A bottle that saves shampoo

Getting to the bottom of a shampoo bottle is never fun. Some of us let the bottle sit upside down. Some of us add water, hoping the remaining shampoo will glide out more easily. And some of us just give up and toss the old bottle.

For Bharat Bhushan and Philip S Brown, engineers at Ohio State University, the solution is a bit more complex. In a paper published Sunday, the scientists reported that they had created a plastic surface that can repel sticky liquids like shampoo and oil.

Creating surfaces that can repel oil and shampoo is hard, and "this is the first time someone's demonstrated that you can do it in a very cheap, very common plastic," said Gareth McKinley, a professor of mechanical engineering at MIT who was not involved in the study.

Materials like shampoo and oil are hard to repel because of surface tension, or the attractive force among the molecules in a liquid. In a fluid with high surface tension such as water or mercury, molecules want to stick together. It's the reason water beads into droplets on a solid surface.

But when a liquid with low surface tension like shampoo meets a solid surface, its molecules want to spread apart and stick to solid molecules rather than to one another.

Surfaces that manage to repel liquids with low surface tension are called superoleophobic. They are difficult to find in nature. Making a superoleophobic surface requires some ingenuity. In particular, such surfaces must be created with two traits. First, they need a particular "roughness" that creates air pockets between the solid surface and the oil droplets. Second, they require a chemical coating with low surface energy.

To achieve roughness, the Ohio State researchers embedded tiny particles of silica into their plastic. To do so, they applied the particles to the polypropylene surface using a liquid that could dissolve the top layer of plastic. For the chemical coating, they used a compound of fluorine, called fluorosilane, to coat the rough polypropylene they created.

Part of the novelty of this approach is how durable the surface is, Bhushan said. Many previous attempts relied on weakly bonded coatings to achieve roughness. This polypropylene is so durable, he added, because the silica structures used to create roughness are embedded in the plastic, rather than sitting on top of it.

The fluorosilane coating the researchers used, however, is not the most environmentally friendly.

Nevertheless, Bhushan is proud of his feat. The rarity of superoleophobic surfaces in nature speaks to how difficult they are to achieve.

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© 2016 The New York Times