Using light to separate oil from water

A MATERIAL that modifies the properties of a surface at the flick of switch makes it possible to separate oil from water, and also direct liquid along a chosen path using only light.

The material – developed by an international team of engineers including from MIT and Saudi Arabia’s King Fahd University of Petroleum and Minerals – changes its wetting properties in response to light, meaning the degree to which it attracts or repels water can be altered in real time. The team started out seeking to develop a system that separates oil from water.

The more thoroughly the oil and water is mixed – that is the finer the droplets are – the harder they are to separate. While electrostatic methods can help in this regard, they are energy-intensive and do not work when the water is highly saline, as is common with produced water from oil wells.

The team hit upon the idea of developing a new system that instead relies on photosensitive surfaces that when activated with light cause droplets of water to coalesce and spread across the surface, dividing them from the oil.

The team modified a common photosensitive material – titanium dioxide that is used in sunscreens. The material is limited in as much as it is only activated by the narrow band of UV light so the team set out to make it responsive to visible light.

They did so by using a layer-by-layer deposition technique to build up a film of polymer-bound titanium dioxide particles on a layer of glass. Then they dip-coated the material with an organic dye producing a surface whose wettability is highly responsive to visible light.

'The coupling of the dye to titania particles allows for the generation of charge carriers upon light illumination. This creates an electric potential difference to be established between the surface and the liquid upon illumination, and leads to a change in the wetting properties.'

'Saline water spreads out on our surface under illumination, but oil doesn't,' says University of Kansas researcher Gibum Kwon. 'We found that virtually all the seawater will spread out on the surface and get separated from crude oil, under visible light.'

Furthermore, the team demonstrated that using a beam of light it is possible to propel a droplet of liquid in any direction across the photo-responsive with great precision. The movement of liquid – for example a blood sample in a diagnostic lab-on-a-chip – could be entirely controlled by the pattern of light projected onto it.

'By systematically studying the relationship between the energy levels of the dye and the wettability of the contacting liquid, we have come up with a framework for the design of these light-guided liquid manipulation systems,' says MIT researcher Kripa Varanasi. 'By choosing the right kind of dye, we can create a significant change in droplet dynamics. It's light-induced motion – a touchless motion of droplets.'

The team says all of the materials involved in the process are widely available, inexpensive, commodity materials, and the processes for making them are commonplace.

Nature Communications: doi.org/b6ff