Clogging, dynamics, and reentrant fluid for active matter on periodic substrates

Active Particles Knead Three-Dimensional Gels into Porous Structures

Colloidal gels are prime examples of functional materials exhibiting disordered, amorphous, yet metastable forms. They maintain stability through short-range attractive forces and their material properties are tunable by external forces. Combining persistent homology analyses and simulations of three-dimensional colloidal gels doped with active particles, we reveal novel dynamically evolving structures of colloidal gels. Specifically, we show that the local injection of energy by active dopants can lead to highly porous, yet compact gel structures that can significantly affect the transport of active particles within the modified colloidal gel. We further show how passive interfaces play a topologically significant role in interacting with active particles in 3D. The results open the door to an unexplored prospect of forming a wide variety of compact but highly heterogeneous and percolated porous media through active doping of 3D passive matter, with diverse implications in designing new functional materials for active ground remediation.

Collective motion of binary chiral particle mixtures with environmental complex noise

The strategies for demixing and sorting mixed-chirality particles are crucial in the biochemical and pharmaceutical industries. However, whether chiral mixed particles can effectively separate in more complex spatial environments remains unresolved. In this paper, we explore the collective motion of binary chiral particle mixtures with environmental complex noise in the binary chiral Vicsek model (BCVM). We discover that the noisy region ratio, λ, significantly influences the separation behavior and spatial distribution of binary mixtures, unveiling system states not observed in uniform environments. Additionally, varying the chirality of particles reveals four distinct phases in our model. In the Vicsek bands phase (small chirality), an increase in λ can, under certain conditions, promote segregation rather than consistently hindering the demixing process. Conversely, for large chirality, localized dynamics and a homogeneous phase emerge, reducing the impact of λ on separation behavior. Notably, when chirality and activity are comparable, macrodrops and microflock phases appear, with a mixed-segregated state transition occurring at a critical λ_{c}. For λ<λ_{c}, chiral separation occurs with particles confined to the noise-free region. However, when λ>λ_{c}, particles gradually migrate to the noisy region, disrupting the separation. Further, we discuss the effects of multiple factors, including chirality, velocity, noise magnitude, particle number, and system size on λ_{c}. We also identify an optimal interaction radius at which λ_{c} reaches its peak value. Our findings may inspire strategies for achieving spontaneous demixing and spatial migration of mixed-chirality particles in complex environments.

Dynamics of self-propelled particles in vibrated dense granular media

We study a system consisting of a few self-propelled particles (SPPs) placed among a crowd of densely packed granular particles that are vertically vibrated in a two-dimensional circular confinement. Our experiments reveal two important findings. First, an SPP exhibits a fractal renewal process within the dense granular medium, which induces a superdiffusive behavior whose diffusion exponent increases with its aspect ratio. Second, the SPPs eventually reach the boundary and form a moving cluster, which transitions from the moving state to the static state as the number of SPPs is increased. These results suggest a simple and effective method of modulating the fluidity and directionality of granular systems via controlling the shape and the number of SPPs.

Clogging transition and anomalous transport in driven suspensions in a disordered medium

We study computationally the dynamics of forced, Brownian particles through a disordered system. As the concentration of mobile particles and/or fixed obstacles increase, we characterize the different regimes of flow and address how clogging develops. We show that clogging is preceded by a wide region of anomalous transport, characterized by a power law decay of intermittent bursts. We analyze the velocity distribution of the moving particles and show that this abnormal flow region is characterized by a coexistence between mobile and arrested particles, and their relative populations change smoothly as clogging is approached. The comparison of the regimes of anomalous transport and clogging with the corresponding scenarios of particles pushed through a single bottleneck show qualitatively the same trends highlighting the generality of the transport regimes leading to clogging.

Modulating drift dynamics of circle swimmers by periodic potentials

We propose a method to modulate the drifting motion of overdamped circle swimmers in steady fluid flows by means of static sinusoidal potentials. Using Langevin formalism, we study drift velocity as a function of potential strength and wavelength with and without diffusional motion. Drift velocity is essentially quantized without diffusion, but in the presence of noise, the displacement per cycle has a continuous range. As a function of dimensionless potential wave number, domains of damped oscillatory and plateau regimes are observed in the drift velocity diagram. At weak potential and fluid velocity less than powered velocity, there is also a regime where drift velocity exceeds the fluid velocity. Methods based on these results can be used to separate biological and artificial circle swimmers based on their dynamical properties.

Partitioning of active particles into porous media

Passive Brownian particles partition homogeneously between a porous medium and an adjacent fluid reservoir. In contrast, active particles accumulate near boundaries and can therefore preferentially partition into the porous medium. Understanding how active particles interact with and partition into such an environment is important for optimizing particle transport. In this work, both the initial transient and steady behavior as active swimmers partition into a porous medium from a bulk fluid reservoir are investigated. At short times, the particle number density in the porous medium exhibits an oscillatory behavior due to the particles' ballistic motion when time t < τ_R, where τ_R is the reorientation time of the active particles. At longer times, t > L²/D^swim, the particles diffuse from the reservoir into the porous medium, leading to a steady state concentration partitioning. Here, L is the characteristic length scale of the porous medium and D^swim = U₀/d(d - 1), where U₀ is the intrinsic swim speed of the particles,  = U₀τ_R is the particles' run, or persistence, length, and d is the dimension of the reorientation process. An analytical prediction is developed for this partitioning for spherical obstacles connected to a fluid reservoir in both two and three dimensions based on the Smoluchowski equation and a macroscopic mechanical momentum balance. The analytical prediction agrees well with Brownian dynamics simulations.

Active matter shepherding and clustering in inhomogeneous environments

We consider a mixture of active and passive run-and-tumble disks in an inhomogeneous environment where only half of the sample contains quenched disorder or pinning. The disks are initialized in a fully mixed state of uniform density. We identify several distinct dynamical phases as a function of motor force and pinning density. At high pinning densities and high motor forces, there is a two-step process initiated by a rapid accumulation of both active and passive disks in the pinned region, which produces a large density gradient in the system. This is followed by a slower species phase separation process where the inactive disks are shepherded by the active disks into the pin-free region, forming a nonclustered fluid and producing a more uniform density with species phase separation. For higher pinning densities and low motor forces, the dynamics becomes very slow and the system maintains a strong density gradient. For weaker pinning and large motor forces, a floating clustered state appears, and the time-averaged density of the system is uniform. We illustrate the appearance of these phases in a dynamic phase diagram.