection and particle sedimentation. While video microscopy and small-angle light scattering were used to record phase transformations, only light scattering measurements have been processed so far to analyze structure evolution in ISS experiments. One important goal of this project is to analyze a rich body of microscopy images available in the NASA Physical Sciences Informatics system to gain better understanding of large-scale structuring in a colloid undergoing phase transition. Results of the image analysis are used for development of phase-field models for structuring in colloids, consistent with ISS experiments. These models will provide a benchmark for comparison and evaluation of existing theories for phase transformations that were developed based on terrestrial experiments influenced by detrimental gravity effects. Effective computational modeling will help improve Earth-based production of advanced materials.

The developed model is based on a coupled diffusion problem for the solid and liquid phase, combined with the consistent boundary conditions involving osmotic pressure balance, including interfacial tension. The motion of the solidification front is modeled based on the Wilson-Frenkel law. The single nuclei model is validated by considering various asymptotic limits, as well as by comparing the results to ones available in the literature. The multiple nuclei model allows, for the first time, for the careful computational investigation of the interaction between evolving nuclei.

This projects is sponsored by NASA grant No. NNX16AQ79G.

Recent Publications

M. A. Lam, B. Khusid, L. Kondic, and W. V. Meyer, Role of diffusion in crystallization of hard-sphere colloids, Phys. Rev. E, 104, 054607, (2021)

Vital for a variety of industries, colloids also serve as an excellent model to probe phase transitions at the individual particle level. Despite extensive studies, origins of the glass transition in hard-sphere colloids discovered about 30 y ago remain elusive. Results of our numerical simulations and asymptotic analysis suggest that cessation of long-time particle diffusivity does not suppress crystallization of a metastable liquid-phase hard-sphere colloid. Once a crystallite forms, its growth is then controlled by the particle diffusion in the depletion zone surrounding the crystallite. Using simulations, we evaluate the solid-liquid interface mobility from data on colloidal crystallization in terrestrial and microgravity experiments and demonstrate that there is no drastic difference between the respective mobility values. The insight into the effect of vanishing particle mobility and particle sedimentation on crystallization of colloids will help engineer colloidal materials with controllable structure.