Crystallization can be described as a physical phenomenon in which disordered molecules in a gas or liquid phase are converted into a highly ordered solid crystal via two stages—that is, nucleation and growth.
Crystallization is a very crucial process in natural sciences and materials because it takes place in an extensive range of materials such as organic compounds, metals, and biological molecules. Hence, crystallization must be thoroughly understood.
To analyze crystallization, colloids containing hard spheres suspended in a liquid are usually employed as a model system. For several years, a huge variation of up to 10 orders of magnitude has been noted between the experimentally measured and computationally simulated nucleation rates of hard-sphere colloids.
This variation has normally been elucidated by the simulations without taking hydrodynamic interactions—that is, the interactions that occur between solvent molecules—into consideration. Scientists at The University of Tokyo’s Institute of Industrial Science, the Sapienza University, and the University of Oxford have now collaborated to additionally investigate this explanation for the variation existing between calculated and real nucleation rates.
Initially, the researchers created a hard-sphere colloidal model that can consistently replicate the experimental thermodynamic behavior of true hard-sphere systems. Subsequently, they simulated the crystallization of the model system, both considering and ignoring the hydrodynamic interactions to explain the impact of those interactions on crystallization behavior.
We initially designed a simulation model that accurately reproduced the real thermodynamics of hard-sphere systems. This confirmed the reliability and suitability of the model for use in further simulations.
Michio Tateno, Study Lead Author, Institute of Industrial Science, The University of Tokyo
The simulation outcomes, achieved using the developed model by considering and ignoring the hydrodynamic interactions, showed that these interactions did not have an impact on the nucleation rate, as opposed to the existing agreement.
The nucleation rate plots against the proportion of hard spheres in the system were found to be identical for calculations, both with and without hydrodynamic interactions. These plots also correlated with the outcomes outlined by another team of researchers.
We performed calculations using the developed model with and without considering hydrodynamic interactions. The calculated rates of crystal nucleation were similar in both cases, which led us to conclude that hydrodynamic interactions do not explain the hugely different nucleation rates obtained experimentally and theoretically.
Hajime Tanaka, Study Senior Author, Institute of Industrial Science, The University of Tokyo
The findings of the research team have clearly demonstrated that hydrodynamic interactions are not the source of the huge variation that exists between simulated and experimental nucleation rates. While the study results provide a better insight into the crystallization behavior, they do not explain the origin of this huge variation.
The article titled “Influence of hydrodynamic interactions on colloidal crystallization” has been published in the Physical Review Letters journal.