Inertial and hydrodynamic effects on the liquid-hexatic transition of active colloids

Abstract

We study numerically the role of inertia and hydrodynamics in the liquid-hexatic transition of active colloids at intermediate activity, where motility induced phase separation (MIPS) does not occur. We show that in the case of active Brownian particles (ABP), the critical density of the transition decreases upon increasing the particle's mass, suggesting that inertia has an effect of promoting hexatic ordering, contrary to self-propulsion which has the opposite effect in the activity regime considered. Active hydrodynamic particles (AHP), instead, undergo the liquid-hexatic transition at higher values of packing fraction φ than the corresponding ABP, suggesting that hydrodynamics have the net effect of disordering the system. At increasing densities, close to the hexatic-liquid transition, we found in the case of AHP the appearance of self-sustained organized motion with clusters of particles moving coherently.

Role of rotational inertia for collective phenomena in active matter

“…Similarly, inertial effects reduce the accumulation near boundaries or obstacles typical of active particles [53][54][55] and hinder the crystallization [56]. In addition, they promote hexatic ordering [57] in homogeneous * lorenzo.caprini@gssi.it phases and, in general, reduce the spatial velocity correlations [58] characterizing dense active systems [48,59,60] both in liquids [61,62] or solid states [58,63]. From the theoretical side, the main conclusion is that translational inertia mainly reduces the typical macroscopic properties characterizing active particles, apparently leading to "less active" systems more similar to their passive counterparts.…”

Active particles driven by competing spatially dependent self-propulsion and external force

“…In spite of its versatility and wide applicability [50][51][52][53][54][55], the ABP model is harder to use to make theoretical progress than its "sister/brother" [56], the active Ornstein-Uhlenbeck particle (AOUP) model [57][58][59][60][61][62][63] . In the former, the modulus of the active force is fixed and its orientation diffuses, while in the latter each component of the propulsion force evolves according to an Ornstein-Uhlenbeck process.…”

Entropons as collective excitations in active solids

“…These active crystals show fascinating phenomena uncommon for equilibrium solids [15][16][17][18] ranging from "traveling" crystals [19][20][21], intrinsic velocity correlations [22][23][24][25] and spontaneous velocity alignment [26,27] to collective rotations [16,28]. Activity also shifts the equilibrium freezing transition significantly [29][30][31][32] and affects the nature of the two-dimensional melting transition [30,[33][34][35][36].…”