Fluidization induced by Magnetic Interactions in Confined Active Matter

Abstract

We investigate magnetic active matter in confined geometries using both experiments with magnetic toy robots Hexbugs and simulations of elongated magnetic active Brownian particles in circular domains. Standard active particles tend to accumulate at boundaries, forming clusters even at relatively low densities. In the presence of magnetic interactions, we provide evidence for a fluidization effect that inhibits clustering and shifts its onset to higher packing fractions. Moreover, magnetic dipolar interactions give rise to novel collective behaviors, such as train-like formations, rotating pairs, and particle vortices.

Figures (5)

• Figure 1: Experimental setup: (a,d) 3D models of the Hexbugs and the magneto-Hexbug. The shell of the magnetic bug is taller to accommodate two or more small neodymium magnets. (b,e) Images of the Hexbug and the magneto-Hexbug showing their shells and the white marker used in the tracking process. (c,f) Temporal profiles of the spped for magnetic and non-magnetic particles.
• Figure 2: Experimental results: (a–c) Experimental frames showing the final configurations for $N = 6, 15,$ and $30$ Hexbugs. (d–f) Experimental frames showing the final configurations for $N = 6, 15,$ and $30$ magneto-Hexbugs. (g) Plot of $\langle \hat{r} \cdot \hat{n} \rangle$, where $\hat{r}$ is the unit vector pointing from the arena center to the particle’s center of mass, and $\hat{n}$ is the unit vector of the particle’s orientation. (h) Plot of $\langle |v \cdot \hat{t}| \rangle / v_0$, where $\hat{t}$ is the unit vector perpendicular to $\hat{r}$ (i.e., locally tangent to the boundary), $v$ is the particle velocity and $v_0$ is the mean velocity measured at the lowest density. Blue curve: experiments with Hexbugs; red curve: experiments with magneto-Hexbugs. Green and red boundaries in (a-f) are used to highlight the final configuration, without or with boundary-clusters respectively.
• Figure 3: Typical rotating structures (blue particles) observed in experiments resulting from dipole-dipole magnetic interactions: (a) a rotating pair and (b) a three-particle vortex.
• Figure 4: Simulation results: (a–c) Simulation frames showing the final configurations for $N = 6, 15$ and $30$ particles. (d–f) Simulation frames showing the final configurations for $N = 6, 15$ and $30$ magnetic particles. In all cases (a-f), each hexbug is represented by its $5$ centers of force, depicted as coloured discs (gray and black for the non-magnetic, red and blue for the magnetic ones); the head of a hexbug is the only disc which is fully visible (black for the non-magnetic, blue for the magnetic ones). (g) Plot of $\langle \hat{r} \cdot \hat{n} \rangle$, where $\hat{r}$ is the unit vector pointing from the arena center to the particle’s center of mass, and $\hat{n}$ is the unit vector of the particle’s orientation. (h) Plot of $\langle |v \cdot \hat{t}| \rangle / v_0$, where $\hat{t}$ is the unit vector perpendicular to $\hat{r}$ (i.e., locally tangent to the boundary), $v$ is the particle velocity and $v_0$ is the mean velocity measured at the lowest density. Blue curve: simulations with non-magnetic particles; red curve: simulations with magnetic particles.
• Figure 5: Comparison between experiments and simulations for magneto-Hexbugs. (a,b) Velocity distributions for $N = 6, 15,$ and $30$ bugs in experiments (a) and simulations (b). Velocities are normalized by $v_0$, defined as the typical body length of a Hexbug divided by the characteristic time it takes to travel this distance. (c–e) Probability distributions of the relative angle $\gamma$ (modulo $\pi$), shown in green for experiments and violet for simulations. The angle $\gamma$ is defined as the difference between the particle orientation in the lab frame and the angle of the vector from the arena center to the particle’s center of mass. The cases with N = 6, 15 and 30 are shown.