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Keywords:

  • colloids;
  • liquids;
  • polymers;
  • surfaces;
  • water

Abstract

Thumbnail image of graphical abstract

Little hovercrafts: The mechanisms of floating and sliding of liquid marbles (see picture) are elucidated. It is demonstrated that floating and sliding liquid marbles are separated from their supporting media by an air layer, resembling Leidenfrost drops. An ESEM study of the marble surface shows a rugged surface facilitating the trapping of air.

The mechanisms of floating and sliding of liquid marbles are studied. Liquid marbles containing CaCl2 and marbles containing NaOH water solutions float on water containing Na2CO3 and an alcoholic solution of phenolphthalein with no chemical reaction. Sliding of liquid marbles, consisting of NaOH water solutions, on polymer substrates coated with phenolphthalein is studied as well. No chemical reaction is observed. These observations supply direct experimental evidence for the suggestion that interfaces are separated by an air layer when marbles roll on solid substrates. It is concluded that a liquid marble rests on hydrophobic particles coating the liquid. In contrast, drops containing an NaOH water solution sliding on superhydrophobic surfaces coated with phenolphthalein leave a colored trace. The mechanism of low-friction sliding of drops deposited on superhydrophobic surfaces and liquid marbles turns out to be quite different: there is no direct contact between liquid and solid in the case of marbles’ motion.