Equilibrium interfacial free energies and Turnbull coefficient for bcc crystallizing colloidal charged sphere suspensions

TL;DR

This study measures the interfacial free energy and Turnbull coefficient for bcc crystallizing colloidal suspensions, revealing larger values than hard sphere models and a decrease with polydispersity, aligning with theoretical predictions.

Contribution

First experimental determination of the Turnbull coefficient for bcc colloidal systems and analysis of interfacial energies across different polydispersities.

Findings

Interfacial free energy exceeds hard sphere reference values.

Interfacial free energy and entropy decrease with polydispersity.

Turnbull coefficient for bcc systems is approximately 0.3.

Abstract

We extend previous analysis of data for the melt-nucleus interfacial free energy, $\gamma$, gained from optical experiments on suspensions of charged colloidal spheres, which crystallize with body centred cubic (bcc) crystal structures. Compiling data from five pure species with different polydispersities and one binary mixture, we find the equilibrium melt-crystal interfacial energy to be considerably larger than the hard sphere reference value. Both this quantity and the entropy of freezing decrease with increasing polydispersity. Moreover, we give a first experimental determination of the Turnbull coefficient for a bcc crystallizing material. The observed value $C_{T, bcc} \approx 0.3$ agrees well with theoretical expectations for bcc systems with short to medium ranged interactions.