The colour-changing properties of living cells that some animals use to camouflage their bodies have been mimicked in a material created by Andrew Salmon and colleagues at the University of Cambridge in the UK. The team made their material by loading tiny water droplets with specialized gold nanoparticles. Through a series of experiments, they showed that these droplets can change colour reversibly when heated by light – and that the movement of the droplets can also be controlled using light.

Animals including cuttlefish, zebrafish and chameleons can actively camouflage themselves in varied environments by changing the colour and patterns on their skin. This is done using chromatophore skin cells, in which pigments are continually shifted around by proteins that expand and contract.

Researchers are keen on mimicking this behaviour using artificial chromatophores, in which the proteins are replaced by light-powered mechanisms. In their study, Salmon’s team created such a structure using tiny water droplets in a thin film of oil. The water droplets contain gold nanoparticles that are coated in a specialized polymer shell.

Hydrophobic transition

Above a critical temperature of 32 °C, the polymer shell undergoes a transition that makes it become hydrophobic. This causes the nanoparticles to cluster together in the droplet as they try to minimize their contact with water. In this configuration, the nanoparticles appear bluer in colour. The transition is reversible so if the temperature dips below 32 °C, the particles are no longer hydrophobic and will move apart, which makes them appear redder (see video).

The process is controlled by illuminating the droplets with focussed beams of light. The nanoparticles are much better at absorbing light than water, which means that the nanoparticles can be heated rapidly by light without unduly heating the water.

Salmon’s team also discovered that the nanoparticles tend to cluster at the bases of the droplets and realized that this can be used to generate locomotion through two separate mechanisms. Firstly, with strong, off-centre laser illumination, they found that high-pressure microbubbles will rapidly form at the bases of the droplets, propelling them forward as they expand. Alternatively, with weaker illumination, local surface tension can be increased towards the base of the droplet. This introduces a surface tension gradient, and a subsequent shear force across the droplet, causing it to move.

The researchers demonstrated these effects both in individual droplets, and in larger, centimetre-scale areas of closely packed droplets in oil films. They now hope to recreate their results on metre-scale sheets with multiple layers. They will also look at different nanoparticle materials and shapes try to use light-triggered swimming to “herd” droplets. With further improvements, their technology could soon become suitable for applications including active camouflage and large-scale dynamic displays.

The research is described in Advance Optical Materials.