Nano-tech coating lets soap products flow freely

Have you every found yourself standing with a near-empty bottle of hand soap, shampoo or laundry detergent, trying to get the last little bit of soapy material out? It can be rather frustrating.

Engineers at The Engineers at The Ohio State University have invented a coating to help soap pour out of plastic bottles more easily. Here, a drop of shampoo slides off a piece of polypropylene. Image from a video by Philip S. Brown, courtesy of The Ohio State University.

Ohio State University researchers have tackled this exact problem. Engineers Bharat Bhushan and Philip Brown have developed the perfect texture for inside plastic bottles to allow for soap products to flow freely.

Plastic bottles are lined 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 using small nanoparticles made of silica or quartz, that when treated, wont stick to soap. This method is extremely effective with the a common plastic packaging material, polypropylene.

The applications for this technology extend beyond grappling with personal annoyance. Manufacturers are very interested in the technology as they make billions of bottles that end up in the garbage with their products still in them, explains Bhushan, Ohio Eminent Scholar and Howard D. Winbigler Professor of mechanical engineering at Ohio State.

Although polypropylene isn’t the most common bottle material, 177 million pounds of it were made into bottles and bottle lids in the United States in 2014 alone, according to the American Chemistry Council (ACC). And as more and more polypropylene bottles hit the market, the more there is a need to recycle, which is where The Ohio State invention comes into play. Before plastic bottles can be recycled, they have to be rinsed completely clean, and Bhushan suspects that he’s not the only person who doesn’t bother.

“We all struggle with shampoo bottles at home,” Bhushan said. “I have a few in my shower right now. Trying to get the last drop out, I put it upside down, and my wife adds water to the bottle and fights with it for a while, and then we give up and just throw it away.”

The coating was specifically designed to deal with the unique challenge posed by soapy products.

The technology comes down to surface tension—the tendency of the molecules of a substance to stick to each other. The organic molecules that make soap “soapy” have a very low surface tension and therefore stick to plastic quite easily, explains Brown, a postdoctoral fellow.

“It was an extra challenge for us to make a surface that could repel surfactant,” he agreed. The goal, which was suggested by a commercial shampoo manufacturer, was to create a shampoo bottle lining that was cheap, effective and environmentally friendly.

Bhushan and Brown created a method to spray-coat a small amount of solvent and ultra-fine silica nanoparticles onto the inside of bottles. Manufacturers change the texture of molded plastics by using solvents to soften the surface of the bottle. The duo realized they could add the silica mixture with the solvent to soften the surface of the polypropylene enough that when the plastic re-hardened, the silica would embed in the surface.

The structures are only a few micrometers–millionths of a meter–high, and covered in even smaller branchlike projections. Due to the shape and properties, the structures don’t need to completely cover the surface, instead, they are planted micometers 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 repellency,” Brown said. This forced the soap droplets to bead and roll off.

www.osu.edu