Self-propelled particles
 SPP model interactive simulation– needs Java |
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Marching locusts – sped up 6-fold. When the density of locusts reaches a critical point, they march steadily together without direction reversals. |
Self-propelled particles (SPP), also referred to as self-driven particles, is a concept used by physicists to describe autonomous agents, which convert energy from the environment into directed or persistent motion. Everyday life examples of such agents are walking, swimming or flying animals. Other biological systems include bacteria, cells, algae and other micro-organisms. Generally, directed propulsion in biological systems is referred to as chemotaxis. There are also artificial systems such as robots or specifically designed particles such as swimming Janus colloids, nanomotors and walking grains.
Self-propelled particles interact according to various social and physical rules, which can lead to the emergence of collective behaviours, such as flocking of birds, swarming of bugs, the formation of sheep herds, etc.
To understand the ubiquity of such phenomena, physicists have developed a number of self-propelled particles models. These models predict that self-propelled particles share certain properties at the group level, regardless of the type of animals in the swarm. It has become a challenge in theoretical physics to find minimal statistical models that capture these behaviours.
Most animals can be seen as SPP: they find energy in their food and exhibit various locomotion strategies, from flying to crawling. The most prominent examples are fish schools, birds flocks, sheep herds, human crowds. At a smaller scale, cells and bacteria can also be treated as SPP. These biological systems can propel themselves based on the presence of chemoattractants. At even smaller scale, molecular motors transform ATP energy into directional motion. Recent work has shown that enzyme molecules will also propel themselves. Further, it has been shown that they will preferentially move towards a region of higher substrate concentration, a phenomenon that has been developed into a purification technique to isolate live enzymes. Additionally, microparticles can become self-propelled when they are functionalized with enzymes. The catalytic reactions of the enzymes direct the particles based on corresponding substrate gradients.
There is a distinction between wet and dry systems. In the first case the particles "swim" in a surrounding fluid; in the second case the particles "walk" on a substrate.
Active colloidal particles are the prototypical example of wet SPP. Janus particles are colloidal particles with two different sides, having different physical or chemical properties. This symmetry breaking allows, by properly tuning the environment (typically the surrounding solution) for the motion of the Janus particle. For instance, the two sides of the Janus particle can induce a local gradient of, temperature, electric field, or concentration of chemical species. This induces motion of the Janus particle along the gradient through, respectively, thermophoresis, electrophoresis or diffusiophoresis. Because the Janus particles consume energy from their environment (catalysis of chemical reactions, light absorption, etc.), the resulting motion constitutes an irreversible process and the particles are out of equilibrium.
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