The investigation of systems comprised of self-propelled particles, known as active particles, has become an area of intense research. In theoretical models, it is commonly assumed that the swimming speed of these particles remains constant. However, in experimental scenarios such as ultrasound propulsion for medical applications, the speed of particles can vary depending on their orientation. A collaborative project between physicists from the University of Münster and the University of Cambridge aimed to explore the effects of this speed dependency on the behavior of systems with many active particles, particularly focusing on cluster formation. Through a combination of computer simulations and theoretical derivations, the team made novel discoveries and published their findings in the journal Physical Review Letters.
Unconventional Cluster Behavior
The spontaneous formation of clusters in systems of active particles is a fascinating phenomenon in physics, especially since it can occur even when the particles do not possess any attractive forces. In their simulations, the researchers anticipated that the particles would tend to remain stationary within these clusters, as is typically observed statistically. To their surprise, however, they discovered a different behavior. The particles exhibited continuous movement, constantly leaving the cluster on one side and reentering from another. This perpetual motion created an ongoing flow within the system. This observation challenges the previously held notion that particles in clusters remain static and highlights the complex dynamics at play.
Shape Variability
Another intriguing aspect of the active particle systems the researchers investigated is the influence of orientation-dependent propulsion speed on cluster shape. Unlike traditional active particles, where clusters tend to be circular, the shape of the clusters in this study varied depending on how strongly the particles’ orientation affected their speed. The experimentalist had the ability to stipulate this relationship, allowing for manipulation of cluster shape. Lead author Dr. Jens Bickmann explains that theoretically, the particles can be arranged into any desired shape. This possibility holds practical significance, as it opens up opportunities for applications such as particle “painting.” In their simulations, the researchers observed clusters forming in the shapes of ellipses, triangles, and squares, further underscoring the experimental potential of these active particle systems.
The research conducted by the team of physicists led by Prof. Raphael Wittkowski and Prof. Michael Cates sheds new light on the behavior of systems comprising active particles with orientation-dependent propulsion speed. Contrary to expectations, the particles in these systems exhibit continuous movement within clusters, leading to a perpetual flow of particles. Additionally, the ability to manipulate the relationship between orientation and speed allows for control over the shape of the clusters formed. These findings have implications for both theoretical and practical applications involving active particle systems. The study opens up new avenues for exploring the dynamics of self-propelled particles and their potential applications in various fields, from medicine to materials science. As our understanding of these systems continues to evolve, we can expect further groundbreaking discoveries in the future.
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