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Self-Propelled Nanomotors
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
Although not everything that moves is alive, and vice versa, it is active if it moves autonomously and consumes energy. A new research area has emerged at the interface of physical and biological sciences that studies active matter. Active systems remain out of thermodynamic equilibrium by consuming energy in the environment (or internally stored) to move [3]. Away from thermodynamic equilibrium (a state where the system does not change, i.e., no flow of matter or energy), active particles interact with each other to form new collective phenomena. These are the subjects of an exciting research area combining physics and biology.
Introduction
Published in A. Šiber, P. Ziherl, Cellular Patterns, 2018
A colony of bacteria in a Petri dish and a school of fish (photograph by H. Sanchez) consist of self‐propelled agents which move in space in a locally coordinated fashion such that the velocity of each of them roughly coincides with the average velocity of their not‐too‐distant neighbors. Despite the different length scales, the two systems share many features and display similar spatiotemporal patterns. Such systems are commonly referred to as active matter so as to emphasize that the characteristic types of motion depend on the internally generated propulsion of each agent as well as on the interaction between the agents. Any active system is inherently non‐equilibrium and depends on the energy supplied; at the molecular scale, invariably in the form of ATP. Equally interesting are the artificial active particles driven either by a suitable external force (e.g., vibration) or by a suitable chemical reaction.
Phase transitions and phase coexistence: equilibrium systems versus externally driven or active systems - Some perspectives
Published in Soft Materials, 2021
While the constituent particles of standard condensed matter range in size from atoms or molecules to mesoscopic size of a few (e.g. the colloidal particles in colloid-polymer mixtures where liquid-gas type phase separation in a phase rich in polymers and a phase rich in colloids can be observed [69–72]), the “particles” constituting “active matter” range in size from mesoscopic objects (colloids, [9,73–75] living bacteria, [6,76] etc.) to macroscopic living animals (flocks of birds [77] etc.). The distinctive feature of these “particles” in active matter is that they are self-propelled, i.e. they convert free energy into directed motion. In all these systems, energy has to be supplied continuously to maintain a steady state: this can be due to chemical processes, e.g. when a spherical colloidal particle has two different hemispheres (“Janus particle”) so that the chemical reaction occurs on one hemisphere only.[78] However, this energy can also be supplied by external fields (light, magnetic fields, etc.) or by mechanical shaking of granular particles.[79] In all such active systems, there is no detailed balance, unlike systems in thermal equilibrium. In the present article, we shall not at all dwell on the mechanisms that create such a motility of particles, but shall only give a qualitative discussion of the consequences of this motility for the behavior of a few popular models.