Self-propelling colloids with finite state dynamics

Abstract

Endowing materials with the ability to sense, adapt, and respond to stimuli holds the key to a progress leap in autonomous systems. In spite of the growing success of macroscopic soft robotic devices, transferring these concepts to the microscale presents several challenges connected to the lack of suitable fabrication and design techniques and of internal response schemes that connect the materials’ properties to the function of the active units. Here, we realize self-propelling colloidal clusters which possess a finite number of internal states, which define their motility and which are connected by reversible transitions. We produce these units via capillary assembly combining hard polystyrene colloids with two different types of thermoresponsive microgels. The clusters, actuated by spatially uniform AC electric fields, adapt their shape and dielectric properties, and consequently their propulsion, via reversible temperature-induced transitions controlled by light. The different transition temperatures for the two microgels enable three distinct dynamical states corresponding to three illumination intensity levels. The sequential reconfiguration of the microgels affects the velocity and shape of the active trajectories according to a pathway defined by tailoring the clusters’ geometry during assembly. The demonstration of these simple systems indicates an exciting route toward building more complex units with broader reconfiguration schemes and multiple responses as a step forward in the pursuit of adaptive autonomous systems at the colloidal scale.

Light, Matter, Action: Shining Light on Active Matter

“…One approach relies on coating one side of the particle with a photocatalytic material (such as platinum, palladium, hematite, or titania) to locally decompose a chemical fuel (usually hydrogen peroxide) in water and create a local concentration to drive the particle’s self-diffusiophoresis ,− or self-electrophoresis. − Alternatively, light absorption in Janus particles half-coated with a light-absorbing material (e.g., gold or carbon) can also lead to self-propulsion directly or indirectly due to the formation of a local temperature gradient across the particle because of selective heating at the absorbing side. ,,− UV light can also tune the mobility of electrically powered semiconductor-dielectric Janus particles by increasing the contrast in the polarizability between the two materials, which results in an increased electrokinetic mobility . Differently from their biological counterparts which can move by body deformation, most of these synthetic micromotors have rigid shapes and only recently have light-responsive reconfigurable microswimmers been proposed. − As in the case of microorganisms, artificial Janus particles can also orient in the light field and feature a biased directional phototactic behavior in a light intensity gradient. ,, …”

“…Structured light with different local polarizations or wavelengths could guide the movement of microvehicles , that can change their motion patters depending on the polarization of the illuminating light (Figure f). Shrinkable microrobots in structured intensity fields . Structured light could spatiotemporally change the propulsion magnitude of microrobots comprising temperature-responsive bodies that shrink upon illumination with infrared light, ,, thus reducing their drag force and increasing their propulsion magnitude or propulsion direction. (Figure g). Propulsion by deformable nanoribbons in spatiotemporally oscillating fields .…”

“…Shrinkable microrobots in structured intensity fields . Structured light could spatiotemporally change the propulsion magnitude of microrobots comprising temperature-responsive bodies that shrink upon illumination with infrared light, ,, thus reducing their drag force and increasing their propulsion magnitude or propulsion direction. (Figure g).…”

Arrested on heating: controlling the motility of active droplets by temperature

“…Recent studies have investigated the control of speed and dynamic states of such microswimmers in response to externally applied stimuli such as temperature [22,23] * c.c.maass@utwente.nl or illumination [24][25][26][27][28][29][30]. On heating, physical intuition suggests that motion should accelerate and destabilise, either by the increase of translational and rotational diffusion with decreasing viscosity, or by increased activity from the molecular thermodynamics driving the motion.…”

Replicating programmed colloidal sequences

“…However, this is only the beginning of this exploration, and while the current methods are still crude, we present some preliminary data that suggest the possibility of achieving exponential self-replication in the near future. Exponential self-replication could revolutionize micromanufacturing and enable the large-scale production of colloid-based micromachines [34,35] or building blocks for new microstructured materials [8,13].…”