Abstract
We analyze the reciprocating locomotion of a photosensitive gel undergoing an oscillating chemical reaction in a temporally constant gradient illumination environment to examine whether periodic migration can arise as an adaptive response of a system’s internal dynamics, analogous to biological functions like growth or breeding, to specific spatial variations in resource distribution, such as food, temperature, or light intensity, caused by geography or seasonal transition. Even in this relatively simple system, kinematic switching in the mode of locomotion under spatially varying illumination levels leads to reciprocating periodic motion, suggesting that such behavior may occur widely in organisms as a response to a variety of spatially distributed environmental stimuli to enhance their survival and reproduction.
Periodic to-and-fro migration is a sophisticated mode of locomotion found in many forms of active matter in nature. Providing a general description of periodic migration is challenging, because many details of animal migration remain a mystery. We study periodic migration in a simpler system using a mechanistic model of a photosensitive, active material in which a stimulus-responsive polymer gel is propelled by chemical waves under the regulation of an illumination gradient sensed by the gel, which plays a role analogous to the environment in periodic animal migration. The reciprocating gel migration results from autonomous transitions between retrograde and direct wave locomotion modes arising from the gradient distribution of the illumination intensity. The local dynamics of the chemical waves modulates the asymmetry between push and pull forces to achieve repeated reorientation of the direction of locomotion. Materials that display similar intelligent, self-adaptive locomotion might be tailored for such functions as drug delivery or self-cleaning systems.