A neat study shows that a sheet of laser light can be used to reflect light-absorbing liquid droplets and manipulate their trajectories. This observation may open up new ways of controlling and studying aerosols.

We think of light as an ephemeral thing with no substance. We appreciate its warming effect when we step outside on a sunny day, but the idea that light can have a mechanical effect, producing forces, seems counter-intuitive. It seems plausible that a small droplet can bounce off a pool of water, unable to break the surface tension. But can we say the same of a droplet hitting a sheet of light? Writing in Applied Physics Letters, Esseling and colleagues1 describe such a phenomenon: an optical 'trampoline'.

In the 1970s, Arthur Ashkin and his co-workers at Bell Laboratories pioneered a new field of research — the manipulation of microscopic particles using light2. Ashkin showed that by using a laser he was able to push objects such as glass beads immersed in water and droplets of liquid dispersed in air along the direction of propagation of the laser beam. This radiation pressure could also be used to trap particles by holding them against gravity or, by using two counter-propagating beams, to confine them where the radiation pressure from each beam balanced.

The idea that light could exert these forces was nothing new: James Clerk Maxwell had predicted3 it as a consequence of his electromagnetic theory nearly 100 years previously. However, observing the effect had proved difficult owing to the problem of distinguishing optical forces from thermal effects. Indeed, William Crookes (of Crookes radiometer fame) was able to demonstrate thermal forces on matter in 1901, long before optical forces were definitively observed.

Ashkin's great insight was to understand that, by using optically transparent microscopic objects, he could rely on the forces generated by scattering, reflection and refraction alone and remove the strong, masking thermal forces. His work has led to many applications, with optical-trapping techniques being widely used for studying minuscule forces and microscopic motion in systems ranging from molecular motors to the evaporation dynamics of aerosols.

Ashkin's techniques are limited in part by the types of object that they can trap. It is challenging to work with particles that strongly absorb the laser light being used, because they tend to heat up, and a process called thermophoresis starts to come into play. By heating one side of an object, a thermal gradient is established that results in it moving away from the hotter region and towards the colder one, driving it along the direction of the laser beam. Although this sounds very much like Ashkin's original experiments, thermal forces are up to 1,000 times stronger than radiation pressure.

Previous optical-manipulation techniques have used thermophoresis to trap and control solid particles such as carbon4. Esseling et al. have now extended this to optically absorbing liquid particles and, by using sheets of laser light instead of simple laser beams, have produced a surface made of light off which liquid droplets can bounce. This behaviour has previously been observed with droplets in emulsions5, but here the manipulation is carried out in air. By using an inkjet printer head, the authors were able to produce droplets of a uniform diameter of 50 micrometres; ink that absorbed at the laser wavelength of 532 nanometres was used. They then fired the droplets at the light sheet. Depending on the power of the laser and the angle of the light sheet, the droplets could be made to pass through the light or to bounce with well-defined, controllable trajectories (Fig. 1). Multiple bounces could be seen at certain light-sheet inclinations, mimicking a rubber ball bouncing down a smooth slope. A horizontal sheet made with a laser power of 1.8 watts and with a peak intensity of 115 μW μm−2 prevented droplets from passing through. Figure 1: Bouncing droplets. Esseling et al.1 have used a sheet of laser light (white) to manipulate the trajectory of liquid droplets for several light-sheet inclinations; red circles represent the tracked centres of the droplets. a, For an inclination of 45°, droplets starting from the top make a single bounce off the light sheet. b, For an inclination of −45°, one bounce is also observed, and some droplets are seen to leak through the light sheet at the initial contact point. c, For an inclination of −20°, the droplets bounce twice, passing through the light sheet at the start of the third 'bounce'. Scale bars, 300 micrometres. (Images taken from ref. 1.) Full size image

This idea opens up avenues for the control of droplet streams. In particular, the ability to shape optical beams means that parabolic light 'bowls' could be created and used to focus droplets in air and carry out controlled chemical reactions; particles in aerosols could be sorted by rapidly altering the light path down which they travel. Furthermore, the behaviour observed suggests that light-absorbing liquid droplets can be trapped in a similar manner to solid particles. An interest in aerosol particles of all types underpins many areas of atmospheric science as well as combustion studies, for example, and new ways to handle aerosols should allow better and more flexible experiments to be carried out.

Another interesting application is in the development of novel forms of optofluidics — a technique in which light and liquid interact, usually on prefabricated, miniaturized devices called microfluidic chips. A recent advance in aerosol manipulation has been to use such devices to probe the properties of aerosols6, and the opportunities afforded by Esseling and colleagues' technique would mean that light could be used to route nearly any type of microscopic particle around such a chip, as well as being used to avoid interactions with the chip's walls (not desirable when using liquid droplets). Their technique may also provide opportunities to analyse particles in confined geometries, such as hollow optical fibres7.

The major open question about this type of technique is how the heating of a droplet caused by its interaction with the light sheet influences the droplet's dynamics and composition. If the goal of manipulating droplets is to investigate the real-world properties of aerosols, then their interaction with the light sheet must have a negligible effect, otherwise the properties of interest will be masked or destroyed. Additionally, many aerosol processes take place in much smaller droplets than those used here. Future experiments will be required to test the limits of the technique. But if all goes to plan, next year's must-have fashion accessory could well be an umbrella made of light.

References 1 Esseling, M., Rose, P., Alpmann, C. & Denz, C. Appl. Phys. Lett. 101, 131115 (2012). 2 Ashkin, A. Phys. Rev. Lett. 24, 156–159 (1970). 3 Maxwell, J. C. A Treatise on Electricity and Magnetism Vol. 2 (Oxford Clarendon Press, 1873). 4 Shvedov, V. G. et al. Phys. Rev. Lett. 105, 118103 (2010). 5 Cordero, M. L., Burnham, D. R., Baroud, C. N. & McGloin, D. Appl. Phys. Lett. 93, 034107 (2008). 6 Horstmann, M., Probst, K. & Fallnich, C. Appl. Phys. B 103, 35–39 (2011). 7 Schmidt, O. A., Garbos, M. K., Euser, T. G. & Russell, P. St. J. Phys. Rev. Lett. 109, 024502 (2012). Download references

Author information Affiliations David McGloin is at the Division of Physics, University of Dundee, Dundee DD1 4HN, UK. David McGloin Authors David McGloin View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to David McGloin.

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