Do you care about lost shampoo? Maybe not. After all, you're on the consumer end of the supply chain: naked, half awake and pounding the butt of a 'poo bottle for the last few drops of hair-cleansing goodness. Unless you use the $100 salon stuff, what difference does it make?
But imagine yourself on the other end of the chain, bottling up shampoo and soap products with the knowledge that some of the product is just there to cling to the sides and make the recycling bin smell better. Not very economically sound, right?
Never keen to waste their product, manufacturers have been improving the container-based delivery of their viscous goods for a while now, from ketchup to toothpaste. But soap and shampoo have always offered a unique problem due to the very chemical properties that make them so cleansing.
What makes soap "soapy" also makes it stick to plastic. It's a surfactant, which means that it acts to lower the surface tension of liquids, allowing grime and oils to wash off our bodies. It works out great for your hair, but it also causes the bottled soap to spread out and hang on for dear life inside the plastic bottle.
Ohio State University engineers Bharat Bhushan and Philip S. Brown have an answer. In a paper published in the journal Philosophical Transactions of the Royal Society A, they lay out an affordable means to line the inside of plastic (specifically polypropylene) shampoo and soap bottles with ultra-fine silica nanoparticles. They use solvent to temporarily soften the inside the bottle, allowing the microscopic y-shaped structures to embed in the plastic.
Zoom in enough and, according to engineers, these structures resemble "shaggy heart-shaped pillows" that overhang the surface of the plastic, creating tiny air pockets. The structures physically prevent the soap from sustaining the droplet shape necessary to drip through and cling to the plastic. They're kind of like those pigeon-repelling spikes you see on walls and buildings. There's simply nowhere to land.
The immediate application here is a better shampoo bottle, but the researchers think the technique might benefit biomedical devices, catheters and anything else that needs to stay clean in the face of viscous substances. You can't always just add water and shake it.