The study of self-propelled particles, known as active particles, is an area of research that has been rapidly expanding. In many theoretical models for active particles, it is commonly assumed that the particles’ swimming speed remains constant. However, in experiments where particles are propelled by ultrasound for medical applications, the propulsion speed is dependent on the orientation of the particles. In a collaborative project led by Prof. Raphael Wittkowski from the University of Münster and Prof. Michael Cates from the University of Cambridge, physicists have investigated the behavior of systems consisting of active particles with orientation-dependent speed and uncovered a series of new effects. Their findings, based on computer simulations and theoretical derivations, have been published in the journal Physical Review Letters.
One intriguing aspect of systems consisting of many active particles is their ability to spontaneously form clusters, even in the absence of attractive interactions between individual particles. The researchers measured the movement of particles in their simulations and made a surprising observation. While it is typically expected that particles in clusters would remain stationary on average, the physicists discovered that particles constantly move out of the cluster on one side and re-enter on the other, resulting in a continuous flow of particles. This dynamic behavior challenges the conventional understanding of cluster stability in active particle systems.
In addition to the unexpected flow of particles, the shape of the clusters in systems with orientation-dependent speed differs from the circular clusters typically observed in active particle systems. The shape of the clusters can be controlled by adjusting the influence of particle orientation on their propulsion speed, providing experimentalists with the ability to manipulate cluster formation. Lead author Dr. Jens Bickmann explains that this flexibility allows researchers to “paint” with the particles, suggesting practical applications for these findings. Through simulations, the physicists observed clusters in the shape of ellipses, triangles, and squares, highlighting the versatility of orientation-dependent propulsion speed in shaping cluster morphology.
The ability to control the shape of clusters in active particle systems has practical implications beyond fundamental physics. Dr. Michael te Vrugt, a co-author of the study, emphasizes the practical importance of these results. By manipulating the propulsion speed of particles based on their orientation, researchers can design systems that arrange themselves into desired shapes. This opens up possibilities for a range of applications where the organization and manipulation of clusters play a crucial role.
The investigation into the behavior of systems made up of active particles with orientation-dependent propulsion speed has revealed new insights and phenomena. The discovery of continuous particle flow within clusters challenges existing assumptions about cluster stability in active particle systems. Moreover, the ability to control cluster shape through orientation-dependent propulsion speed presents exciting possibilities for practical applications in fields such as materials science and bioengineering. This study underscores the importance of considering the effects of propulsion speed on the behavior of active particle systems, expanding our understanding and opening up new avenues of research in this rapidly growing field.