A shampoo bottle that empties to the last drop  


New Delhi : Emptying the last bit left in a shampoo or detergent bottle can be annoying; but no more. Researchers, including an engineer of Indian origin, 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 appears in the journal Philosophical Transactions of the Royal Society. 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. Bharat Bhushan and Philip Brown took a lot of pain 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. '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 Bhushan, Ohio Eminent Scholar and Howard D Winbigler Professor of mechanical engineering at the Ohio State University. Coatings already exist to help food, but not soap, pour out of their containers, he said. "Compared to soaps, getting ketchup out of a bottle is trivial,” he said. “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. Their goal, which was suggested by a commercial shampoo manufacturer, was to create a shampoo bottle lining that was cheap, effective and environmentally friendly. Soap and shampoo clean our skin and hair by bonding chemically with both oil and water, so the surface oils that were on our bodies wash off when we rinse. The same goes for dishes. During clothes washing, detergent performs double-duty, releasing oils and also helping water penetrate fabrics. It's that tenacity that makes the last drops of surfactant cling to the insides of bottles. 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. "You end up with air pockets underneath, and that's what gives you liquid repellency," Brown said. Instead of spreading out on the surface, the soap droplets form beads and roll right off.

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