Morphology, Polarization Patterns, Compression, and Entropy Production in Phase-Separating Active Dumbbell Systems

TL;DR

This study investigates how polar patterns, topological defects, and compression interact during MIPS in a 2D active dumbbell system. By comparing soft (LJ-like) and rigid (Mie) inter-dumbbell interactions under varying density $\rho$ and activity $Pe$, the authors reveal that softer interactions promote bead sliding and stronger compression, leading to blurred hexatic order and extended polarization across patches. They show that isolated clusters host inward-pointing defects that drive domain compression and generate nontrivial density profiles, with compression amplified in softer systems. Entropy production analyses uncover thermodynamic signatures of defects: grain-boundary regions exhibit elevated irreversibility, while aster defects yield flat and spiral defects yield increasing entropy profiles, offering a thermodynamic route to defect identification. Overall, the results clarify how interaction strength and defect-induced compression shape cluster evolution in polar active matter and link particle-based models to recent continuum polar field theories.

Abstract

Polar patterns and topological defects are ubiquitous in active matter. In this paper, we study a paradigmatic polar active dumbbell system through numerical simulations, to clarify how polar patterns and defects emerge and shape evolution. We focus on the interplay between these patterns and morphology, domain growth, irreversibility, and compressibility, tuned by dumbbell rigidity and interaction strength. Our results show that, when separated through MIPS, dumbbells with softer interactions can slide one relative to each other and compress more easily, producing blurred hexatic patterns, polarization patterns extended across entire hexatically varied domains, and stronger compression effects. Analysis of isolated domains reveals the consistent presence of inward-pointing topological defects that drive cluster compression and generate non-trivial density profiles, whose magnitude and extension are ruled by the rigidity of the pairwise potential. Investigation of entropy production reveals instead that clusters hosting an aster (spiral) defect are characterized by a flat (increasing) entropy profile mirroring the underlying polarization structure, thus suggesting an alternative avenue to distinguish topological defects on thermodynamical grounds. Overall, our study highlights how interaction strength and defect-compression interplay affect cluster evolution in particle-based active models, and also provides connections with recent studies of continuum polar active field models.

Figures (10)

• Figure 1: Sketch of typical active dumbbell systems. Each dumbbell is composed of two beads, tail and head, connected together. Self-propulsion (arrows) acts on each bead along the axis of its dumbbell (dashed line) with constant magnitude $F_a$ in the tail--head direction (Equation \ref{['eq:act_force']}). All beads interact through a shifted and truncated pairwise potential (Equation \ref{['eq:pot_n']}) of Lennard--Jones (a) or Mie (b) kind. In the first case, head and tail are kept together through a FENE force (Equation \ref{['eq:fene']}, green zig-zag line), thus their distance can slightly oscillate. In the second, a RATTLE scheme, which keeps head--tail distance fixed, is instead implemented (thick green line).
• Figure 2: Growth regimes and hexatic order. (a,b). Characteristic domain size $R(t)$ for $Pe=100,~200$ in the Mie and LJ configurations, respectively. (c--p). Snapshots of an enlarged area of the system in the Mie at $Pe=100$ (f--n), LJ at $Pe=200$ (g--j), and LJ at $Pe=100$ (k--n) configurations at $\rho\sim 0.4$. Beads are colored according to their $\psi_{6i}$ values (right bar).
• Figure 3: The local polarization density field. (a--c,e--g). Local polarization density field $\vec{p}_i$ with $\sigma_{cg}=10\sigma_d$ in an enlarged area of the system at $Pe=100$ in the Mie and at $Pe=200$ in the LJ configuration at $\rho\sim 0.4$, respectively. Extraction times and enlarged areas are as in Figure \ref{['fig:fig2']} for the three respective larger times. The field is represented as arrows colored according to their magnitude $p_i=|\vec{p}_i|$ (bottom bar). Arrows such that $p_i<0.2$ are removed. Backgrounds report corresponding hexatic snapshots from Figure \ref{['fig:fig2']}. (d,h). Distribution of the field magnitude $p_i$ in the Mie and LJ configurations, respectively. The inset in (d) reports the distributions obtained at $Pe=10$ with axes as in the main panel. All curves are generated at increasing times (bottom legend), collecting data from $10$ independent simulation runs.
• Figure 4: The local density field. (a--c,e--g). Local density field $\phi_i$ with $\sigma_{cg}=10\sigma_d$ in an enlarged area of the system at $Pe=100$ in the Mie and at $Pe=200$ in the LJ configuration at $\rho\sim 0.4$, respectively. Extraction times and enlarged areas are as in Figure \ref{['fig:fig3']}. The field is colored according to the bottom bar. (d,h). Distribution of the local density field $\phi_i$ in the Mie and LJ configurations, respectively. Curves are generated at increasing times (bottom legend), collecting data from $10$ independent simulation runs. Vertical lines highlight the location of $\phi_{low}$ and $\phi_{high}$ at the largest time sampled.
• Figure 5: Topological defects in isolated clusters. (a--f). Local polarization density magnitude field $p_i$ with $\sigma_{cg}=10\sigma_d$ in an enlarged area of the system at $Pe=100$ in the Mie and at $Pe=200$ in the LJ configurations at $\rho\sim 0.4$, respectively. Extraction times and enlarged areas are as in Figure \ref{['fig:fig3']}. The field is colored according to the right bar. (g--i,j--m). Representative instances of topological defects in the LJ and Mie configurations extracted in (a--f) from regions delimited by rectangles with matching colors. Overall representation is as in Figure \ref{['fig:fig3']}. (n). Schematic depiction of the nucleation mechanism driving the formation of topological defects.