Diffusing wave paradox of phototactic particles in traveling light pulses

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.

Recent Advances in Nano‐ and Micromotors

“…Further, Ag/poly(methyl methacrylate) Janus micromotors were able to self‐propel under pulsating and continuous visible light, achieving instantaneous (peak) velocities of 20–25 µm s −1 . Travelling light pulses also promoted the movement of phototactic particles, which were demonstrated to be able to track local light gradients . Further, inorganic Zn x Cd 1– x Se‐Cu 2 Se‐Pt p–n junction nanowire were explored as visible‐light‐driven micromotors with velocities up to 10 µm s −1 …”

Realization of a motility-trap for active particles

“…An important class of particles for which effective and versatile trapping schemes are currently not available are self-propelled active particles (APs) [9][10][11][12][13][14][15] . Such systems receive considerable attention due to their resemblance with motile microorganisms [16][17][18] , allowing to identify general conditions under which collective behavior can emerge 19,20 . In addition, APs are currently discussed as autonomous microrobots that may find use as autonomous vehicles to deliver loads to specific targets [21][22][23] .…”

Lorentz forces induce inhomogeneity and flux in active systems

“…The width of a lane can be approximated as d = L/(2π) ∼ 0.1 cm. Note that for the chosen parameters the diffusive Hall-constant is of the order, κ = qB/γ ∼ 0.1 (29) implying that Lorentz force becomes comparable to frictional force on the particle. Finally, assuming that the diffusion coefficient of the tracer particle is D p ∼ 10 −13 m 2 /s, a reasonable estimate for large tracer particles (∼ µm), we obtain from Eq.…”

“…[23]. Whereas in these studies activity has been time independent, one can induce fluxes by making the swim speed space and time dependent, as has been shown in theoretical and numerical studies [24][25][26][27][28][29]. In these systems the active particles 'surf' on an activity wave resulting in active fluxes.…”