‘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 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. This liquid marble is completely mobile and rolls freely on solid (and water) surfaces.
Professor Quéré and co-workers have shown that small liquid marbles roll down hill faster than larger ones, which is the opposite of what would be expected for solid marbles.
Liquid marbles occur naturally and are used by galling aphids as a waste disposal system for the honeydew they secrete – read more about “How Aphids Lose Their Marbles” here.
Electrowetting of non-wetting liquids and liquid marbles
G. McHale, D.L. Herbertson, S.J. Elliot, N.J. Shirtcliffe and M.I. Newton, Langmuir. 23 (2) (2007) 918-924
Liquid Marbles: Principles and applications
G. McHale and M.I. Newton, Soft Matter. 7 (12) (2011) 5473-5481