Soap that slides off the bottles

We all know how hard it is to get the last bit of shampoo out of the bottle. You turn it upside down, you bang on the bottom, but you never can get everything to come out.

The obvious, annoying problem here is that soap sticks to plastic, but new research by Ohio State University engineers Philip S. Brown and Bharat Bhushan suggests a possible solution. The duo has invented a treatment that makes polypropylene—the "number 5" plastic so often used for shampoo and detergent containers—repel soap. The treatment spikes the plastic surface with y-shaped structures made of silica, a component of glass and sand, which prevent the soap from touching and sticking to the plastic. Instead, according to a university press release, the soap rolls along on the silica spikes and pockets of air. The new surface also repels water and oil.

"It's what you'd call a first-world problem, right? 'I can't get all of the shampoo to come out of the bottle,'" Bhushan says in the release. "But manufacturers are really interested in this, because they make billions of bottles that end up in the garbage with product still in them." In addition to reducing waste, containers made out of soap-resistant material would be easier to clean, making for more efficient plastic recycling. 

This slippery polypropylene is one of many recent advancement in oil- and water-resistant materials. For example, the coating called LiquiGlide, originally developed in an MIT lab, makes surfaces permanently wet and slick so that normally recalcitrant condiments like mayonnaise and ketchup slip right out of bottles treated with the stuff. The product and company of the same name made the news last year when Elmer's agreed to use the product in its glue bottles. Meanwhile, researchers at University of California, Los Angeles (UCLA), are developing ultra-water resistant surfaces out of etched silica. 

To understand how such slippery surfaces work, you have to think about surface tension, which is reflected in how much a substance will bead up on a given surface. Shampoo sticks to a plastic bottle in part because it has a lower surface tension than the bottle itself, Bhushan told PopMech in an email. Rather than forming droplets and rolling away, the soap tends to spread out and stick because, according to the laws of nature, the substance of lower surface tension winds up on, well, the surface. Texture also plays a role: Rougher surfaces, on which a substance has more contact with air, tend to be more slippery. For example, water rolls off the lotus leaf, the prototypical water-resistant surface, because it is covered in microscopic bumps. This called the lotus effect, and it's a principle at work in the etched silica from UCLA and also the new surface from OSU.

To create their sleek surface, the OSU researchers treated the polypropylene with nanoparticles of silica and another substance (fluorosilane) that further lowers the surface energy. The result? A substance that has low surface energy and is also rough, due to the y-shaped silica structures.

The researchers tested the slipperiness of their specially coated surfaces by adding droplets of an oil (hexadecane), which rolled right off. Next they tried shampoo, and at first, it dribbled away, too. However, they noted that if they left the shampoo on the surface for longer—and they did not quantify how much longer, Bhushan said—it starts to stick. 

"More work is required to fully achieve shampoo repellency for polypropylene surfaces in constant contact with shampoo such as the inside of a typical consumer bottle," the authors wrote in a paper detailing their results, which was published today in the scientific journal Philosophical Transactions of the Royal Society A. Before the shampoo revolution, the slippery-bottle tech may need some tweaking.