DOI: 10.1002/cphc.200800746 On the Mechanism of Floating and Sliding of Liquid Marbles Edward Bormashenko,* [a] Yelena Bormashenko, [a] Albina Musin, [a] and Zahava Barkay [b] 1. Introduction The discovery of the self-cleaning water-repelling properties of lotus leaves inspired intensive research in the field of “non- stick” surfaces in the past decade. [1–4] Non-stick solid/liquid in- terfaces posess promising technological applications in medi- cine,biotechnology, chemical, mechanical engineering and an- alytics. [1–6] There are two main approaches for the design of non-stick surfaces. The first is the biomimetic approach, imitating the highly de- veloped hierarchical relief of biological objects such as the lotus leaves already mentioned. [2–12] The “lotus effect”-based approach uses modification of the solid surface promoting air trapping and a consequent increase in the apparent contactangle. [2–12] The alternative approach exploits modification of the liquid in- terface with hydrophobic colloidal particles, such as lycopodium, clay or polyvinylidene fluoride. [13–24] These particles, absorbed at the liquid/airinterface, lead to the formation of liquid “mar- bles”—liquid drops covered with micro- or nanoparticles with a low surface energy. [13–24] Liquid marbles are also found naturally. For example, aphids convert honeydew droplets into marbles. [17] This approach, while also allowing various technological applica- tions (including the ferrofluidic ones discussed in ref. [23]), is much lessstudied.Qur and coauthors already noted that liquid marbles demonstrate very unusual friction when deposit- ed on solid substrates. [13, 14] It was supposed that there is no direct contact between the liquid and solid and that the liquid marble really sits on the hydrophobic particles as depicted in Figure 1. [13, 14] Our study supplies direct microscopic and macro- scopic experimental evidence of the fact that liquid and solid surfaces are separated by an air layer when a liquid marble is deposited on the solid surface. Experimental Section Floating of Marbles: The water marbles were manufactured accord- ing to experimental procedures presented in refs. [23, 24] . For man- ufacturing marbles we used polyvinylidene fluoride (PVDF) nano- beads.PVDF nanobeads with a molecular weight (M w ) of 534 000, a melting temperature (T m ) of 165.0 8C, and a density of 1.74 g cm 3 , were supplied by Aldrich. The average diameter of the PVDF parti- cles was established with SEM imaging as 130 nm (see images of PVDF beads in ref. [11]). Severaliquids(liquid 1 in Figure 1 a) were coated with a PVDF powder, including 0.1 m water solutions of CaCl 2 and Na 2 CO 3 , and a 0.3 m NaOH water solution.0.1 m water solutions ofNa 2 CO 3 and CaCl 2 and water containing an alcohol solution of phenolphthalein (0.1 % wt.) were used as supporting liquids (liquid 2 in Figure 1 a). Sliding ofMarbles and Drops: For the manufacturing of marbles, drops with a volume of 10–200 mL of a 0.3 m NaOH water solution were coated with PVDF nanobeads (see Section 2.1). The rolling of 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 polymersubstrates coated with phenolphthalein is studied as well.No chemical reaction is observed. These observations supply directexperimental evidence for the suggestion that interfaces are separated by an air layerwhen marbles roll on solid sub- strates. It is concluded that a liquid marble rests on hydrophobic particlescoating theliquid.In contrast,dropscontaining an NaOH water solutionsliding on superhydrophobic surfaces coated with phenolphthalein leave a colored trace. The mecha- nism of low-friction sliding of drops deposited on superhydropho bic 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. Figure 1. a) Floating of a marble. b) Sliding of a marble. [a] Dr.E.Bormashenko, Y.Bormashenko, A.Musin ArielUniversity Center of Samaria Chemical and Biotechnological Engineering Faculty Applied Physics P.O.Box 3, Ariel,Faculty, 40700 (Israel) Fax :ACHTUNGTRENNUNG(+72) 3 906 6621 E-mail : edward@ariel.ac.il [b] Dr.Z.Barkay Wolfson Applied Materials Research Center TelAviv University, Ramat-Aviv, 69978 (Israel) 654 2009 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim ChemPhysChem 2009, 10, 654 – 656