pubs.acs.org/Langmuir © 2010 American Chemical Society
Janus Droplets: Liquid Marbles Coated with Dielectric/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 polytetrafluoroethylene (dielectric), is presented. Droplets composed of two hemispheres (Janus droplets) characterized by various physical and chemical properties are reported first. Watermelon-like striped liquid marbles are reported. Janus droplets 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 during the past decade.1-6 The term “Janus” is used to describe particles in which the surfaces of both hemispheres are different from a chemical point of view. De Gennes coined the term Janus for such particles in his Nobel lecture.7 By combining a hydrophilic hemisphere with a hydrophobic one, amphiphilic Janus particles could be useful for the stabilization of water-in-oil or oil-in-water emulsions.8 Janus particles have been used for the development of dual-functionalized optical, electronic, and sensor devices.9,10 Janus particles are usually nano- or micrometrically scaled solid beads. Our letter describes novel Janus droplets coated partially with dielectric and partially with semiconductor particles. The Janus droplets presented here are based on so-called liquid marbles. Liquid marbles are droplets wrapped with hydrophobic (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, 128, 9408–9412. (5) Nisisako, T.; Torii, T.; Takahashi, T.; Takizawa, Y. Adv. Mater. 2006, 18, 1152–1156. (6) Nisisako, T.; Torii, T. Adv. Mater. 2007, 19, 1489–1493. (7) de Gennes, P. G. Rev. Mod. Phys. 1992, 64, 645–648. (8) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21–41. (9) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. J. Mater. Chem. 2005, 15, 3745–3760. (10) Vanakaras, A. G. Langmuir 2006, 22, 88–93. (11) Aussillous, P.; Quere, D. Nature 2001, 411, 924–927. (12) Aussillous, P.; Quere, D. Proc. R. Soc. London, Ser. A 2006, 46, 973–999. (13) Mahadevan, L. Nature 2001, 411, 895–896. (14) McHale, G.; Herbertson, D. L.; Elliott, S. J.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2007, 23, 918–924. (15) Bhosale, P. S.; Panchagnula, M. V.; Stretz, H. A. Appl. Phys. Lett. 2008, 93, 034109. (16) Bhosale, P. S.; M. V. Panchagnula, M. V. Langmuir 2010, 26, 10745–10749. (17) Gao, L.; McCarthy, Th. J. Langmuir 2007, 23, 10445–10447. (18) Dupin, D.; Armes, S. P.; Fujii, S. J. Am. Chem. Soc. 2009, 131, 5386–5387. (19) Fujii, S.; Kameyama, S.; Armes, S. P.; Dupin, D.; Suzaki, M.; Nakamura, Y. Soft Matter 2010, 6, 635–640. (20) Dandan, M.; Erbil, H. Y. Langmuir 2009, 25, 8362–8367. (21) Zhao, Y.; Fang, J.; Wang, H.; Wang, X.; Lin, T. Adv. Mater. 2010, 22, 707. (22) Xue, Yu.; Wang, H.; Zhao, Y.; Dai, L.; Feng, L.; Wang, X.; Lin, T. Adv. Mater. 2010, 22, 4814–4818. (23) Eshtiaghi, N.; Liu, J. S.; Shen, W.; Hapgood, K. P. Powder Technol. 2009, 196, 126–132. (24) Nguyen, Th. H.; Hapgood, K. P.; Shen, W. Chem. Eng. J. 2010, 162, 396–405. (25) Tian, J.; Arbatan, T.; Li, X.; Shen, W. Chem. Commun. 2010, 46, 4734– 4736.
Langmuir 2011, 27(1), 7–10
or hydrophilic particles11-34 and are characterized by extremely low friction between the droplet and solid support, which is due to air pockets separating the marbles from the substrate.11-13,28,35 Various applications of liquid marbles have been reported, including gas sensing, revealing water pollution, micro- and ferrofluidic devices, microreactors, micropumps, and so forth.21,22,24,26,36 In our letter, we introduce marbles composed of two liquid hemispheres, one of which is coated with hydrophobic material and the other of which is coated with a hydrophobic material, thus giving rise to the Janus droplets. We also demonstrate that Janus droplets could be activated by an electric field.
2. Experimental Section Janus marbles were produced by a two-step process. In the first stage, carbon black- and polytetrafluoroethylene (PTFE)-coated marbles were prepared. PTFE 100-200 nm powder was supplied by Aldrich (for SEM images of PTFE beads, see ref 27.); Tm = 321 °C and density = 2.15 g/cm3. Carbon black (Vulcan XC72R) was supplied by Cabot. The physical and chemical properties of this kind of carbon black were studied to a great extent because of its widespread use in fuel cells as supports for electrocatalysis and also for manufacturing carbon nanotubes.37,38 Its BET surface area is 250 m2/g; according to elemental analysis data, Vulcan (26) Kim, Sh.-H.; Lee, S. Y.; Yang, S. M. Angew. Chem., Int. Ed. 2010, 49, 2535– 2538. (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. ChemPhysChem 2009, 10, 654–656. (30) Bormashenko, E.; Bormashenko, Y.; Musin, A. J. Colloid Interface Sci. 2009, 333, 419–421. (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 2007, 173, 860–866. (38) Antonucci, P. L.; Pino, L; Giordano, N.; Pinna, G. Mater. Chem. Phys. 1989, 21, 495–506.
Published on Web 12/03/2010
DOI: 10.1021/la103653p
7
Letter
Bormashenko et al.
Figure 2. Cassie wetting occurring on the liquid/carbon black aggregates’ interface. Figure 1. PTFE- (white) and carbon black-coated (black) 20 μL liquid marbles.
XC72R contains 95.92% C, 1.05% S, 1.05% O, 0.25% H, and 0.25% N.38 The average dimension of the carbon black particles was established to be 30 nm with high-resolution scanning electron microscopy.33 PTFE and carbon black were separately poured and spread on a superhydrophobic surface, manufactured as explained in detail in ref 39. Water drops of a fixed volume of 20 μL were deposited with a precise microdosing syringe onto a superhydrophobic surface covered with either a layer of PTFE or a layer of carbon black powder. Slight tilting of the superhydrophobic surface caused the drop to roll and become coated with the powder. Thus, water marbles, such as depicted in Figure 1, were formed.32,33 Carbon black- and PTFE-coated marbles were merged in the second stage, as described in detail in section 3.
Figure 3. Scheme for manufacturing Janus marbles. (A) Carbon black- and PTFE-coated marbles are inserted into the vibrating spherical dish. (B) Merged marbles give rise to the Janus marble.
3. Results and Discussion Liquid marbles coated with Janus solid beads have already been reported.26 Our approach exploits the fact that liquid marbles can be coated not only with hydrophobic but also with hydrophilic particles, such as polyvinylidene fluoride, graphite, and carbon black.20,32,33 Aussillous and Quere demonstrated that there are two different scenarios for marble formation (i.e., a particle comes from either air or liquid12). In both cases, the surface energy ΔG of the liquid/particle/air system decreases. When the smooth spherical particle comes from air, the energy gain is given by ΔG1 ¼ - πb2 γð1þcos θY Þ2
ð1Þ
For the particle coming out of liquid, we have ΔG2 ¼ - πb2 γð1 - cos θY Þ2
ð2Þ
where θY is the Young angle, inherent in the particle/liquid/air system, γ is the surface tension at the liquid/vapor interface, and b is the radius of the particle. In both cases, a particle lowers its energy by sticking to the interface regardless of the contact angle.12 Marbles coated with strongly hydrophobic particles (θY >90°) and marbles coated with hydrophilic beads (θY
