Like getting your money’s worth, even down to the very last drop? Of course you do, especially when it comes to that annoying last splodge of shampoo that refuses to budge from the bottom of the bottle as you frantically try to squeeze it out.
Now, scientists from Ohio State University have developed a way to create the perfect texture for the inside of bottles that lets soaps, such as shampoo or washing detergent, flow freely over it. This could mean the end of filling the bottle with a bit of water and swishing it around to get a frothy, watery liquid at the end of it.
Unlike food products like tomato ketchup, which are mostly water based and can slide out of your plastic bottle onto your fish and chips with ease, soapy products contain surfactants, which help lift the dirt and grime off your body but also stick to plastic. The new technique, published in the journal Philosophical Transactions of the Royal Society, uses a spray that lines the inside of the plastic bottles with y-shaped silica structures only a few micrometres high, which cradle droplets above tiny air pockets so that the soap never actually touches the side of the bottle.
Previous efforts at solving the problem have involved manually carving shapes into the bottle using photolithography, the same technique to create computer chips, but this surfaces its own problems:
“That’s expensive and time consuming,” says co-author Philip Brown. “Plus, they end up with fragile little overhangs that snap off. We embedded a hard material directly into the polymer surface, so we know it’s durable.”
It might sound like a first-world problem but with more than 20 million kilograms of soapy bottles piling up every year in the US alone, it causes a serious headache for recycling because they need to be completely clean on the inside before they can be processed.
“We all struggle with shampoo bottles at home,” says Bharat Bhushan, co-author of the study. “I have a few in my shower right now. Trying to get the last drop out, I put it upside down, and my wife adds water to the bottle and fights with it for a while, and then we give up and just throw it away.”
This latest discovery will make shampoo bottles considerably easier and cheaper to recycle and further development aims to use the technique in medical devices such as catheters, and even on car headlamps and smartphone cases.