UK scientists have re-shaped micrometre-sized emulsion droplets using light. 

Andrew Ward at the Rutherford Appleton Laboratory, Didcot, UK, together with Colin Bain at Durham University, UK, and colleagues coaxed (normally spherical) emulsion droplets into a range of geometric shapes with optical tweezers, which use tightly focused laser light to manipulate microscopic objects. 

Oil or emulsion droplets tend to form spheres in order to minimise the surface area, and hence the energy, for a given volume. These droplets aren't usually deformed by optical tweezers, because of their high surface tension. Ward and colleagues realised that if they significantly reduced the surface tension of a droplet, using surfactants, optical forces would be able to overcome surface tension and change the shape of the droplet. Using this principle, the researchers forced the spherical oil drops into unusual droplet shapes like triangles and squares.

The team plan to extend this technique to droplets made of oils that can be polymerised. 'This may offer an interesting method of manufacturing micron-sized particles dispersed in aqueous media,' said Ward. However, he added, 'the polymerisation of shaped droplets will have to be extremely fast to freeze the imposed shapes before the interfacial properties of the solid-water surfaces reform the minimum energy spherical shapes.' 

Julian Eastoe, a surfactant chemist at the University of Bristol, UK, described the work as a 'pioneering and elegant approach for controlling structure in fluid colloidal systems'. He added, 'Bain's group have shown how to make triangular and square emulsion droplets, which would have been unthinkable until now.'

Sarah Corcoran