Sometimes scientists aim to answer long-standing mysteries, like the shared evolutionary origin of fur and feathers. Sometimes they can perform technological marvels, like detecting the gravitational waves from black holes on a collision course, or reading voters’ brain waves to determine their preference for political candidates.
But, occasionally, the problem at hand is a bit more pedestrian.
Consider the struggle of trying to get that very last dab of soap out of a shampoo bottle. Perhaps you’re the type to pound the bottom of the bottle, or you rely on gravity to draw the soap down into an explosive final burp. Or you dilute what remains to a weak froth. None of these, in the eyes of Ohio State University engineering professor Bharat Bhushan, are ideal.
“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 said in a statement. “But manufacturers are really interested in this,” he added, “because they make billions of bottles that end up in the garbage with product still in them.”
Bhushan and his colleague, Ohio State mechanical engineer Philip Brown, say they have created a new coating that could decrease the amount of soap that finds its way to the landfill. In a study published Monday in the journal Philosophical Transactions of the Royal Society, the two scientists outline how they borrowed from nature — specifically, the slippery leaves of the lotus plant — to create a better shampoo bottle surface.
The property of shampoos and soaps that makes them valuable cleaners also means the chemicals want to adhere to their containers. Compared with water, soapy liquids have low surface tensions. Whereas water is attracted to other water molecules, and therefore pours out in a ready cascade, soap molecules bind to water on one side and cling to particles of oil or grease on the other. Or, in the case of a bottle, the container surface. The Ohio State University engineers needed to cheaply create a coating that the soap molecules’ sticky tails would not find so attractive. So they cribbed from nature: the lotus leaf, which is riddled with little pockets of air.
To create a synthetic lotus leaf, the researchers sprayed quartz nanoparticles over plastic. What was once smooth is now rough and craggy. Under magnification, it looks as though the surface has been studded with rows of candy hearts carved from volcanic rock. These puckered surfaces entrap air, which provides little purchase for grippy soap.
“You end up with air pockets underneath,” Bhushan said, “and that’s what gives you liquid repellency.”
Massachusetts Institute of Technology mechanical engineer Gareth McKinley told the New York Times that this was the first time anyone has been able to create a soap-repelling surface that works with common polypropylene plastic. However, McKinley sees a possible a wrinkle — to get the quartz particles to embed themselves into polypropylene, the Ohio State researchers use a fluorine-containing compound. The industry is trying to move away from fluorinated materials, McKinley said to the Times, because of their environmental toxicity.
Despite the use of fluorine compound, Bhushan hopes this bottle coating will lead to improved recycling, as the containers would have less soap inside. If successful, the quartz spray would join MIT’s LiquiGlide — a pebbled coating that propels mayonnaise out of its bottle on a bed of liquid lubricant — in transitioning out of the lab and into the supermarket aisle.