Tuning higher order structure in colloidal fluids

XY Wu and K Skipper and YS Yang and FJ Moore and FC Meldrum and CP Royall, SOFT MATTER, 21 (2025).

DOI: 10.1039/d4sm00889h

Colloidal particles self assemble into a wide range of structures under external AC electric fields due to induced dipolar interactions Yethiraj and Van Blaaderen, Nature, 2003, 421, 513. As a result of these dipolar interactions, at low volume fraction the system is modulated between a hard-sphere like state (in the case of zero applied field) and a "string fluid" upon application of the field. Using both particle-resolved experiments and computer simulations, we investigate the emergence of the string fluid with a variety of structural measures including two-body and higher-order correlations. We probe the higher- order structure using three-body spatial correlation functions and a many-body approach based on minimum energy clusters of a dipolar- Lennard-Jones system. The latter constitutes a series of geometrically distinct minimum energy clusters upon increasing the strength of the dipolar interaction, which are echoed in the higher-order structure of the colloidal fluids we study here. We find good agreement between experiment and simulation at the two-body level. Higher-order correlations exhibit reasonable agreement between experiment and simulation, again with more discrepancy at higher field strength for three-body correlation functions. At higher field strength, the cluster population in our experiments and simulations is dominated by the minimum energy clusters for all sizes 8 <= m <= 12.

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