On the Mechanism of Floating and Sliding of Liquid Marbles

Abstract

The mechanisms of floating and sliding of liquid marbles are studied. Liquid marbles containing CaCl(2) and marbles containing NaOH water solutions float on water containing Na(2)CO(3) 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.

Formation of liquid marbles and wetting transitions

“…Liquid marbles were prepared as described earlier [17][18][19][20][21][22][23]. 20 lL water and water/ethanol solution drops were deposited with a precise micro-dosing syringe onto a superhydrophobic surface, prepared by the hot embossing process with use of a polyethylene substrate and PTFE nanobeads, as described in Ref.…”

“…Liquid marbles, introduced in the pioneering study carried out by Quéré et al [1,2] are droplets enwrapped by hydrophobic particles or aggregates of hydrophilic particles [31,32]. The marbles are separated from a solid or liquid support by air pockets in a way similar to that of Leidenfrost droplets [1,2,19]. These air pockets determine the extremely low friction properties of liquid marbles [33].…”

Interpretation of elasticity of liquid marbles

“…Stimulus (pH, UV and IR) responsive liquid marbles were reported by Dupin, Fujii et al [28][29][30]. It is noteworthy that liquid marbles retain non-stick properties on a broad diversity of solid and liquid supports [31]. Actually, liquid marbles are separated from the support by air cushions in a way similar to Leidenfrost droplets [32].…”

Liquid marbles: Physics and applications

“…Such liquid marbles are susceptible to rupture. The existence of this air layer was also experimentally confirmed by Bormashenko et al (2009a). They performed an elegant experiment in which a marble of liquid NaOH was allowed to come in contact with a substrate containing phenolphthalein.…”

“…Because there is no direct interaction between the liquid and the surface, the effective surface tension of these liquid marbles will differ from the surface tension of the pure liquid. Liquid marbles are even able to float on liquids without mixing with them as shown by Bormashenko et al (2009a). They merely cause a depression on the liquid surface.…”