Materials known as yield-stress fluids, including gels and pastes, can be found in consumer products such as food, condiments, and cosmetics, and in products in the energy and pharmaceuticals industries. Unlike other fluids such as water and oils, these materials will not start to flow on their own, even when their container is turned upside-down. Starting the flow requires an input of energy, such as squeezing the container.
But that squeezing has its own effects; for example, bread-making machinery typically includes scrapers that constantly push the sticky dough away from the sides of its container but that constant scraping can result in over-kneading and a denser loaf. A slippery container that requires no scraping could thus produce better-tasting bread. This can have great impact on pharmaceuticals — the use of mechanical scrapers to propel drug materials through mixing tanks and pipes can interfere with the effectiveness of the medicine because the shear forces involved can damage the proteins and other active compounds in the drug.
Researchers developed slippery coatings called liquid-impregnated surfaces that could eliminate production waste that results from material that sticks to the insides of processing equipment. They might also improve the quality of products ranging from bread to pharmaceuticals, and even improve the efficiency of flow batteries, a technology that could foster renewable energy by providing inexpensive storage for generated electricity.
The new surfaces are based on a combination of a specially textured surface and a liquid lubricant that coats the surface and remains trapped in place through capillary action and other intermolecular forces associated with such interfaces. The fundamental design principles can achieve almost 100 percent friction reduction for these gel-like fluids.
The new process begins by making a surface that is textured at the nanoscale, either by etching a series of closely spaced pillars or walls on the surface, or mechanically grinding grooves or pits. The resulting texture is designed to have such tiny features that capillary action — the same process that allows trees to draw water up to their highest branches through tiny openings beneath the bark — can act to hold a liquid, such as a lubricating oil, in place on the surface. As a result, any material inside a container with this kind of lining essentially only comes in contact with the lubricating liquid, and slides right off instead of sticking to the solid container wall.