Liquid Marbles Swallowing One Another and Extraneous Objects Edward Bormashenko*a,b, Revital Balterb,c, Doron Aurbachc, Anton Starostind, Viktor Valtsiferd, Vladimir Strelnikovd. aAriel University, Physics Faculty, 40700, Ariel, Israel bAriel University, Chemical Engineering and Biotechnology Department , 40700, Ariel, Israel cDepartment of Chemistry, Bar-Ilan University, Ramat-Gan, Israel dInstitute of Technical Chemistry of Ural Division of Russian Academy of Science, Perm, Russia.

*Corresponding author: Edward Bormashenko E-mail: [email protected]

Abstract Forced coalescence of liquid marbles is reported. Liquid marbles were connected by a glass rod hydrophilized with cold radiofrequency plasma. The process resulted in formation of Janus-marbles. “Sandwich” marbles enclosing solid foamed polystyrene particles and built from immiscible liquids are reported. A broad variety of organic and non-organic powders gives rise to Janus and “sandwich” marbles.

Keywords: liquid marbles; ; colloidal particles; coalescence; plasma treatment; Janus marbles; composite marbles.

1. Introduction Liquid marbles, shown in Fig. 1, are non-stick droplets encapsulated with micro- or nano-scaled solid particles [1-4]. Since liquid marbles were introduced in the pioneering works of Quèrè et al., they have been exposed to intensive theoretical and experimental research [5-11]. An interest in liquid marbles arises from both their very unusual physical properties and their promising applications. Liquid marbles present an alternate approach to superhydrophobicity, i.e. creating a non-stick situation for a liquid/solid pair. Usually superhydrophobicity is achieved by a surface modification of a solid substrate. In the case of liquid marbles, the approach is opposite: the surface of a liquid is coated by particles, which may be more or less

4-1 hydrophobic [12]. Marbles coated by graphite and carbon black, which are not strongly hydrophobic, were also reported [13-14]. A variety of media, including organic and ionic liquids and liquid metals, could be converted into liquid marbles [15-17]. Liquid marbles were successfully exploited for: [17-21], pollution detection [22], gas sensing [23], electrowetting [24], blood typing [25] and optical probing [26]. Respirable liquid marbles for the cultivation of microorganisms and Daniel cells based on liquid marbles were reported recently by Shen et al. [27-28]. Stimulus (pH, UV and IR) responsive liquid marbles were reported by Dupin, Fujii et al. [29-31]. The stability of marbles is crucial for their microfluidics and sensing applications. Marbles possessing increased mechanical and time stability were prepared by Matsukuma et al. It is noteworthy that liquid marbles retain non-stick properties on a broad diversity of solid and liquid supports [33-34]. Actually, liquid marbles are separated from the support by air cushions in a way similar to Leidenfrost droplets [35]. The state-of-the-art in the study of properties and applications of liquid marbles is covered in recent reviews [36-38]. In our paper we demonstrate new possibilities in micro-manipulation of liquids with the use of liquid marbles. 2. Experimental Nine kinds of powders were used for manufacturing marbles. Eight kinds of powders were hydrophobic. SEM images of the powders are presented in Fig. 1. (PVDF) nanobeads with the average diameter of particles equal to 130 nm, (PTFE) powder (100–200 nm), and (PE) spectrophotometric grade powder (10 µm) were supplied by Aldrich. Lycopodium (the average diameter of particles was about 30 µm) was supplied by Fluka. Two kinds of hydrophobized SiO2 powder (SiO2-mezo, the average diameter of particles was about 7 µm and SiO2 -nano, the average diameter of particles was about 25 nm) were supplied by Nanotech Ltd (Perm, Russia). The silicon-oxide powder was produced under a multistage process including three main stages. In the first stage, the preliminary preparation of the powder, namely powdering and drying has been carried out. The second stage included treatment of the powder particles with the polyalkylhydrosiloxane compound, resulting in the grafting of functional hydrophobic groups onto the powder surface. The third stage was the final heat treatment of the powder at 120–200°C, resulting in formation of a stable hydrophobic coating. The hydrophobic-coating content on the surface of the silicon

4-2 oxide was established as 5 wt%. Fluorinated decyl polyhedral oligomeric silsequioxane (FD-POSS) was synthesized according to the procedure described in detail in Ref. 37. FD-POSS powder is strongly hydrophobic and was successfully used by Lin et al. for manufacturing marbles of both aqueous solutions and organic liquids [17]. Liquid marbles were also manufactured with the hydrophilic powder of carbon black, supplied by Cabot Corporation [14]. The average diameter of particles, specified above, was established with SEM imaging, carried out with high resolution SEM (JSM-6510 LV). Distilled water (the electric conductance of 0.6 mS) was used for manufacturing marbles. For the purposes of visualization of the coalescence of marbles, water was colored with potassium permanganate. Organic Liquids: Dimethyl Sulfoxide (DMSO, pure for synthesis), Toluene and Hexadecane (chemical purity solvents) were supplied by Sigma Aldrich. 3. Results and discussion 3.1. Liquid marbles swallowing one another Liquid marbles were manufactured as described in the Experimental Section. Typical marbles are depicted in Fig. 2. It was demonstrated that the powder coating of marbles demonstrate elastic properties [40]. Liquid surfaces coated with solid particles behave as two dimensional elastic solids (and not liquids) when compressed [40]. The stretching modulus and bending stiffness of such surfaces were reported recently [40]. Hence, when we place liquid marbles one close to another, and even press them slightly, they do not coalesce, as shown in Fig. 3. This is true for all kinds of powders used in our study. However we can force the process of coalescence of liquid marbles. We connected liquid marbles a cylindrical glass rod with a diameter of 0.4 mm and a length of 8 mm. The rod was hydrophilized with the cold radiofrequency plasma treatment, according to the protocol described in detail in Ref. 41, 42. The plasma treatment resulted in an essential increase of the surface energy of the glass rod [43-44]. Thus, the wetting parameter  of the liquid/glass pair became positive [45-46]:

   SA  ( SL   ) (1) where  SA, SL, are the surface tensions at the solid/air (vapor), solid/liquid and liquid/air interfaces respectively [45-46]. Hence it was energetically favorable to water to wet the rod completely [45-46]. The process of wetting the rod gave rise to

4-3 the forced coalescence of marbles, depicted in Fig. 4. The Lycopodium-coated marble was colored with red for the purposes of visualization. It should be mentioned that marbles swallowed the glass rod. In spite of this, they formed a one-piece non-stick droplet, which could be easily displaced as an entire liquid marble containing a glass rod inside it. The described process of the forced coalescence of liquid marbles opens a pathway for manufacturing so-called Janus marbles, reported in Ref. 21, 47. Janus marbles are liquid marbles built of hemispheres coated by various powders, which may be more or less hydrophobic [21, 47]. The powders may also demonstrate different dielectric properties. This occurs when one of the hemi-spheres is coated by dielectric powder and the second one is coated with a semiconductor (carbon black), as reported in Ref. 21. Such Janus marbles coated with dielectric/semiconductor powders may be activated with an electric field [21]. Fig. 5 presents the manufacturing of Janus marbles under this forced coalescence. Carbon black and PVDF-coated marbles were connected with a cold plasma hydrophilized glass rod, as described above. The process resulted in the manufacturing of a Janus marble enclosing the glass rod, shown in Fig. 5. 3.2. “Sandwich” liquid marbles We communicate one more experimental situation where a liquid marble “swallows” an extraneous object, which may be a solid particle or liquid. Shen et al. reported numerous biological and biomedical applications of liquid marbles [25, 27, 48]. Thus, it seems attractive to prepare liquid marbles encompassing a solid container, which may embody cells, microorganisms, chemical reagents, etc. We manufactured such marbles containing millimeterically-scaled particles of foamed polystyrene (PS) with a diameter of 2 mm. Spherical particles of foamed polystyrene (PS) were pre-treated with cold radiofrequency air plasma for 1 minute (according to the protocol described in detail in Ref. 41, 42). Plasma treatment increased the surface energy of PS and hydrophilized it. Plasma treated PS balls were wetted with water and coated with hydrophobic particles as described in the Experimental Section. Thus, “sandwich” liquid marbles, depicted schematically in Fig 6A, were manufactured. Fig. 6B represents a 30 µl water marble containing a hydrophilized foamed PS ball. The marble is coated with PVDF powder, shown in Fig. 1C. However, “sandwich” marbles may be manufactured with all powders specified in the Experimental Section. “Sandwich” marbles may be supported not only by a solid substrate but also by

4-4 liquids, as discussed in Ref. 34. Fig. 7 depicts stable floating of a 30 µl marble containing a foamed PS particle. We also developed a process of manufacturing liquid “sandwich” marbles built from different immiscible organic liquids. Synthesis of FD-POSS powder allows the manufacturing of marbles containing organic liquids [17, 39]. In the first stage, a marble containing an organic liquid coated with FD-POSS powder was prepared, as described in the Experimental Section. In the second stage, the other organic liquid was injected with a precise micro-syringe into the bulk of the marble, as shown in Fig. 8A. The process resulted in the manufacturing of a composite marble containing two immiscible liquids, such as the “sandwich” marble built of hexadecane and DMSO demonstrated in Fig. 8B. We obtained composite marbles built of toluene/DMSO, hexadecane/water and hexadecane/DMSO pairs. Similar composite (compound) droplets and liquid marbles were reported recently [20, 49]. It is noteworthy that they could be actuated by an electric field [20]. Conclusions Liquid marbles are fascinating non-stick droplets presenting an alternative approach to the phenomenon of superhydrophobicity. Our paper reports the methods allowing the manufacturing of Janus and composite (compound) liquid marbles. Janus marbles built from semi-hemispheres coated with different particles were obtained under forced coalescence of liquid marbles. The forced coalescence of marbles is achieved when they are connected with a cold plasma-hydrophilized glass rod. The process results in the formation of stable non-stick droplets containing the glass rod inside it. We also report the possibility to prepare liquid marbles containing millimeterically-scaled particles of foamed polystyrene, hydrophilized with the plasma treatment. The paper presents “sandwich” marbles containing two immiscible liquids coated by a common powder shell. A variety of organic and non-organic powders gives rise to Janus and “sandwich” marbles. Floating of “sandwich” marbles is reported. Acknowledgements The work was financially supported by the Russian Foundation for Basic Research (Grant Nr. 13-03-96112, Grant Nr. 13-03) and of Ministry of Education of Perm Region (Agreement № C-26/203 of 09.12.2011). The work was also financially supported by the ACS Petroleum Research Fund (Grant 52043-UR5).

4-5 The authors are grateful to Mrs. Yelena Bormashenko for her kind help in preparing this manuscript. We thank Dr. Roman Grinyov for the SEM images.

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Fig. 1. SEM images of the powders: A. FD-POSS, scale bar is 2 µm; B. Al2O3, scale

bar is 5µm; C. PVDF, scale bar is 1 µm; D. SiO2-nano, scale bar is 5 µm;

E. Lycopodium, scale bar is 5 µm; F. Carbon Black, scale bar is 1 µm; G. SiO2- mezo, scale bar is 10 µm; H. PE, scale bar is 5 µm; I. PTFE, scale bar is 1 µm.

Fig. 2. Images of 20 µl water marbles coated by different powders.

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Fig. 3. 10 µl PVDF (white) and lycopodium (yellow)-coated marbles do not coalesce even when pressed one to another.

Fig. 4. Forced coalescence of 30 µl water marbles connected with the plasma hydrophilized glass rod. The red marble is coated with lycopodium, the white marble is coated with PVDF. A. Marbles are connected with a glass rod (depicted in yellow). B. Water wets the rod and marbles approach one to another. C. The final study of the coalescence.

Fig. 5. Manufacturing of Janus marbles connected with the plasma hydrophilized glass rod. The “white” marble is coated with PVDF, the “black” one is coated by carbon black. The volume of marbles is 60 µl. The scale bar is 1 mm. A. Marbles are connected with a glass rod (depicted in yellow). B. Water wets the rod and marbles approach one to another. C. The final study of the coalescence results in the manufacturing “Yin Yang-like” Janus marbles.

4-11 powder

foamed PS ball

liquid

PS particle

A.

B Fig. 6. “Sandwich” water marbles containing a spherical particle of foamed PS. A. Sketch of the “sandwich” marble. B. Image of the “sandwich” marble. The PS particle is colored with red for purposes of the visualization. The marble is coated with PVDF powder.

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PS particle

Fig. 7. Floating 30 µl sandwich water marble containing a spherical particle of foamed PS. The PS particle is colored with red for purposes of the visualization. The marble is coated with PVDF powder.

FD-POSS powder

DMSO

Liquids:

Hexadecane or

Toluene

A

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B Fig. 8. Liquid “sandwich” marbles. A. Sketch depicting manufacturing of the “sandwich” liquid droplet. B. 10 µl hexadecane marble containing a 5 µl DMSO droplet. The DMSO is colored with red for visualization.

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