DOI: 10.1021/la103653p 7Langmuir 2011, 27(1), 7–10 Published on Web 12/03/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Janus Droplets: Liquid Marbles Coated withDielectric/Semiconductor Particles

Edward Bormashenko,*,† Yelena Bormashenko,† Roman Pogreb,† and Oleg Gendelman‡

†Ariel University Center of Samaria, The Research Institute, Applied Physics Department,Department of Chemistry and Biotechnology Engineering, POB 3, Ariel 40700, Israel, and

‡Faculty of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel

Received September 12, 2010. Revised Manuscript Received November 18, 2010

The manufacturing of water droplets wrapped with two different powders, carbon black (semiconductor) and poly-tetrafluoroethylene (dielectric), is presented. Droplets composed of two hemispheres (Janus droplets) characterized byvarious physical and chemical properties are reported first. Watermelon-like striped liquid marbles are reported. Janusdroplets remained stable on solid and liquid supports and could be activated with an electric field.

1. Introduction

Janus particles have been subjected to intensive research duringthe past decade.1-6 The term “Janus” is used to describe particlesin which the surfaces of both hemispheres are different from achemical point of view. DeGennes coined the term Janus for suchparticles in his Nobel lecture.7 By combining a hydrophilic hemi-sphere with a hydrophobic one, amphiphilic Janus particles couldbe useful for the stabilization of water-in-oil or oil-in-wateremulsions.8 Janus particles have been used for the development ofdual-functionalized optical, electronic, and sensor devices.9,10

Janus particles are usually nano- or micrometrically scaledsolid beads. Our letter describes novel Janus droplets coatedpartially with dielectric and partially with semiconductor particles.The Janus droplets presented here are based on so-called liquidmarbles. Liquid marbles are droplets wrapped with hydrophobic

or hydrophilic particles11-34 and are characterized by extremelylow friction between the droplet and solid support, which is due toair pockets separating the marbles from the substrate.11-13,28,35

Various applications of liquidmarbles have been reported, includ-ing gas sensing, revealing water pollution, micro- and ferrofluidicdevices,microreactors, micropumps, and so forth.21,22,24,26,36 In ourletter, we introduce marbles composed of two liquid hemispheres,one of which is coated with hydrophobic material and the otherof which is coated with a hydrophobic material, thus giving riseto the Janus droplets. We also demonstrate that Janus dropletscould be activated by an electric field.

2. Experimental Section

Janusmarbles were produced by a two-step process. In the firststage, carbon black- and polytetrafluoroethylene (PTFE)-coatedmarbles were prepared. PTFE 100-200 nm powder was suppliedby Aldrich (for SEM images of PTFE beads, see ref 27.); Tm =321 �C and density= 2.15 g/cm3. Carbon black (VulcanXC72R)was supplied by Cabot. The physical and chemical properties ofthis kind of carbon black were studied to a great extent because ofits widespread use in fuel cells as supports for electrocatalysis andalso for manufacturing carbon nanotubes.37,38 Its BET surfacearea is 250 m2/g; according to elemental analysis data, Vulcan

(1) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708–4710.(2) Hong, L.; Jiang, Sh.; Granick, St. Langmuir 2006, 22, 9495–9499.(3) Roh, K.-H.; Martin, D. C.; Lahann, J. Nat. Mater. 2005, 4, 759–763.(4) Nie, Zh.; Li, W.; Seo, M.; Xu, Sh.; Kumacheva, E. J. Am. Chem. Soc. 2006,

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M. I. Langmuir 2007, 23, 918–924.(15) Bhosale, P. S.; Panchagnula, M. V.; Stretz, H. A.Appl. Phys. Lett. 2008, 93,

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(27) Bormashenko, E.; Pogreb, R.; Bormashenko, Y.; Musin, A.; Stein, T.Langmuir 2008, 24, 12119–12122.

(28) Bormashenko, E.; Pogreb, R.;Whyman,G.;Musin, A.; Bormashenko, Ye.;Barkay, Z. Langmuir 2009, 25, 1893–1896.

(29) Bormashenko, E.; Bormashenko, Ye.; Musin, Al.; Barkay, Z. Chem-PhysChem 2009, 10, 654–656.

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(31) Bormashenko, E.; Musin, A. Appl. Surf. Sci. 2009, 255, 6429–6431.(32) Bormashenko, E.; Pogreb, R.; Whyman, G.; Musin, A. Colloids Surf., A

2009, 351, 78–82.(33) Bormashenko, E.; Pogreb, R.; Musin, A.; Balter, R.; Whyman, G.;

Aurbach, D. Powder Technol. 2010, 203, 529–533.(34) McEleney, P.; Walker, G. M; Larmour, I. A.; Bell, S. E. J. Chem. Eng. J.

2009, 147, 373–382.(35) Bormashenko, Ed.; Bormashenko, Ye.; Gendelman, O. Langmuir 2010, 26,

12479–12482.(36) Bormashenko, Ed.; Balter, R.; Aurbach, D. Appl. Phys. Lett. 2010, 97,

091908.(37) Carmo,M.; dos Santos, A. R.; Poco, J. G. R.; Linardi, M. J. Power Sources

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Letter Bormashenko et al.

XC72R contains 95.92% C, 1.05% S, 1.05% O, 0.25% H, and0.25% N.38 The average dimension of the carbon black particleswas established to be 30 nm with high-resolution scanning elec-tron microscopy.33

PTFE and carbon black were separately poured and spread ona superhydrophobic surface, manufactured as explained in detailin ref 39. Water drops of a fixed volume of 20 μL were depositedwith a precise microdosing syringe onto a superhydrophobic sur-face coveredwith either a layer ofPTFEor a layer of carbonblackpowder. Slight tilting of the superhydrophobic surface caused thedrop to roll and become coated with the powder. Thus, watermarbles, such as depicted in Figure 1, were formed.32,33 Carbonblack- and PTFE-coated marbles were merged in the secondstage, as described in detail in section 3.

3. Results and Discussion

Liquid marbles coated with Janus solid beads have alreadybeen reported.26Ourapproach exploits the fact that liquidmarblescan be coated not only with hydrophobic but also with hydro-philic particles, such as polyvinylidene fluoride, graphite, andcarbon black.20,32,33 Aussillous and Qu�er�e demonstrated thatthere are twodifferent scenarios formarble formation (i.e., a particlecomes from either air or liquid12). In both cases, the surface energyΔG of the liquid/particle/air system decreases. When the smoothspherical particle comes from air, the energy gain is given by

ΔG1 ¼ -πb2γð1þcos θYÞ2 ð1Þ

For the particle coming out of liquid, we have

ΔG2 ¼ -πb2γð1- cos θYÞ2 ð2Þ

where θY is the Young angle, inherent in the particle/liquid/airsystem, γ is the surface tension at the liquid/vapor interface, andb is the radius of the particle. In both cases, a particle lowers itsenergy by sticking to the interface regardless of the contactangle.12 Marbles coated with strongly hydrophobic particles(θY >90�) and marbles coated with hydrophilic beads (θY<90�)have both been reported.11,12,20,21,32,33

Weassumed that an inherently hydrophilic particle (or an aggre-gate of particles) can trap air, thus the Cassie-Baxter wettingregimemay be realized at the particle/liquid interface, as shown inFigure 2.33 In this case, the Young contact angle in eqs 1 and 2should be replaced with the apparent angle, which could beobtuse.33 ESEM and optical microscopy studies of the marbles’boundaries show that hydrophilic particles form porous aggre-gates, as depicted in Figure 2 (see also images in ref 29), and itis reasonable to suggest that the Cassie-Baxter wetting regime

occurs at the liquid/particle aggregate interface. We deposited10 μLwater droplets onto a thick carbon black powder layer. Theapparent water contact angle in this case was established as 150�;this finding supports the idea that the Cassie-Baxter wettingregime takes place at the liquid/carbon black interface.33

Tomanufacture Janus droplets, we merged PTFE- and carbonblack-coated marbles as shown in Figure 3. Carbon black- andPTFE-coatedmarbles were put in a spherical dish and vibrated ata frequency of 1 Hz and an amplitude of 5 mm under ambientconditions. This process resulted in manufacturing compositeJanus liquid marbles, as depicted in Figure 4 and shown in theSupporting Information. The marbles remained stable whendeposited on solid and liquid (water) supports. A floating liquidmarble is presented in Figure 5. It should be mentioned that afloating Janusmarble puncturedwith a needle broke up along theplane separating the PTFE- and carbon black-coated hemi-spheres. Thus, PTFE and carbon black deposits were eventuallyseparated, as shown in Figure 5. The consequent merging of twoJanus marbles gave rise to a watermelon-like marble, as depictedin Figure 6. It should be mentioned that carbon black particlescould be partially removed with a blade from the surface of thecomposite marble. Thus, water clearings were formed, as showninFigure 6. The “shaved”marble remained stable. The stability ofthe shavedmarble is explained by the fact that the effective surfacetensions of carbon black- and PTFE-coated marbles are close tothat of water.32,33

Figure 1. PTFE- (white) and carbon black-coated (black) 20 μLliquid marbles.

Figure 2. Cassie wetting occurring on the liquid/carbon blackaggregates’ interface.

Figure 3. Scheme for manufacturing Janus marbles. (A) Carbonblack- and PTFE-coated marbles are inserted into the vibratingspherical dish. (B) Merged marbles give rise to the Janus marble.

Figure 4. Composite 40 μL Janus liquid marble.

(39) Bormashenko, Ed.; Stein, T.; Whyman, G.; Bormashenko, Ye.; Pogreb, R.Langmuir 2006, 22, 9982–9985.

DOI: 10.1021/la103653p 9Langmuir 2011, 27(1), 7–10

Bormashenko et al. Letter

The reported composite marbles are composed of hemispherespossessing very different electrical properties: PTFE is a dielectricmaterial but carbon black is a semiconductor.40 It has alreadybeen demonstrated that Janus particles could be activated by anelectric field.41,42 It has been shown that a dielectrophoretic forceimposed on solid Janus particles oriented them so that the planebetween the hemispheres aligned in the direction of the electricfield.41We activated Janusmarbles with an experimental setup, asdepicted inFigure 7. Inour case, Janus droplets depositedonglassslides were rotated by an electric field. The marble starts itsrotation when the electric field attains a value ofE≈ 5� 105 V/m.The sequence of images demonstrating the marble’s rotation isshown in Figure 7. This could be understood from simple scalingconsiderations. The electric energy of themarble with a volumeVis given by Wel = (ε0εE

2/2)V, whereas the surface energy is sup-plied byWsurf = γeff4πR

2, where ε and γeff are the dielectric per-mittivity and the effective surface tension of the marble, respec-tively, andR is the radius of the marble. Equilibrating the electricand surface energies Wel = Wsurf yields the estimation

E�=ffiffiffiffiffiffiffiffiffiffi6γeffε0εR

sð3Þ

Substituting γeff ≈ 70 mJ/m2 (as mentioned above, carbon black-and PTFE-coated marbles are characterized by similar valuesof their effective surface tension, see refs 32 and 33,), ε ≈ 80, and

R ≈ 2 � 10-3 m supplies E* ≈ 5.5 � 105 V/m, which is in goodagreement with the value of the electric field corresponding to theonset ofmarble rotation.The radius of themarble,R≈ 2� 10-3m,is close to the capillary length lcap = (γeff/Fg)1/2, where F is thedensity of the droplet. Hence, the gravitational energy of themarble is of the same order of magnitude as the surface energy.This means that the marble’s behavior is governed by a complexinterplay of surface effects, electrical effects, and gravity. More-over, when the electric field attains a value ofE≈ 5� 105 V/m thedeformation of the marble is not negligible.43 Thus, it could berecognized that the behavior of Janus liquid marbles subjected to

Figure 5. (Top) Floating 50 μL Janus marble. (Bottom) PTFE(white) and carbon black deposits produced by puncturing thefloating Janus marble. Figure 6. (Top)Watermelon-likemarbleobtainedbymerging two

Janusmarbles. (Bottom) Shaved Janusmarble inwhich the carbonblack is partially removedwith a razor andwater clearings are seen.

Figure 7. (Top) Electrical actuation of liquid marbles. (Bottom)Sequenceof imagesdemonstrating the rotationof the Janusmarbleby the growing electric field. (A) Zero electric field, (B) E= 5.8�105 V/m, (C) E= 6.5 � 105 V/m, and (D) E= 6.7 � 105 V/m.

(40) Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Roy, Ch. Appl. Surf. Sci. 2003,217, 181–193.(41) Gangwal, S.; Cayre, O. J.; Bazant,M. Z.; Velev, O.D.Phys. Rev. Lett. 2008,

100, 058302.(42) Dong, L.; Huang, J. P.; Yu, K. W.; Gu, G. Q. J. Appl. Phys. 2004, 95,

621–624.

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Letter Bormashenko et al.

an electric field ismuchmore complicated than that of solid Janusmicrobeads.41

Moreover and quite remarkably, the carbon black side of themarble always rotated upward, as demonstrated in Figure 7(image D), regardless of the direction of the electric field betweenthe electrodes. In other words, the behavior of the marble was thesame for opposite polarities of the battery.

This rather unexpected resultmay be explained in the followingway. Themajor difference between carbon black andTeflon is theelectrical conductivity; the former is close to a semiconductor andthe latter is almost a perfect insulator. As is well known,44 thepolarizability of the conductor is proportional to the cube of itslinear size whereas the polarizability of the insulator is propor-tional to its volume. It is difficult to estimate the size of carbonclusters formed at the marble surface, but one can qualitativelyimagine that these clusters are polarized as a whole and interactrather strongly. That is why the effective surface energy of thehemisphere coveredwith carbon black in the external electric fieldgrows in a more significant manner that the second one coveredwith Teflon. Therefore, the balance between the gravity and the

surface tension is violated; the black hemisphere tends to shrinkand the center of mass of the marble moves toward the whitehemisphere. Finally, the system tends to minimize its generalenergy; therefore, themarble flipswith thewhite side down.Whenthe electric field attained a value of E ≈ 7 � 105 V/m, the Janusmarble was destabilized and destroyed.

Conclusions

Janus droplets (i.e., liquidmarbles coated partially with carbonblack and partially with polytetrafluoroethylene particles) arereported. The process of manufacturing Janus and watermelon-like composite marbles is presented. Janus marbles are stablewhen deposited on solid and liquid supports. Janus droplets couldbe rotated with an electric field and demonstrate a potential formicrofluidics applications, microsensors and actuators, and thestabilization of emulsions.

Acknowledgment. We are grateful to Professor M. Zinigradfor his generous support of our experimental activity.We are alsograteful to Revital Balter for her help in preparing this letter.

Supporting Information Available: Movie demonstratingthe manufacturing of Janus marbles. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

(43) Bormashenko, E.; Pogreb, R.; Stein, T.; Whyman, G.; Hakham-Itzhaq,M.Appl. Phys. Lett. 2009, 95, 264102.(44) Landau, L. D.; Lifshitz, E. M. Electrodynamics of Continuous Media;

Pergamon Press: Oxford, U.K., 1984.