It’s one of life’s little annoyances: that last bit of shampoo that won’t quite pour out of the bottle. Or the last bit of hand soap, or dish soap, or laundry detergent.
Now researchers at The Ohio State University have found a way to create the perfect texture inside plastic bottles to let soap products flow freely. They describe the patent-pending technology in a paper to appear in the journal Philosophical Transactions of the Royal Society on June 27.
The technique involves lining a plastic bottle with microscopic y-shaped structures that cradle the droplets of soap aloft above tiny air pockets, so that the soap never actually touches the inside of the bottle. The “y” structures are built up using much smaller nanoparticles made of silica, or quartz – an ingredient in glass—which, when treated further, won’t stick to soap.
The key is surface tension—the tendency of the molecules of a substance to stick to each other. Ketchup and other sauces are made mostly of water, and water molecules tend to stick to each other more than they stick to plastic. But surfactants – the organic molecules that make soap “soapy” – are just the opposite: They have a very low surface tension and stick to plastic easily.
With further development, the university hopes to license the coating technique to manufacturers—not just for shampoo bottles, but for other plastic products that have to stay clean, such as biomedical devices or catheters. They have already applied the same technique to polycarbonate, a plastic used in car headlights and smartphone cases, among other applications.