pubs.acs.org/Langmuir © 2009 American Chemical Society
“Freezing” of Nanoconfined Fluids under an Electric Field Guoxin Xie, Jianbin Luo,* Shuhai Liu, Dan Guo, and Chenhui Zhang State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Received September 10, 2009. Revised Manuscript Received November 1, 2009 The problem of the solidlike transition of fluids in a nanogap has drawn much fundamental and practical attention. Here, we directly observed the disappearance of the fluidity of liquids confined within a gap with a surface separation of >10 nm under an EF in a ball-plate system, which is called the “freezing” of liquids. The flow of the nanoconfined liquid became very weak as the EF intensity was increased to a critical value and was correlated with the liquid polarity and the film thickness. It is deduced that the EF can induce more liquid molecules to be aligned to form more ordered layers in the nanogap.
The properties of a liquid confined to nanometer-sized gaps or pores are totally different from those of the liquid in the bulk. Typically, liquids undergo a solidlike transition adjacent to solid surfaces with a dramatic increase in viscosity, which has been extensively studied with the surface forces apparatus (SFA),1-5 the atomic force microscope (AFM),6 and relative optical interference intensity (ROII).7 The gap spacing between two solid surfaces where the solidlike transition of the confined liquid occurred was generally several nanometers. The properties of thin liquid films on the nanoscale under an electric field (EF) have attracted considerable attention because of the potential applications in many fields (e.g., nanofluidics,8 nanobiotechnology,9 and nanotribology10-12). EFs can be used to drive flow,13 move analytes,14 and separate DNA species15,16 in nanochannels. However, what will happen to nanoconfined liquids when exposed to an EF has been rarely discussed. The phase change of liquid water into ice was observed inside a gap of nanometer spacing under the control of EFs and gap distance with a scanning tunneling microscope (STM).17 In this letter, the disappearance of the fluidity of confined liquids in a gap (>10 nm) under an EF in a ball-plate system is shown. The work was conducted by using an instrument that permits the use of a ROII method,7,10 which has been proven to be *Corresponding author. E-mail:
[email protected]. (1) Granick, S. Science 1991, 253, 1374. Zhu, Y.; Granick, S. Langmuir 2003, 19, 8148. (2) Klein, J.; Kumacheva, E. Science 1995, 269, 816. (3) Drummond, C.; Israelachvili, J. Phys. Rev. E 2001, 63, 041506. Drummond, C.; Alcantar, N.; Israelachvili, J. Phys. Rev. E 2002, 66, 011705. (4) Lang, X. Y.; Zhu, Y. F.; Jiang, Q. Langmuir 2007, 23, 1000. (5) Patil, S.; Matei, G.; Oral, A.; Hoffmann, P. M. Langmuir 2006, 22, 6485. (6) Sun, G.; Bonaccurso, E.; Franz, V.; Butt, H. J. J. Chem. Phys. 2002, 117, 10311. (7) Luo, J. B.; Wen, S. Z.; Huang, P. Wear 1996, 194, 107. (8) Schoch, R. B.; Renaud, P. Rev. Mod. Phys. 2008, 80, 839. (9) Baldessari, F.; Santiago, J. G. J. Nanobiotechnology 2006, 4, 12. (10) Luo, J. B.; Shen, M. W.; Wen, S. Z. J. Appl. Phys. 2004, 96, 6733. (11) Luo, J. B.; He, Y.; Zhong, M.; Jin, Z. M. Appl. Phys. Lett. 2006, 89, 013104. (12) Xie, G. X.; Luo, J. B.; Liu, S. H.; Zhang, C. H.; Lu, X. C.; Guo, D. J. Appl. Phys. 2008, 103, 094306. (13) Gong, X. J.; Li, J. Y.; Lu, H. J.; Wan, R. Z.; Li, J. C.; Hu, J.; Fang, H. P. Nat. Nanotechnol. 2007, 2, 709. (14) Zhang, Y.; Gamble, T. C.; Neumann, A.; Lopez, G. P.; Brueck, S. R.; Petsev, D. N. Lab. Chip 2008, 8, 1671. (15) Henrickson, S. E.; Misakian, M.; Robertson, B.; Kasianowicz, J. J. Phys. Rev. Lett. 2000, 85, 3057. (16) Salieb-Beugelaar, G. B.; Teapal, J.; Nieuwkasteele, J.; Wijnperl, D.; Tegenfeldt, J. O.; Lisdat, F.; Berg, A.; Eijkel, J. C. T. Nano Lett. 2008, 8, 1785. (17) Choi, E. M.; Yoon, Y. H.; Lee, S.; Kang, H. Phys. Rev. Lett. 2005, 95, 085701.
Langmuir 2010, 26(3), 1445–1448
Figure 1. Scheme of the experimental rig. When an external voltage is applied to the liquid film, one end of the dc power source remains in contact with the Cr layer on the glass disk, and the other end is in contact with the steel ball.
an effective tool for investigating liquids in a nanogap. As schematically shown in Figure 1, a liquid film is formed between the surface of a glass disk coated with a semireflective chromium (Cr) layer with a surface roughness Ra of 20 nm (i.e., entering region III in Figure 3), the intermediate liquid molecules predominate as a result of the saturated thickness of the bounding layer because the polarization density decays dramatically a finite distance (several molecular layers) away from the electrified walls.25 Therefore, a less significant contraction of the bright tail could be observed. In addition, if the time t for liquid molecules to pass through the contact region is too short for them to be absorbed and arranged, then this might be another factor when the entraining speed is increased to a high value. In summary, a freezing phenomenon with an obvious flow reduction in liquids through a nanogap under an EF (>108 V m-1) has been reported. A dramatic reduction can be found when the EF intensity increases to a critical value, and the complete disappearance of the flow can be achieved at a gap distance larger than 10 nm. A solidlike transition among liquid molecules taking place near the confining walls has been proposed to account for the reduction in liquid flow. The present work seems to have immediate implications in such areas as nanopore media, interfacial slip,26 tip-based nanolithography,27 and ultrathin film lubrication in high-density-storage hard disks. Acknowledgment. We thank Professor Jacob Klein for stimulating discussions. We also acknowledge financial support from the National Natural Science Foundation of China (50721004) and from the National Key Basic Research Program of China (2009CB724200). Supporting Information Available: Formulas of the EF lines of a sphere-on-flat configuration and an estimation of the critical EF intensity for orientating a considerable number of molecules inside the nanogap. This material is available free of charge via the Internet at http://pubs.acs.org. (25) Rasaiah, J. C.; Isbister, D. J.; Eggebrecht, J. J. Chem. Phys. 1981, 75, 5497. (26) Klein, J. Phys. Rev. Lett. 2007, 98, 056101. (27) Snow, E. S.; Campbell, P. M. Appl. Phys. Lett. 1994, 64, 1932.
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