Diffusing wave paradox of phototactic particles in traveling light pulses

Celia Lozano,Clemens Bechinger

doi:10.1038/s41467-019-10535-z

Abstract

Cells navigate through complex surroundings by following cues from their environment. A prominent example is Dictyostelium, which is directed by chemotaxis towards regions with higher concentrations. In the presence of traveling chemical waves, however, amoebae migrate counter to the running wave. Such behavior, referred to as diffusing wave paradox, suggests the existence of adaptation and directional memory. Here we experimentally investigate the response of phototactic self-propelled microparticles to traveling light-pulses. Despite their entirely memory-less (i.e., strictly local) response to the environment, we observe the same phenomenological behavior, i.e., particle motion counter to the pulse direction. Our findings are supported by a minimal model which considers active particle reorientations within local light gradients. The complex and robust behavior of synthetic active particles to spatially and temporally varying stimuli enables new strategies for achieving collective behavior and can be used for the design of micro-robotic systems with limited signal-processing capabilities.

Highlights

Cells navigate through complex surroundings by following cues from their environment
active particles (APs) are fabricated from colloidal spheres with diameter σ = 3.25 μm, which are half-coated by carbon cap with 50 nm thickness
As a result of Brownian rotational motion, the direction of the propulsion fluctuates on a time scale 1/Dr∼5 s set by the inverse rotational diffusion coefficient Dr Under homogeneous light illumination, the APs perform an isotropic persistent random walk which is confined to two dimensions due to gravity and hydrodynamic interactions with the walls of the sample cell[37,39]

Summary

Introduction

Cells navigate through complex surroundings by following cues from their environment. It has been demonstrated that it results from the finite adaptation time of the organisms to variations of the surrounding chemical concentration[11,14,15,16] Such time-delayed response to spatio-temporal stimuli leads to a slightly different motional response of amoebae to the front and back of entirely symmetric cAMP waves[17]. This explains their aggregation into multi-cellular collectives, and their astonishingly effective migration within complex cellular tissues[18] and artificial mazes[11,19]

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Results

Conclusion