Getting the last bit of hand soap, shampoo or laundry detergent out of the bottle can be an annoying task but thanks to researchers at Ohio State University it could soon be a thing of the past.

This is because they 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 that appeared in the journal Philosophical Transactions of the Royal Society on June 27.

How they solved the problem

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.

Engineers Bharat Bhushan and Philip Brown went to a lot of trouble to solve this problem but the solution they found is actually simpler and less expensive than alternatives under development elsewhere. And it works for a common plastic used to package foodstuffs and household goods: polypropylene.

Coatings already exist to help food, but not soap, pour out of their containers, according to Bhushan, Ohio Eminent Scholar and Howard D. Winbigler Professor of mechanical engineering at Ohio State.

“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 key, he explained, 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, explained Brown, a postdoctoral fellow.

“It was an extra challenge for us to make a surface that could repel surfactant,” he agreed.

The method to make it work

Bhushan and Brown came up with a method to spray-coat a small amount of solvent and ultra-fine silica nanoparticles onto the inside of bottles. Manufacturers already use solvents to change the texture of moulded plastics, because they cause the surface of the plastic to soften a little.

By mixing the silica and solvent, the researchers were able to soften the surface of the polypropylene just enough that when the plastic re-hardened, the silica would be embedded in the surface.

The structures are only a few micrometers–millionths of a meter–high, and covered in even smaller branchlike projections.

They don’t cover the inside of the bottle completely, either, but instead are planted a few micrometers apart. The main branches of the “y” overhang the plastic surface at an angle less than 90 degrees–steep enough that water, oils and even surfactant can’t physically sustain a droplet shape that would fall in between the branches and touch the plastic.

“You end up with air pockets underneath, and that’s what gives you liquid repelling effect,” Brown said.

Instead of spreading out on the surface, the soap droplets form beads and roll right off.