Active systems are commonly found in Nature, such as in bacterial colonies and in flocks of birds. Usually active particles come together with passive particles in a system. These systems show various collective patterns and one of them is the segregation of active and passive particles. Self-propelled particle-based models have been used to study the segregation phenomena between active and passive particles in an active system. In this regard, the Active Brownian Particle (ABP) model is the most commonly studied model. Studying the segregation of active and passive particles in an active system is important as the knowledge about segregation is used in drug delivery in biomedical procedures and the purification of pharmaceutical ingredients.
The problem with most studies on segregation between active and passive particles is that they focus on the overdamped limit where particle inertia is neglected. The drag from the surrounding fluid justifies the overdamped limit. Studies on a thermal overdamped active systems show that particles align spontaneously, forming velocity patterns.
Mr. Naveen Kumar Agrawal 
Prof. Pallab Sinha Mahapatra
In this study, Mr. Naveen Kumar Agrawal and Prof. Pallab Sinha Mahapatra, from the Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, India, have observed the velocity patterns without considering thermal fluctuations, but have included particle mass in the dynamics.
The role of alignment force in segregation is not well understood in the literature. In this study, the alignment interaction of the particles is considered to investigate the role of the collective motion in the segregation of active and passive particles in a fixed enclosure. The particles are mixed randomly, and it is observed that active particles collect in the centre and the passive particles move to the boundaries or peripheries. The study is further done to observe the segregation of particles in a monodispersed system and a bidispersed system.
The following parameters are considered in this experiment:
1. Segregation index:
The segregation between active and passive particles is quantified by calculating the standard deviation of the active particle’s concentration.
2. Energy dissipation:
The interaction of the particles with the fluid causes energy dissipation.
3. Rotational order parameter:
The collective phase is studied, where the particles align locally and move in circular trajectories forming a global vortex motion.
4. Gini coefficient:
The Gini coefficient gives the measure of particle dispersion in the domain.
On comparing a monodispersed system and a bidispersed system, it was found that segregation of active and passive particles is better in a monodispersed system while the mixed state is favoured in bidispersed systems.
In conclusion, the binary active-passive particle system was studied. It was found that with the onset of collective motion, segregation followed. The particle-particle interaction was found to increase with higher packing fractions, hindering the particle’s mobility and preventing segregation. This study gives a fundamental understanding of the collective behaviour in a binary system in the presence of alignment interaction. The role of collective motion in segregation was also studied. This reserach could find use in the study of particle sorting in the presence of cells.
Prof. Luca Brandt from the Department of Fluid Mechanics, KTH Royal Institute of Technology, Stockholm, Sweden, explains at length the significance of this study in the following comments: “The study of the collective dynamics of complex systems, as example bacterial colonies, large flocks of birds and artificial self-propelling particles, typically focuses on phenomena as concerted motion and segregation. Most existing literature deals with purely active systems, whereas active-passive mixtures have been less studied. However, practical systems often consist of active and passive particles, e.g., dead bacteria in biofilms or non-motile swimmers. This heterogeneity is a primary motivation for the investigation of the dynamics of a binary mixture of active and passive particles, aiming to understand the complex behavior of such active/passive systems. Complex phenomena such as collective motion and segregation have been shown to occur because of two distinct interactions: tendency of alignment and attraction/repulsion. This work includes both these interactions, modelled as an alignment force and a repulsion force between the particles and strives to reproduce segregation among the particles.
In most of the existing literature, collective motion and segregation have been examined individually. Yet, experimental forays in the field of embryonic development show that the collective motion accelerates the cell segregation and numerical investigation, employing velocity-dependent interactions. They have also shown that coherent motions facilitate cell segregation. Although the focus of several recent studies, the role of the alignment force in the segregation is not well understood. This gap is addressed in the current work by examining the link between collective motion and segregation. The authors observe that as the system is forced to undergo collective motion, it demonstrates a tendency towards segregation. Further, the authors investigate the effect of the size inhomogeneity between the two types of particles. The results suggest that a mono-dispersed system readily segregates, whereas the inclusion of size differences between the active and passive particles suppresses the segregation phenomenon.
The present work on active and passive systems contributes to our current understanding by providing evidence of the intricate relationship between collective motion and segregation. The work provides, in particular, new details on the behavior of heterogeneous soft particles in a group. This study is crucial for an improved understanding of natural active systems, i.e., mixtures of active and passive bodies and motivates further research on complex active systems.”
Article by Akshay Anantharaman
Here is the original link to the paper:
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.104.044610
