The study of self-propelled particles, known as active particles, has gained significant attention in the field of research. One common assumption in theoretical models is that these particles have a constant swimming speed. However, in experimental scenarios where particles are propelled using ultrasound for medical applications or other similar methods, the propulsion speed of the particles depends on their orientation. This dependency raises intriguing questions about how it influences the collective behavior of systems with many active particles, specifically with regards to the formation of clusters. A collaborative project conducted by a team of physicists led by Prof. Raphael Wittkowski from the University of Münster and Prof. Michael Cates from the University of Cambridge aimed to answer these questions. Their groundbreaking study, utilizing computer simulations and theoretical derivations, uncovered a range of new effects in systems consisting of active particles with orientation-dependent speed. The findings were published in the journal Physical Review Letters.
One intriguing aspect of systems with many active particles is their ability to spontaneously form clusters even without any attractive forces between the individual particles. As the researchers analyzed the movement of particles in simulations, they stumbled upon a particularly surprising result. Contrary to their expectations, the particles within these clusters did not simply stay in place on average, as commonly observed. Instead, the team observed an incessant movement of particles, with them continuously exiting the cluster on one side and reentering from the other, creating a perpetual flow of particles. This discovery adds an exciting dimension to the understanding of cluster dynamics in active particle systems.
The Influence of Particle Orientation
Another remarkable finding of the study is the impact of particle orientation on cluster shapes. In typical active particle systems, clusters tend to be circular. However, in the examined particles, the shape of the cluster is determined by the degree to which particle orientation influences the propulsion speed, allowing experimentalists to manipulate the cluster formation. Lead author Dr. Jens Bickmann explains that “theoretically, at least, we can make the particles arrange themselves into any shape we want. We can paint with them, so to speak.” This observation opens up practical applications in shaping clusters, such as the formation of ellipses, triangles, and squares. Dr. Michael te Vrugt, a co-author of the study, highlights the significant practical implications of this finding.
The experimental investigation of systems with active particles heavily relied on a combination of computer simulations and theoretical derivations. By simulating the behavior of particles with speed-dependent orientation, the researchers were able to observe and analyze the complex dynamics of cluster formation. The theoretical derivations complemented the simulations by providing a deeper understanding of the underlying principles and mechanisms at play in these systems. This synergy between simulation and theory is crucial for advancing our knowledge of active particle dynamics.
The research conducted by Prof. Wittkowski, Prof. Cates, and their team serves as a milestone in exploring the behavior of systems comprised of active particles with orientation-dependent speed. The new effects observed in the study offer valuable insights into cluster dynamics and pave the way for further investigations. The ability to control cluster shapes has practical implications in fields where the manipulation of particle arrangements is desirable. This groundbreaking research opens up new possibilities for the development of innovative technologies and advancements in various scientific disciplines. The ongoing advancements in the study of active particles continue to captivate researchers and drive discoveries that reshape our understanding of complex systems.