Light, Matter, Action: Shining Light on Active Matter

Abstract

Light carries energy and momentum. It can therefore alter the motion of objects on the atomic to astronomical scales. Being widely available, readily controllable, and broadly biocompatible, light is also an ideal tool to propel microscopic particles, drive them out of thermodynamic equilibrium, and make them active. Thus, light-driven particles have become a recent focus of research in the field of soft active matter. In this Perspective, we discuss recent advances in the control of soft active matter with light, which has mainly been achieved using light intensity. We also highlight some first attempts to utilize light's additional properties, such as its wavelength, polarization, and momentum. We then argue that fully exploiting light with all of its properties will play a critical role in increasing the level of control over the actuation of active matter as well as the flow of light itself through it. This enabling step will advance the design of soft active matter systems, their functionalities, and their transfer toward technological applications.

Figure 1

Actuation of active matter by different properties of light. Different properties of light (intensity, wavelength, polarization, and momentum) can be employed to control active matter. These schematics represent the main properties of light (inner circle) and some prominent examples of actuation (outer circle). (Top) Intensity of light typically correlates with the magnitude of the respective particle’s propulsion mechanism, so that the speed of the active particles can be adjusted by intensity. (Right) Different wavelengths can address different parts of heterogeneous active Janus particles, enabling control over propulsion direction and magnitude. (Bottom) Nanomotors consisting of nanowires with a high dichroic ratio preferentially absorb polarized light, enabling polarotactic active movement controlled by the polarization state of the incident light. (Left) Light also carries momentum, which can propel matter: for example, microvehicles bearing metasurfaces that scatter light directionally can be accelerated via transfer of light momentum.

Figure 2

Active matter systems controlled by light intensity. Light intensity has been employed to control active matter systems at all length scales. (a) On the molecular scale, light-responsive nanomotors can undergo a photochemical isomerization around the central double bond upon irradiation with UV light that results in helicity inversion (from right-handed to left-handed). This motor is very effective at inducing helical organization in a liquid-crystal film, which can be harnessed to move microparticles placed on top of it (i–iv). Adapted with permission from ref (35). Copyright 2006 Springer Nature. (b–e) Microscopic systems. (b) Light-activated Janus colloids self-organize into clusters under blue light but dissolve when the light source is turned off. Adapted with permission from ref (29). Copyright 2013 American Association for the Advancement of Science. (c) Bacteria, genetically modified to swim smoothly with a light-controllable speed, can be arranged into complex and reconfigurable density patterns such as a portrait of Mona Lisa using a simple digital light projector. Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (30). Copyright 2018 eLife Sciences Publications Ltd. (d) Self-propelling droplets of chiral nematic liquid crystals in surfactant-rich water propel in a screw-like motion. Photoinvertible chiral dopants allow conversion between right-handed and left-handed trajectories upon UV irradiation. Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (31). Copyright 2019 Springer Nature. (e) Electronically integrated micromotors consisting of a body containing standard silicon electronics and surface electrochemical actuator legs are able to walk by directing laser light to its photovoltaics that alternately bias the front and back legs. Adapted with permission from ref (36). Copyright 2020 Springer Nature. (f) On the macroscale, phototactic robots can respond to light gradients, e.g., by adjusting their speed in response to the measured light intensity. Adapted with permission under a Creative Commons CC-BY 3.0 License from ref (34). Copyright 2016 Springer Nature.

Figure 3

Active matter systems controlled by wavelength. (a) Forward trajectory of Au-coated anatase TiO₂ Janus particles in a H₂O₂ solution upon illumination with UV light (top, magenta trajectory) and reverse direction upon illumination with green light (bottom, green trajectory). Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (19). Copyright 2020 Springer Nature. (b) AzoTAB-stabilized Janus emulsions under bright-field blue light irradiation self-assemble toward a localized UV light spot (filled circle). Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (114). Copyright 2022 Springer Nature. (c–e) Potential future uses of wavelength as control strategy for active particles. (c) Elongated particles (e.g., ellipsoids) with two different metal patches can be steered using two wavelengths of light. (d) U-shaped particle with three metal patches can work as cargo carriers, with full control over their planar movement. (e) In Janus particles coated with stimuli-responsive polymer brushes decorated with plasmonic nanoparticles, external stimuli such as temperature, pH, or salt concentration can collapse the polymer and shift the absorbance spectrum, affecting the particle mobility.

Figure 4

Active matter systems controlled by polarization. (a) Polarotaxis in photoresponsive algae (Euglena gracilis) leads to movement perpendicular to the polarization of light. Adapted with permission from ref (134). Copyright 2021 American Physical Society. (b) Nanomotors consisting of nanowires with a high dichroic ratio preferentially absorb polarized light, enabling polarotactic active movement and steering controlled by the polarization state of the incident light. Adapted with permission from ref (20). Copyright 2019 John Wiley and Sons. (c,d) Potential future uses of polarization as control mechanism in active particles. (c) Plasmonic nanocrescents feature polarization-dependent resonances and near-field enhancement at their tips. For a fixed wavelength, the propulsion strength of active particles driven by similar nanostructures therefore would depend on their orientation relative to the polarization of light, leading to predominant motion in the direction where the absorption of a given polarization is stronger. (d) Electronically integrated micromotors equipped with polarizing filters in front of photovoltaic components could allow the polarization-dependent control of specific actuators to steer the particle’s self-propulsion with the light polarization.

Figure 5

Active matter systems controlled by transfer of light momentum. (a) Plasmonic linear nanomotor driven by momentum transfer of light. Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (148). Copyright 2020 American Association for the Advancement of Science. (b) Microvehicles containing a directional scattering metasurface. The transfer of momentum leads to straight propulsion under linearly polarized light and circular motion under circularly polarized light. These metavehicles can also be used to move some microorganisms present in the solution (bottom panel). Adapted with permission from ref (21). Copyright 2021 Springer Nature. (c) Light-driven microparticles containing four chiral plasmonic resonators are maneuvered by adjusting the optical power for each resonator using two overlapping unfocused light fields at 830 nm (orange arrow) and 980 nm (red arrow) with right- and left-handed circular polarization, respectively. Adapted with permission from ref (149). Copyright 2022 Springer Nature. (d–g) Potential future uses of momentum transfer in active matter. (d) Active particles either propelled by light or by chemical fuels can explore a speckle light pattern according to some nontrivial random motion statistics (e.g., by a Fickian yet non-Gaussian diffusion). (e) Active momentum-driven particles with different shapes can generate emergent collective self-assembly behaviors, as already theoretically modeled.^, (f) Active birefringent Janus particles with a metal cap on one side can combine the propulsion of Janus particles with the orientation in polarized light of birefringent particles. Such particles would move along the polarization direction of linearly polarized light or show a circular motion in circularly polarized light. (g) Solar sails propelled by the light of the sun could self-assemble in space into complex devices, e.g., space telescopes.

Figure 6

Active matter systems interacting with structured light. (a) Janus particles in a critical mixture of water–lutidine align such that they move along the gradient of light toward low light intensities. Directed particle transport over arbitrarily long distances can then be achieved using periodic sawtooth-like light profiles. Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (74). Copyright 2016 Springer Nature. (b) Structured light can guide the motion waves among photochemically activated colloids. Adapted from ref (156). Copyright 2022 American Chemical Society. (c) Structured UV light can create spatiotemporal chemical landscapes by releasing caged chemo-attractants, which guide the movement of sperms. Adapted with permission under a Creative Commons CC-BY 4.0 License from ref (158). Copyright 2015 Springer Nature. (d) Spatiotemporally structured light induces intrabody shape changes in microrobots consisting of photoactive liquid-crystal elastomers, which enables self-propulsion by generating a traveling-wave motion. Adapted with permission from ref (109). Copyright 2016 Springer Nature. (e) Temporally structured light enables rapid dynamic switching between the configurations of helical composite hydrogel microrobots, enabling translational movement near a solid surface. Adapted with permission from ref (72). Copyright 2017 John Wiley and Sons. (f–i) Potential future uses of structured light to actuate and control active matter. (f) Metavehicles are able to change their motion direction (linaer or circular) depending on the polarized polarization of the illuminating light.^, Structured light landscapes with different local polarizations could guide the movement of such microvehicles. (g) Microrobots could comprise temperature-responsive bodies that shrink upon irradiation with IR light, thus reducing their drag force and increasing their propulsion magnitude. Structured light with different local wavelengths could spatiotemporally change its propulsion magnitude. (h) Spatiotemporally structured light could locally bend hydrogel nanoribbons^,, and induce a snake-like motion, which could be exploited to propel microparticles. (i) Microwalkers could be driven by hydrogel-gold nanoparticle composites, which serve as artificial muscles and joints in response to light. Using spatiotemporally structured light, each artificial element could be addressed individually to enable microscale artificial walking.