Water is a fabulous cleaning agent, but it’s even better when tiny nanoscale particles, such as detergent surfactants, are dispersed in it. Much of this cleaning boost comes from the enhanced capacity of the detergent solution to spread out on surfaces, but scientists haven’t fully understood the details.
In the May 8 Nature, Darsh T. Wasan and Alex D. Nikolov of the Illinois Institute of Technology in Chicago report the mechanism by which a particle-containing fluid spreads out and readily lifts oil droplets off a surface. This knowledge could improve the design of a variety of detergents, says Wasan.
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It could also be useful for the commercial extraction of oil from the ground or the remediation of oil-fouled soil, comments Manoj Chaudhury of Lehigh University in Bethlehem, Pa. Or, he suggests, cosmetics manufacturers might employ the new findings to discover new ways to stabilize emulsions that are made of oily droplets suspended in water.
When a fluid containing tiny, uniform spheres spreads across a surface and encounters an oil drop–or an air bubble, for that matter–the spheres concentrate and organize themselves, packing tightly into what effectively serves as a wedge between the sphere and the solid surface, says Wasan. Consequently, the rest of the fluid has a lower concentration of nanospheres than the wedge does and will naturally flow toward it to equalize the concentration. This flow, in combination with the wedge structure, lifts the oil drop, and fluid flows underneath it.
Wasan and Nikolov used video microscopy to observe this process directly. First, they placed charged, 1-micrometer latex spheres in water and watched as the particles arranged themselves into a crystal-like structure wedged into the space where an air bubble met a solid surface. Next, the researchers observed a fluid containing charged, 8-nanometer-wide surfactant spheres, called micelles, as it spread under an oil drop and lifted it from the surface.
In a final experiment, the researchers added salt to a micelle-filled fluid and were surprised to find that it could no longer lift an oil drop. This happened, Wasan and Nikolov suggest, because the salt altered the micelles and they no longer packed into the wedge area.
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The results could be of value to industry, suggests Wasan. For example, most commercial detergents contain surfactant or micelle particles of various sizes, but the new findings suggest that a fluid may be most effective at removing oil if the particles are all the same, small size for tight packing.
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