Impossible, right? Squeezing the bottle every which way you can until your fingers go numb doesn't do it. In fact, the only way to get the last bit of shampoo out is the cut the bottle so you can scoop it out. Our guess is that, rather than go through all that trouble, you're simply throwing it out and opening a fresh bottle.
It's annoying and a waste of product and money.
Luckily, a group of researchers at the Ohio State University have found a way to create a texture inside plastic bottles that lets soap products flow freely. The bottle is lined with microscopic y-shaped structures that cradle soap droplets above tiny air pockets, so 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.
It's a solution that cuts back on frustration and waste. We're not only talking shampoo, either. It works for foodstuffs and household goods that are packaged in polypropylene bottles. And the team points out that it's actually simpler and less expensive than alternatives under development elsewhere.
"It's what you'd call a first-world problem, right? Not being able to get all the shampoo 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," says Bharat Bhushan, Ohio Eminent Scholar and Howard D. Winbigler professor of mechanical engineering at Ohio State.
The key, he explains, is surface tension — the tendency of the molecules of a substance to stick to one another. Ketchup and other sauces are made mostly of water, and water molecules tend to stick to one another more than they stick to plastic.
But the organic molecules that make soap "soapy" — called surfactants — are just the opposite. They have a very low surface tension and stick to plastic easily, adds Philip Brown, a postdoctoral fellow. "It was an extra challenge for us to make a surface that could repel surfactant."
The duo's goal, which was suggested by a commercial shampoo manufacturer, was to create a shampoo bottle lining that was cheap, effective and environmentally friendly.
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 high and covered in even smaller branchlike projections. They look like shaggy heart-shaped pillows, but they're as hard as glass. They don't cover the inside of the bottle entirely, either, but are instead 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 falls in between the branches and touches the plastic.
"You end up with air pockets underneath, and that's what gives you liquid repellency," Brown says.
Instead of spreading out on the surface, the soap droplets bead up and roll right off.
Researchers have known for some time that a support structure with the right angle of overhang would solve this problem. Some have actually tried to carve the shapes into plastic manually using photolithography.
"That's expensive and time consuming," Brown explains. "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."
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. They have already applied the same technique to polycarbonate, a plastic used in car headlights and smartphone cases.
The researchers describe this patent-pending technology in a paper to appear this week in the journal Philosophical Transactions of the Royal Society.
Video by Philip S. Brown and Joe Camoriano, courtesy of the Ohio State University.