Super Plastic Both Attracts and Repels Water

A new, practical method for making surfaces with patterns of areas that strongly attract and strongly repel water could lead to a highly efficient method for capturing clean water. This versatile material could also find uses in fabricating new types of devices for medical tests and chemical synthesis.

Scientists have reported numerous applications of water-attracting (superhydrophilic) and water-repelling (superhydrophobic) surfaces, including fog-free eyeglasses and windshields, and self-cleaning cloth and glass. Now a group of researchers in MIT’s materials science and engineering department has combined those opposing characteristics on a single surface, by using a simple and versatile fabrication process.

[For images of this new dual-quality material, click here.]

Robert Cohen, Michael Rubner, and colleagues started by assembling a nano-structured film made of alternating layers of positively and negatively charged polymers and silica nanoparticles. The film’s structure and a coating of waxy fluorinated silane cause water to bead on it, forming near-perfect spheres that easily roll off. To add the superhydrophilic regions (to which water droplets cling), the researchers applied a naturally hydrophilic polymer to selected areas.

In dry regions of the world, without easy access to clean water, such a material could be used for collecting water. In this application, the hydrophilic areas of the material would attract moisture in the air, collecting water drops that accumulate, until they spill over into the hydrophobic regions and roll into a collecting channel. Currently, in countries with limited access to clean water, the inhabitants typically use large polypropylene fiber meshes to harvest water from fog.

The new technology “would provide a more than tenfold increase in water capture compared to the inefficient nets that are used currently,” says Andrew Parker, a biologist at Oxford University and the Natural History Museum in London, who has studied the desert beetle that inspired the MIT work. If the new material “could be added simply to the roofs of houses in areas subjected to desert fogs,” says Parker, “then a water supply could be gained with little effort.”

Rubner’s lab is also taking the technique further. “When we harvest water, we have chemistry built into the hydrophilic area so that it has an antibacterial agent to kill off bacteria and other things that cause harm,” Rubner says. This decontaminates the water as it accumulates so that the collected water is safe for use. Applying this technique, the researchers have been able to kill common harmful bacteria in four minutes, he says.

The coating could also find uses in biomedical applications to make microfluidic chips. Typically, microfluidic devices contain enclosed micrometer-wide channels etched into silicon, glass, or plastic plates. Then pressure or electric fields drive tiny volumes of fluids, typically nanoliters, along these channels for diagnostic tests and genetics research. For instance, to test for the presence of a certain protein in blood you could take blood in one channel and direct it to another channel containing a chemical reagent that identifies the protein.

Compared with conventional microfluidics, a microfluidic chip based on the new surface would have the advantage of easier mixing, Rubner says. Right now, the chips need pumps and valves that move the liquid around to induce mixing. “In our case you can mix the liquids by just controlling the amount of liquid you put on the surface,” he says. With a pipette, you could add precise amounts of fluid into two hydrophilic grooves placed close to each other. As you add more fluid, the droplets bulge out at the edge of the grooves because of the surrounding hydrophobic area. Eventually, the bulging surfaces touch and mix. Being able to confine liquids to a small region could provide densely packed reaction sites with more control over the reaction, he says, since adjacent drops won’t mix unless they are forced to.

While the exact uses of this new material are still uncertain, it opens up many possibilities, says Kenneth Wynne, a chemical engineering professor at Virginia Commonwealth University. “Patterning ultra-hydrophilic patches on a ultra-hydrophobic surface in this way is new and useful,” he says.