Evolution of Self‐Propelled Objects: From the Viewpoint of Nonlinear Science

Abstract

A variety of moving objects driven by chemical energy have been reported. In this Minireview, we focus on self-propelled objects driven by interfacial tension and explain three types of basic mechanisms for such self-propelled motion, that is, driven by a) surface tension difference, b) contact angle difference, and c) axisymmetric swirling flow in a droplet. Simple behavior induced from the basic mechanisms is then extended by coupling to a chemical reaction or increasing the number of moving objects. Even though the chemicals used here are still simple, the extended systems could show characteristic nonlinear behavior, such as reciprocating motion, oscillatory motion, and spatiotemporal pattern formation. Combining the dynamical information about these characteristic motions with the knowledge of molecular structures will lead to the development of more advanced self-propelled objects. We believe this Minireview can help chemists in investigating self-propelled objects displaying various functional motions observed in a biological system.

Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks

“…As far as we are aware, the point-source model was first introduced by Mikhailov and Meinköhn [53,55] to describe an interfacial swimmer which releases a chemical diffusing in the underlying bulk fluid. More recent works consider the one-dimensional situation [5,7,56] relevant when swimmers are confined within a narrow channel. A similar approach was also used to interpret molecular simulations of a symmetric autophoretic swimmer [36].…”

“…Among the systems relying on this working principle, two important classes are the autophoretic and interfacial swimmers [2]. Since the mid-2000s, both have received sustained attention [3][4][5][6][7].…”

“…The motion, however, is only transient. Steady propulsion in artificial systems is generally made possible with a volatile surfactant that avoids saturation by quickly evaporating from the surface [7].…”

“…With the current interest in active matter, both autophoretic and interfacial swimmers have attracted attention as model systems to investigate collective and nonlinear effects, including clustering [24][25][26], oscillations, synchronization and pattern formation [5,27], density waves [28], swarming [29], and self-assembly [30]. Yet, in spite of a wealth of investigations [4,5,7], some questions remain on the mechanism of individual self-propulsion. One aspect lacking a complete picture so far is the motion of isotropic swimmers permitted by spontaneous symmetry breaking.…”

Autonomous mesoscale positioning emerging from myelin filament self-organization and Marangoni flows

“…Typically, spatial control in synthetic systems involves external stimuli that contain spatial information, such as concentration gradients 27 , external fields [28][29][30] , or localized photocontrol 31 , although self-organizing patterns emerging from initially homogeneous conditions have been established via reaction diffusion 32,33 and convection 34 . Furthermore, chemical gradients-either applied top-down or emerging spontaneously -can drive motion in self-organization processes [35][36][37][38] . For example, Marangoni flows driven by surface tension gradients caused by dissolution of surface-active chemicals have been reported to direct the ordering of swarms of floating droplets or camphor boats at aqueous interfaces 39,40 .…”