Unveiling Crystalline Order from Glassy Behavior of Charged Rods at Very Low Salt Concentrations

H Anop and LD Compare and F Nallet and A Giacometti and E Grelet, PHYSICAL REVIEW LETTERS, 134, 118101 (2025).

DOI: 10.1103/PhysRevLett.134.118101

Charged colloids can form ordered structures like Wigner crystals or glasses at very low concentrations due to long-range electrostatic repulsions. Here, we combine small-angle x-ray scattering (SAXS) and optical experiments with simulations to investigate the phase behavior of charged rodlike colloids across a wide range of salt concentrations and packing fractions. At ultralow ionic strength and packing fractions, we reveal both experimentally and numerically a direct transition from a nematic to a crystalline smectic-B phase, previously identified as a glass state. This transition, bypassing the smectic-A intermediate phase, results from minimizing Coulomb repulsion and maximizing entropic gains due to fluctuations in the crystalline structure. This demonstrates how long-range electrostatic repulsion significantly alters the phase behavior of rod-shaped particles and highlights its key role in driving the self-organization of anisotropic particles.

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