Engineers at the Massachusetts Institute of Technology (MIT) have created a system that could make it possible to control how water moves over a surface by using only light. This could pave the way for advanced technologies like reprogramming microfluidic diagnostic devices or field systems that can separate water from oil at a drilling rig.

The system has been reported in the journal "Nature Communications." It was developed by Kripa Varanasi, MIT associate professor of mechanical engineering, Gareth McKinley, School of Engineering Professor of Teaching Innovation, Gibum Kwon, former postdoc, Divya Panchanathan, graduate student, Seyed Mahmoudi, former research scientist, and Mohammed Gondal at the King Fahd University of Petroleum and Minerals in Saudi Arabia.

Initially, the project's goal was to find ways of separating oil from water. One example was to treat the frothy mixture of briny water and crude oil produced from specific oil wells since the more thoroughly these mixtures are combined, the harder they are to separate.

Others use electrostatic methods but these are energy-intensive and are not effective when the water is highly saline. Instead, the team shifted the focus on the use of "photoresponsive" surfaces, whose responses to water can be modified by exposure to light.

They created surfaces whose interactions with water could be activated by light. This property is known as "wettability."

The researchers found that they are able to directly separate the oil from the water by causing individual droplets of water to coalesce and spread across the surface. With this, the more that these water droplets fuse together, the more they separate from the oil.

An example of photoresponsive materials is titanium dioxide, also known as titania. It is the active ingredient in most sunscreens. These materials primarily respond to ultraviolet light, though.

The team used a layer-by-layer deposition technique to build up a film of polymer-bound titania particles on a layer of glass. Afterwards, they dip-coated the material using a simple, organic dye.

This resulted to a surface that was highly responsive to visible light and produced a change in wettability when it was exposed to sunlight. The material was found to be very effective at separating the oil and water.