Investigating systems consisting of self-propelled particles, also known as active particles, is currently a rapidly growing area of research. In theoretical models for active particles, it is commonly assumed that the particles’ swimming speed remains constant. However, in many experimental scenarios, such as particles propelled by ultrasound for medical applications, the propulsion speed depends on the orientation of the particles. Understanding how this dependency affects the behavior of systems consisting of many particles, particularly the formation of clusters, is the focus of a collaboration between physicists from the University of Münster and the University of Cambridge.
Through a combination of computer simulations and theoretical derivations, a team of physicists led by Prof. Raphael Wittkowski and Prof. Michael Cates, have explored the behavior of systems consisting of many active particles whose speed is orientation-dependent. Their study has uncovered a series of new effects, highlighting the complex dynamics of such systems. Published in the journal Physical Review Letters, the research sheds light on the formation and movement of clusters in active particle systems.
One fascinating aspect of systems consisting of many active particles is their ability to spontaneously form clusters, even in the absence of attraction between individual particles. When observing the movement of particles in simulations, the researchers made a surprising discovery. Under normal circumstances, particles within clusters tend to stay in place on a statistical average. However, in this study, the physicists found that the particles constantly moved out of the cluster on one side and returned on the other side, creating a permanent flow of particles. This observation challenges previous assumptions about the behavior of particles within clusters.
In addition to the dynamic flow of particles, the researchers also observed that the shape of clusters in these active particle systems varied depending on the strength of orientation-dependent propulsion. Typically, clusters formed by active particles are circular in shape. However, in the particles examined in this study, the shape of the cluster was influenced by the degree to which particle orientation affected their propulsion speed. This aspect of the research opens up the possibility of manipulating particle arrangements into various shapes. Theoretical control over cluster shape has potential practical applications in fields such as materials science and engineering.
The ability to control the shape of clusters in active particle systems has practical importance beyond theoretical physics. The researchers observed various shapes, including ellipses, triangles, and squares, during their simulations. This finding suggests that active particles can be used to create predetermined patterns or structures, allowing for precise control and manipulation. From a practical standpoint, these findings have implications for the development of novel materials and technologies.
The study conducted by the team of physicists from the University of Münster and the University of Cambridge has provided valuable insights into the behavior of active particle systems in which the propulsion speed depends on particle orientation. The discovery of the continuous flow of particles within clusters and the ability to manipulate cluster shape offer exciting opportunities for further research and practical applications in a variety of fields.