Persistence-Driven Void Formation in Active-Passive Mixtures

Abstract

It is well established that dilute active dopants can melt an arrested amorphous solid by enhancing cage breaking and accelerating structural relaxation. Yet it remains unclear whether increasing persistence simply amplifies this effective melting or instead reorganizes the fluidization mechanism itself. Here we show that, in a minimal active-passive mixture, increasing persistence drives a crossover from homogeneous fluidization to a localized mechanical instability, demonstrating that sustained active forcing restructures relaxation in space rather than merely strengthening it. Persistent dopants accumulate stress and nucleate voids as their mechanically perturbed regions overlap. In this regime, rearrangements localize at void boundaries, and active and passive particles exhibit comparable mobility, producing dynamics reminiscent of crowd mosh pits. Persistence therefore reorganizes fluidization through stress accumulation and confinement, revealing a distinct nonequilibrium localization mechanism in disordered solids.

Figures (13)

• Figure 1: Characterization of the system at fixed active fraction $\phi_a = 10^{-1}$ as a function of the persistence length $l_p$. (a) Mean rearrangement fraction $\langle \phi_r \rangle$. (b) Rearrangement fluctuations $\chi_r = \langle \phi_r^2 \rangle - \langle \phi_r \rangle^2$. The first peak marks the arrest–fluidization crossover; at larger $l_p$, a second peak signals the onset of a dynamically distinct void-forming regime. (c) Mean fraction of undercoordinated particles $\langle \phi_{n<4} \rangle$. A plateau between the two peaks is followed by renewed growth near the second peak, consistent with void nucleation. Shaded bands indicate the standard deviation; solid lines are guides to the eye. Triangle and diamond symbols mark the homogeneous-fluid and void-forming regimes, respectively, and correspond to the representative snapshots. In the snapshots, passive particles are shown in purple and active particles in yellow.
• Figure 2: Phase diagram of the active–passive mixture in the $(l_p,\phi_a)$ plane. Dark blue denotes arrested dynamics, light blue a homogeneous liquid, and purple the void-forming (mosh-pit-like) regime. Boundaries are determined from the rearrangement fluctuations $\chi_r = \langle \phi_r^2 \rangle - \langle \phi_r \rangle^2$. The arrest–fluidization boundary corresponds to the first peak in $\chi_r$, and the homogeneous–void-forming boundary to the second peak. White dashed lines show power-law fits of the form $l_p = C \phi_a^{b}$ to the extracted peak locations. Symbols denote simulation points.
• Figure 3: Characterization of the two fluid regimes at $\phi_a = 10^{-1}$. Passive particles are shown in purple, active particles in yellow. (a) Probability distribution of the coarse-grained rearrangement fraction $P(\bar{\phi}_r)$ for $l_p = 9\times 10^{1}$ (green triangles; homogeneous fluid) and $l_p = 2\times 10^{3}$ (blue diamonds; void-forming regime), computed over a time window $\Delta t = 10^{2}$. Inset: mean per-particle rearrangement rate $\langle r_i \rangle$ as a function of distance from the void center (in units of $L_b/2$, where $L_b$ is the box size). The dashed line marks the void radius; $\langle r_i \rangle$ peaks sharply at the void boundary and is suppressed in the bulk. In the homogeneous fluid, the same analysis referenced to a random point yields an approximately flat profile. (b) Ratio of effective diffusion coefficients $D_{\mathrm{eff}}^{p}/D_{\mathrm{eff}}^{a}$ versus $l_p$ at $\phi_a = 10^{-2}$ (purple circles) and $\phi_a = 9\times 10^{-2}$ (yellow squares).
• Figure 4: Localized emergent chirality. Probability distribution of passive-particle chirality $P(\mathrm{C}_i)$ for three persistence lengths $l_p = 2\times 10^{-1}$ (purple circles), $2\times 10^{0}$ (blue triangles), and $2\times 10^{3}$ (yellow squares). At small persistence, $P(\mathrm{C}_i)$ is centered near zero, indicating no systematic local rotation. At large persistence, $P(\mathrm{C}_i)$ develops two symmetric peaks near $\mathrm{C}_i \simeq \pm 1$, revealing intermittent clockwise and counterclockwise rotations on timescales shorter than $\tau_r$. Time-ordered snapshots (separated by $2\Delta t$) illustrate a representative event: a passive particle (pink) undergoes a transient rotation while dragged by neighboring active particles (yellow); all other particles are shown in black.
• Figure S1: Void-onset persistence length $l_p^\star$ as a function of the active fraction $\phi_a$, determined from the phase boundaries shown in Fig. 2 of the main text. Symbols with error bars denote numerical measurements; error bars reflect statistical uncertainty in determining the onset boundary. The solid line shows the continuum prediction $l_p^\star \propto \phi_a^{-1}$, while the dashed line corresponds to a linear fit to the data. The agreement supports the stress-diffusion scaling $V_{\rm infl} \sim D_\sigma \tau_r$ and the resulting collective overlap criterion.