Entropy forces of nanoparticles in self-propelled systems

Abstract

Entropy force is fairly ubiquitous in nature, but it is not practically beneficial for most cases, thus how to reduce the entropic force of the system is very important. In this paper, by employing the overdamped Langevin dynamics simulations, we explore the entropy force between two large nanoparticles (or two nanorods) immersed in a self-propelled system. Self-propelled particles can be regarded as active matter, and the active matter is an interesting subject which has been studied theoretically and experimentally over the past few years. A great many biological and physical systems can be referred to as active matter systems, including molecular motors, swimming bacteria, self-propelled colloids, motile cells, and macroscopic animals. Active matter obtains energy from an external system under non-equilibrium conditions, and active particles with suitably designed constructions are able to convert energy input into the desired control of function, which has wide potential applications in a diversity of fields, such as drug delivery in medicine. Self-propelled particles without angular velocity would gather around the nanoparticles (or nanorods) under the effect of entropy force, which can induce large entropy force between nanoparticles. The interaction force between two nanoparticles is large enough, owing to the asymmetry of the system, and entropy force also depends on the distance between two nanoparticles (or two nanorods). For the case of self-propelled particles with an angular velocity, the entropic effect is weak, and the larger the angular velocity, the weaker the entropic force is. Moreover, nanoparticles will no longer assemble together because of their weak entropic forces. Meanwhile, the entropy force between two nanorods can be tuned from a long repulsion into a long range attraction by changing the distance between two nanorods. The present investigation can help us understand the entropy forces in non-equilibrium systems.