How slippery surfaces allow sticky pastes and gels to slide

A Massachusetts Institute of Technology research team that has already conquered the problem of getting ketchup out of its bottle has now tackled a new category of consumer and manufacturing woe: how to get much thicker materials to slide without sticking or deforming. The slippery coatings the team has developed, called liquid-impregnated surfaces, could have numerous advantages, including eliminating production waste that results from material that sticks to the insides of processing equipment.

Almost 100 percent friction reduction

These surfaces are based on principles initially developed to help foods, cosmetics, and other viscous liquids slide out of their containers. The new paper explains the fundamental design principles that can achieve almost 100 percent friction reduction for gel-like fluids. Such materials are ubiquitous. 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. By using the new coatings, in some cases it’s possible to achieve a 100 percent reduction in the drag the material experiences — equivalent to “infinite slip,”  says one of the researchers.

Like the earlier version of slippery surfaces, 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 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.

Further information on the website of the Massachusetts Institute of Technology.