Inverse Solidification Induced by Active Janus Particles

Abstract

Crystals melt when thermal excitations or the concentration of defects in the lattice is sufficiently high. Upon melting, the crystalline long-range order vanishes, turning the solid to a fluid. In contrast to this classical scenario of solid melting, here a counter-intuitive behavior of the occurrence of crystalline long-range order in an initially disordered matrix is demonstrated. This unusual solidification is demonstrated in a system of passive colloidal particles accommodating chemically active defects-photocatalytic Janus particles. The observed crystallization occurs when the amount of active-defectinduced fluctuations (which is the measure of the effective temperature) reaches critical value. The driving mechanism behind this unusual behavior is purely internal and resembles a blast-induced solidification. Here, the role of "internal micro-blasts" is played by the photochemical activity of defects residing in the colloidal matrix. The defect-induced solidification occurs under non-equilibrium conditions: the resulting solid exists as long as a constant supply of energy in the form of ion flow is provided by the catalytic photochemical reaction at the surface of active Janus particle defects. The findings could be useful for the understanding of the phase transitions of matter under extreme conditions far from thermodynamic equilibrium.

Impact of surface charge on the motion of light-activated Janus micromotors

“…The mechanism of motion for Ag/AgCl/PS micromotors was previously discussed in the literature [ 33 , 34 , 38 ], wherein it was shown that the Janus particles are driven by the decomposition of AgCl, which is supported by plasmonic excitations in Ag clusters. The reaction triggers the simultaneous release of protons and Cl ions.…”

Phase Diagram of Active Brownian Spheres: Crystallization and the Metastability of Motility-Induced Phase Separation

“…Moreover, additional examination of active phase behavior in 3D may prove insightful for further understanding the role of dimensionality in the rich phase behavior (such as "bubbly liquids" [31,88,89]) reported in 2D. Finally, while active freezing has primarily been experimentally interrogated in 2D [90][91][92], we hope that our study will aid in guiding ongoing efforts [93] to realize 3D active crystals.…”

Which interactions dominate in active colloids?

“…More recently, in ref. [138] visible-light-driven Ag/AgCl− based Janus particles have been fixed to a glass substrate such that they do not move but create a phoretic field which acts on the particles in their vicinity. When embedding the Janus particles, here called "active defects", in a "matrix" of passive colloids at moderate density (where they form an amorphous state with a liquid-like structure), they repel the SiO 2 colloids and push them together which leads to crystallization.…”