Dual Droplet Functionality: Phototaxis and Photopolymerization.

The use of phototaxis to move droplets in liquids offers the opportunity to emulate natural processes such as the controlled transport of materials in fluidic environments and to undertake chemistry at specific locations. We have developed a photoactive organic droplet, whose movement in aqueous solution is driven by a photoinitiator, as a result of a light-induced reaction within the droplet generating a Marangoni flow. The photoinitiator not only drives the droplet motion but can also be used to initiate polymerization following transfer of the droplet to a specific location and its merging with a monomer-containing droplet. The same light is used to control the transport of the droplet and the polymerization. The efficacy of this droplet transport and reactor system has been demonstrated by the site-specific underwater polymerization of N-isopropylacrylamide to repair a leaking vessel and the adhesion of two materials together.

Moving Droplets in 3D Using Light.

The emulation of the complex cellular and bacterial vesicles used to transport materials through fluids has the potential to add revolutionary capabilities to fluidic platforms. Although a number of artificial motile vesicles or microdroplets have been demonstrated previously, control over their movement in liquid in 3D has not been achieved. Here it is shown that by adding a chemical "fuel," a photoactive material, to the droplet, it can be moved in any direction (3D) in water using simple light sources without the need for additives in the water. The droplets can be made up of a range of solvents and move with speeds as high as 10.4 mm s-1 toward or away from the irradiation source as a result of a light-induced isothermal change in interfacial tension (Marangoni flow). It is further demonstrated that more complex functions can be accomplished by merging a photoactive droplet with a droplet carrying a "cargo" and moving the new larger droplet to a "reactor" droplet where the cargo undergoes a chemical reaction. The control and versatility of this light-activated, motile droplet system will open up new possibilities for fluidic chemical transport and applications.