Having gathered moss, water drops roll

Ordinarily, water drops dribble like tears down a slanting surface, or they simply get stuck. Now a modified type of water drop rolls like a ball. It may even morph into a hoop or peanut shape.

Coated with powdered moss, a drop of water floats on water. Aussillous and Quéré/Nature

These “liquid marbles” represent a “fundamentally new state for a drop,” says physicist David Quéré. He and Pascale Aussillous, both of the Collège de France in Paris, created the curious, millimeter-size marbles by coating water drops with a powder made from the water-repellent moss Lycopodium. The drops float on water, the researchers report in the June 21 Nature.

Such powdered drops may already exist in nature. Quéré says he has heard of a phenomenon in southern France known as “summer ice”–a slick coating that forms when rain sprinkles dust-and-pollen-covered roads.

Because the Lycopodium-powdered drops dart around quickly and leave no trace, they may aid chip-size labs that shunt liquid chemicals around in microscale plumbing, Quéré says (SN: 8/15/98, p. 104). He and Aussillous also have used electric and magnetic fields to propel the liquid marbles.

From a pure-physics perspective, the drops seem to obey their own laws: Among solid balls, bigger ones roll faster down an incline. Yet, with the drops, the little ones get to the bottom first. A slight flattening of the squishy drops causes disproportionately large friction on bigger drops compared with smaller ones, Quéré says. For such semisolid drops, he adds, “the things we knew about moving drops are totally changed.”

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