Unforeseen Actions in Active Particles
Physicists have demonstrated that the reliance of active particles’ propulsion speed on their orientation leads to the formation of clusters in various shapes within multi-particle systems. This discovery could potentially unlock the potential for programmable matter.
Unforeseen Actions in Active Particles: systems composed of self-propelled particles
Exploring systems composed of self-propelled particles, known as active particles, is an increasingly prominent field of research. While theoretical models often assume constant swimming speeds for these particles, in many experiments, such as those using ultrasound for medical applications, the propulsion speed varies with orientation.
A team of physicists, led by Prof. Raphael Wittkowski from the University of Münster (Germany), in collaboration with Prof. Michael Cates from the University of Cambridge (United Kingdom/England), has become the first to investigate the impact of this orientation-dependent propulsion speed on the behavior of multi-particle systems, particularly its influence on cluster formation.
Employing a combination of computer simulations and theoretical analyses, the research team investigated systems composed of numerous active particles whose speed is orientation-dependent. In the process, they unveiled a series of novel effects. These findings have been published in the journal Physical Review Letters.
The spontaneous formation of clusters
A noteworthy aspect from a physics perspective is the spontaneous formation of clusters within systems of many active particles, even in the absence of any mutual attraction among the individual particles. The researchers conducted simulations to monitor particle movements and observed an unexpected phenomenon.
Lead author Dr. Stephan Bröker from the Institute of Theoretical Physics at the University of Münster explains that typically, particles in such clusters remain stationary on a statistical average. Therefore, the researchers expected the same in this case. However, they discovered that the particles continually exit the cluster on one side and re-enter from the other side, resulting in an ongoing flow of particles.
Furthermore, another notable distinction from the typical scenario is the shape of the clusters formed in systems of active particles. In conventional cases, these clusters are usually circular. However, in the particles under examination, the cluster’s shape depends on the degree to which the orientation of the particles affects their propulsion speed.
Unforeseen Actions in Active Particles:The simulations
This influence can be controlled by experimentalists. As co-lead author Dr. Jens Bickmann points out, “Theoretically, we can make the particles arrange themselves into any shape we want, like painting with them.” Therefore, the simulations conducted by the researchers revealed various shapes, including ellipses, triangles, and squares.
Dr. Michael te Vrugt, a member of the Wittkowski team and co-author of the study, emphasizes the practical significance of these findings: “For technical applications, such as realizing programmable matter, it is crucial to be able to control the self-assembly of particles. With our approach, this is indeed possible.”
Background: Active particles are prevalent in biology, seen in phenomena like swimming bacteria or flying birds. Nowadays, there is also the capability to engineer artificial active particles, such as nano- and micro-robots. One potential application is their use within the human body for precise drug delivery.
Read the original article on sciencedaily.
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