On the scale of curing cancer or stopping global warming, extracting the last drop of shampoo barely registers. But it is a frustration most of us are familiar with. Now, an answer has been found in the bottle material.
"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.' But manufacturers are really interested in this, because they make billions of bottles that end up in the garbage with product still in them," said Professor Bharat Bhushan of Ohio State University in a statement. The problem isn't just the waste of all those dollops of hair products, but the way the shampoo interferes with recycling. Similar problems apply to hand soap and detergent.
In Philosophical Transactions of the Royal Society A, Bhushan has announced a solution. Instead of modifying the shampoo, he has changed the bottle, lining the inside with tiny silica structures that create air pockets between the walls and the shampoo.
The idea is not entirely novel. Coatings have been designed that help get the last drops of foods such as ketchup from bottles. Bhushan told IFLScience that these are not yet available in the supermarket as “it takes 1-3 years to develop a commercial product from the science discoveries in the lab.”
Bhushan added: "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."
Soaps and shampoos are surfactants with much lower surface tensions than foods. Surfactants are molecules with one end that binds to water and another end to oils. They can grip the grease molecules we wish to remove and also get carried away by water.
This wonderful trait turns sour, however, when you want to get a surfactant out of a plastic bottle. The oil-gripping end also sticks to the walls. By keeping an air bubble between the shampoo and the wall, Bhushan's structures prevent this, allowing the shampoo to slide out under gravity.
The novel part of Bhushan's work is not the air bubble, something that has been demonstrated before, but the mechanism for producing it. Past work has been done carving shapes in the bottle's plastic in the same way that computer chips are cut; great as a proof of concept, but prohibitively expensive for mass production and not very durable.
Instead, Bhushan added silica nanoparticles and solvents to widely used plastics called polypropylene and polycarbonate, softening the material so that the silica embedded before re-hardening. The fact that the nanoparticles only incorporate near one surface reduces the danger they will affect the plastic's stability or strength.
Mass market products like shampoo might be where the money is, but Bhushan also has hopes of making a greater contribution to humanity. For medical equipment such as catheters, keeping the inside clean and free of obstruction can be a matter of life and death, and the same technique may prove applicable.
A comparison of laundry detergent on ordinary polycarbonate and polycarbonate treated with silica nanoparticles. Peter Brown Ohio State University