Getting that last bit of shampoo or washing up liquid out of a bottle is a total pain. Researchers have now engineered a surface coating that allows thick and soapy products to slide right out — meaning you’ll never have to store your shampoo bottle upside down ever again. In a new paper, Ohio State University researchers Bharat Bhushan and Philip S. Brown describe a new surface coating that enables sticky liquids like shampoo and oil to slide cleanly out of a bottle. Importantly, this super-repellent surface works with polypropylene, a common plastic that’s used to package food and other consumer products.
Surface coatings already exist to help food pour of their containers, but as Bhushan pointed out in a press statement, “Compared to soaps, getting ketchup out of a bottle is trivial. Our coating repels liquids in general, but getting it to repel soap was the hard part.”
The issue has to do with surface tension, which is the tendency of molecules to stick to each other. Ketchup and other sauces have a lot of water in them, and water molecules like to hold on to each other more than they like to stick to plastic. Soapy compounds, on the other hand, have a very low surface tension and stick to plastic quite easily. Scientists have struggled to find the right formula to enable these surfactants to just slide off. At the same time, any solution would also have to be environmentally friendly and cheap.