Liquid Marbles

reflected liquid marble,

‘Hydrophobic’ is usually taken to mean ‘water-fearing’, but hydrophobic grains stick to the surface of water. If the grains forming a hydrophobic surface are loose, rather than fixed, a droplet of water rolling across the surface will become encapsulated by the grains. This is known as a liquid marble can be formed in which a ‘ball’ of water is encased in a granular coating. A droplet of water rolling across the surface becomes encapsulated by the grains., as illustrated schematically in the image below.

A schematic picture of a liquid marble

Liquid marbles are completely mobile and rolls freely on solid (and water) surfaces, as seen below.

A liquid marble rolling on a surface
Liquid ‘marble’ rolling on a surface. (Courtesy Professor David Quéré).
slowly-rolling liquid marble
Slow liquid marble (side view). (Courtesy Professor David Quéré).
Fast-rolling liquid marble
Fast liquid marble (side view). (Courtesy Professor David Quéré).

Professor Quéré and co-workers have shown that small liquid marbles roll down hill faster than larger ones, as shown below. This is the opposite of what would be expected for solid marbles.

A race between liquid marbles
Marble race. (Courtesy Professor David Quéré).

Liquid marbles occur naturally and are used by galling aphids as a waste disposal system for the honeydew they secrete – read more about gall aphids here.