Technology & Science

Soap slides out of plastic bottles spray-coated with silica nanoparticles

The aggravation of not getting the last drop of shampoo out of the bottle may soon be a thing of the past, thanks to research on a microscopic coating that makes soap droplets bead and flow more easily.

Ohio State researchers get around problem of surfactants, which make soap cling to surfaces

Shake, shake, shake it out (Philip S. Brown and Joe Camoriano/Ohio State University)

Tipping a bottle of shampoo upside down and shaking hard to use it up may some day be a thing of the past, according to researchers at Ohio State University.

They're working to develop a microscopic coating that makes the soap droplets bead and easily roll off the sides of a plastic container. 

To achieve this slip, they lined a plastic bottle with microscopic y-shaped structures that cradle the droplets of soap aloft above tiny air pockets.

With this technique in place, they say the soap never actually touches the inside of the bottle, so it flows freely.

The "y" structures are built up using nanoparticles made of treated quartz or silica, the main ingredient in glass.

The molecules that make soap "soapy" — called surfactants — also make them stick to plastic because they lower surface tension between two liquids, or between a liquid and a solid.

While surfactants play a key role as wetting and foaming agents, as well as dispersing dirt particles, they make the last drop of the liquid containing these compounds cling to the insides of bottles.

The microscopic coating solves the problem, said engineer and researcher Philip Brown, who described their patent-pending technology in a paper to appear Monday in the journal Philosophical Transactions of the Royal Society.

Brown worked with fellow engineer Bharat Bhushan to come up with a method to spray-coat ultra-fine silica nanoparticles, mixed with a small amount of solvent, onto the inside of bottles. 

Manufacturers already use solvents to change the texture of molded 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 became embedded in the surface.

Brown and Bhushan say their work should do more than solve a trivial problem for manufacturers and consumers. With further development, the university hopes to license the coating technique for other plastic products that have to stay clean, such as biomedical devices or catheters.