Emergent ultra–long-range interactions between active particles in hybrid active–inactive systems

Abstract

Particle-particle interactions determine the state of a system. Control over the range of such interactions as well as their magnitude has been an active area of research for decades due to the fundamental challenges it poses in science and technology. Very recently, effective interactions between active particles have gathered much attention as they can lead to out-of-equilibrium cooperative states such as flocking. Inspired by nature, where active living cells coexist with lifeless objects and structures, here we study the effective interactions that appear in systems composed of active and passive mixtures of colloids. Our systems are 2D colloidal monolayers composed primarily of passive (inactive) colloids, and a very small fraction of active (spinning) ferromagnetic colloids. We find an emergent ultra-long-range attractive interaction induced by the activity of the spinning particles and mediated by the elasticity of the passive medium. Interestingly, the appearance of such interaction depends on the spinning protocol and has a minimum actuation timescale below which no attraction is observed. Overall, these results clearly show that, in the presence of elastic components, active particles can interact across very long distances without any chemical modification of the environment. Such a mechanism might potentially be important for some biological systems and can be harnessed for newer developments in synthetic active soft materials.active matter | nonequilibrium | colloids | monolayer | elasticity A ctive-matter systems have received much interest due to their emergent nonequilibrium phase and collective dynamical behavior. This interest is well founded as some of the most ubiquitous and important biological systems or processes can exhibit such emergent nonequilibrium behavior, which is perhaps most recognizable in macroscopic examples ranging from schools of aquatic organisms like rays or fish (1-3), herds of livestock (4), flocks of birds (5-7), and even a mosh pit at a heavy metal concert (8). Active-matter systems are additionally unique in that such phenomena spans multiple length scales from meter to nanometer. At these smaller length scales, it is clear that such dynamical adaptation and phase separation are necessary to perform many vital biological processes. Dense crowds of cells move collectively through tissue during development and in many of the immune response processes, i.e., wound healing (9). Sea urchin sperm cells have been found to phase separate and organize into arrays of vortices when the density of spermatozoa is large enough (10). In fact, a myriad of biological systems, and experimental systems with biological components, have reported swarming (11-13), flocking (6, 14, 15), spiraling (13, 16), and many more nonequilibrium steady states (17,18). It is clear that, in all these systems, a combination of the activity, shape of the active agents, and the environment lead to effective out-ofequilibrium interactions that determine their steady states. These active-matter systems have ...

Aggregation dynamics of active rotating particles in dense passive media

“…Therefore, we also explore the effect of the spinners' activity on the aggregation dynamics by applying different rotational frequencies, Re = 0.1, 0.72 and 3.58, to spinners embedded in passive matrixes of φ A = 0.8. In agreement with our previous observations for the spinner-spinner interaction in passive environments [22,23], we observe there exists a minimum threshold of loading stress, or spinner activity, for the spinner attractive interaction to be important. Spinners rotating at Re smaller than 0.1 do not aggregate.…”

Hydrodynamic clustering and emergent phase separation of spherical spinners

“…5a). The effects of the secondary flow has been attributed to the repulsion between two spheres spinning side-by-side [35,36,46] as well as to an attraction of a single spinner towards a flat wall along the vorticity direction [43]. The stabilisation of the clusters, requires the formation of structures via hydrodynamic interactions at Re ∼ 1.…”

Dynamic Coupling at Low Reynolds Number

“…Theg eometry of the suspended objects was expected to play asignificant role in determining the detailed features of such dynamic interactions-which could drive the self-assembly of particles into larger structures without any external interventions.Steimel et al considered atwo-dimensional monolayer of passive colloidal particles and showed that the addition of asmall amount of spinning ferromagnetic colloid facilitated attractive long-range interactions between the active colloid and the tracers,aided by the elasticity of the medium. [140] Theinduced particle interaction depended on the spinning frequency and had am inimum actuation timescale below which no attraction was observed. Ther esults showed that active particles could, in the presence of elastic components,i nteract across very long distances without any chemical modification of the environment.…”