Liquid Pearls and Marbles

FIGURE 9.12. Ideal textured surface obtained by a molding process. (From. J. Dico, D. Marzolin, and U. Quore, in Evrophysics Letters.. 7. p. 220 (1999). FDP Sciences. Reprodticed by permission.) 

Problem Show that the fakir state is metastabh on a surface such as in Fig. 9.12. in most cascis. [For that, c-onipare the iiiterfacial eiu-rgy of th( Wenzel state and that of tlu Chtssie states].  

FIGURE 9.13. Naturally occurring super-hydrophobic surfaces, as seen in an electron microscope. Shown are a drosera loaf (left) and a lotus leaf (right). The horizontal bars represent 5 i.m and 20 im, respectively. (From C. Neinhuis and W. Barthlott, in Annals of Botany, 79, p. 667 (1997), published by Academic Press. Reproduced by permission.) 

Finally, there is a third category of pearls—the so-called Leidenfrost drops. The method to create them involves placing a water drop on a very hot plate (typically 300°C). The drop will retain a spherical shape because of the vapor film that supports it. Needless to say. the drop evaporates, but because the vapor film is a good thermal insulator, the evaporation is slow (of the order of a minute for a millimeter-size drop). This phenomenon has been known for a long time (Leidenfrost, 1756), and has been discussed by Boiiassc under the name spheroidal state. Nevertheless, many questions persist (such as the thickness of the underlying film and the lifetime of the drop). 

FIGURE 9.14. Liquid marble obtained by niixing clay (a super-hydrophobic powder) and water. The resulting object, a millimeter in size in the present case, can subsequently be transferred onto a substrate (here glass) that would normally be wetted by water, were it not for the protection of the powder shell. (From P. Aussillous and D. Quere, in Nature, p. 924 (2001). (c) 2001 McMillan Magazines, Ltd. Reproduced by permission.) 

---