Langmuir 2008, 24, 1173-1182
1173
New SFA Techniques for Studying Surface Forces and Thin Film Patterns Induced by Electric Fields† Hongbo Zeng,‡ Yu Tian,‡,§ Travers H. Anderson,‡ Matthew Tirrell,‡ and Jacob N. Israelachvili*,‡ Department of Chemical Engineering, Materials Department, and Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, and State Key Lab of Tribology, Department of Precision Instruments, Tsinghua UniVersity, Beijing 100084, China ReceiVed June 11, 2007. In Final Form: August 14, 2007 We describe two ways to measure normal and/or lateral forces between two surfaces in a surface forces apparatus (SFA) while an electric field is applied between the surfaces. The first method involves depositing thin conductive layers on the exposed substrate (usually mica) sheets; the second involves using the optically reflecting silver layers on the back surfaces of the sheets as the electrodes. Two types of experiments were performed using these new techniques: (1) measuring the effects of an electric field on the rheology of an ∼40-µm-thick film of zeolite particles suspended in silicone oil and (2) a dynamic study of electric field-induced pattern formation of a thin polymer film. In the first study, under an electric field of strength ∼106 V/m the shear force or effective viscosity of the colloid suspension was found to be two orders of magnitude higher than in the absence of the field, when the expected bulk value was measured. In the dynamic study, the initially uniform film transformed into a 2-D honeycombed network of depressed cells bounded by elevated ridges that grew slowly with time in a way consistent with previously derived theories. The new techniques should be applicable to studies of other systems and interactions, such as double-layer forces, micro- and nanoelectrorheology, electric field-induced ordering of particles, and the effects of electric fields on adhesion, friction, and lubrication.
Introduction During the last three decades, several different techniques have been developed for measuring surface and intermolecular forces both in air and in liquids: the surface forces apparatus (SFA),1 the osmotic stress device,2 the force balance,3 and the atomic force microscope (AFM).4 These techniques have allowed accurate measurements of surface forces from the macroscopic to the molecular (subnanoscopic) scale, leading to an improved understanding of these forces as well as their implications in colloidal behavior, adhesion, friction, dynamic (nonequilibrium) interactions, and so forth.5-8 The way in which distances or surface separations, D, are measured during force measurements can be divided into direct and indirect, where in the latter, D is inferred from displacements measured somewhere away from the actual contact or “interaction zone”. In the surface forces apparatus (SFA) technique1,7,9-14 and modified versions of the SFA technique,15-21 angstrom positioning and movement are achieved by piezoelectric crystals, and †
Part of the Molecular and Surface Forces special issue. * Corresponding author. E-mail:
[email protected]. ‡ University of California. § Tsinghua University. (1) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (2) Leneveu, D. M.; Rand, R. P.; Parsegian, V. A. Nature 1976, 259, 601603. (3) Derjaguin, B. V.; Rabinovich, Y. I.; Churaev, N. V. Nature 1978, 272, 313-318. (4) Binnig, G.; Quate, C. F.; Gerber, C. Phys. ReV. Lett. 1986, 56, 930-933. (5) Bhushan, B. Springer Handbook of Nanotechnology; Springer: Berlin, 2004. (6) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 18311836. (7) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: Amsterdam, 1992. (8) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239-241. (9) Tabor, D.; Winterton, R. H. S. Proc. R. Soc. London, Sect. A 1969, 312, 435. (10) Israelachvili, J. Nature, Phys. Sci. 1971, 229, 85.
optical interferometry is used for positional sensing, allowing for normal distances (surface separations) to be controlled and measured to within 0.1 nm and forces to be controlled and measured to within 10-8 N. The use of macroscopic surfaces in SFA experiments,5,7,22-31 with radii on the order of 1 cm, allows for optical interferometry to be used to directly measure not only absolute distances or surface separations but also the local surface geometry or radius, the true contact area, the thickness of adsorbed layers and their refractive index, and dynamic surface deformations during force measurements, all within the interaction zone. (11) Israelachvili, J. N.; Tabor, D. Proc. R. Soc. London, Sect. A 1972, 331, 19. (12) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500-514. (13) Klein, J.; Kamiyama, Y.; Yoshizawa, H.; Israelachvili, J. N.; Fredrickson, G. H.; Pincus, P.; Fetters, L. J. Macromolecules 1993, 26, 5552-5560. (14) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437-443. (15) Klein, J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 99. (16) Parker, J. L.; Christenson, H. K.; Ninham, B. W. ReV. Sci. Instrum. 1989, 60, 3135-3138. (17) Israelachvili, J. N.; Mcguiggan, P. M. J. Mater. Res. 1990, 5, 2223-2231. (18) Peachey, J.; Vanalsten, J.; Granick, S. ReV. Sci. Instrum. 1991, 62, 463473. (19) Heuberger, M. ReV. Sci. Instrum. 2001, 72, 1700-1707. (20) Connor, J. N.; Horn, R. G. ReV. Sci. Instrum. 2003, 74, 4601-4606. (21) Cho, Y. K.; Granick, S. J. Chem. Phys. 2003, 119, 547-554. (22) Klein, J.; Kumacheva, E. Science 1995, 269, 816-819. (23) Israelachvili, J. J. Colloid Interface Sci. 1973, 44, 259-272. (24) Israelachvili, J. N. J. Colloid Interface Sci. 1986, 110, 263-271. (25) Horn, R. G.; Israelachvili, J. N.; Pribac, F. J. Colloid Interface Sci. 1987, 115, 480-492. (26) Zeng, H. B.; Maeda, N.; Chen, N. H.; Tirrell, M.; Israelachvili, J. Macromolecules 2006, 39, 2350-2363. (27) Zeng, H. B.; Tirrell, M.; Israelachvili, J. J. Adhes. 2006, 82, 933-943. (28) Maeda, N.; Chen, N. H.; Tirrell, M.; Israelachvili, J. N. Science 2002, 297, 379-382. (29) Zeng, H. B.; Zhao, B. X.; Tian, Y.; Tirrell, M.; Israelachvili, J. Langmuir 2007, 23, 6126-6135. (30) Zhao, B. X.; Zeng, H. B.; Tian, Y.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 19624-19629. (31) Zeng, H. B.; Zhao, B. X.; Tian, Y.; Tirrell, M.; Leal, L. G.; Israelachvili, J. Soft Matter 2007, 3, 88-93.
10.1021/la7017242 CCC: $40.75 © 2008 American Chemical Society Published on Web 10/16/2007
1174 Langmuir, Vol. 24, No. 4, 2008
This kind of information is not directly accessible using AFM techniques that involve much smaller objectssnanoscopic tips or microscopic probessof unknown local geometry5-8 and an indirect measurement of surface separation. The extended surfaces used in SFA experiments also allow for a uniform electric field to be applied across the interaction zone, which is the subject of this article. One of the most interesting current problems in surface and colloid science is understanding the effects of external electric fields on surface and interparticle interactions.32-40 In the presence of an electric field, the flow properties of some fluids can be controlled and greatly changed (so-called electrorheology), and patterned structures such as chains of particles can form in colloidal suspensions.41,42 Patterns can be induced in thin polymer films by an electric field. The formation of desired patterns has the potential to eliminate complex steps in conventional imprinting techniques, such as the photoresist and exposure stages, in the fabrication of microelectromechanical systems (MEMS).32-35,39,40,43-48 Electric field gradients also show great promise in the design of DNA separation devices.36-38 By applying an electric field across the surfaces in an SFA experiment, its effect on adhesion, rheology, tribology, and film morphology should be directly measurable and visualized, either directly (via a normal microscope) or interferometrically using the optical multiple beam interferometry (MBI) technique employing fringes of equal chromatic order (FECO). Previous studies by Vanderlick and co-workers49-51 have shown how applying a voltage between one of the surfaces and the aqueous solution can modify the double-layer force between a gold (electrode) surface and a mica surface. In a related approach, Horn and co-workers52,53 changed the surface potential of a mercury droplet and measured how this affected the dynamic deformations of the surface. However, until now, no electric field has been applied between the two interacting surfaces in an SFA. In the study reported here, an electric field was applied between two conducting (electrode) surfaces in an SFA, and two kinds of experiments were performed to assess the possibilities of the new technique with regard to (1) electrical field effects on the rheology of a colloid suspension and (2) a real-time dynamic study of electric field-induced patterning of thin polymer films. (32) Schaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Nature 2000, 403, 874-877. (33) Wu, N.; Pease, L. F.; Russel, W. B. AdV. Funct. Mater. 2006, 16, 19921999. (34) Wu, N.; Russel, W. B. Appl. Phys. Lett. 2005, 86, 241912. (35) Schafer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Europhys. Lett. 2001, 53, 518-524. (36) Randall, G. C.; Doyle, P. S. Macromolecules 2005, 38, 2410-2418. (37) Randall, G. C.; Doyle, P. S. Macromolecules 2006, 39, 7734-7745. (38) Olson, D. J.; Johnson, J. M.; Patel, P. D.; Shaqfeh, E. S. G.; Boxer, S. G.; Fuller, G. G. Langmuir 2001, 17, 7396-7401. (39) Tsori, Y.; Tournilhac, F.; Leibler, L. Nature 2004, 430, 544-547. (40) Leach, K. A.; Gupta, S.; Dickey, M. D.; Willson, C. G.; Russell, T. P. Chaos 2005, 15, 047506. (41) Parthasarathy, M.; Klingenberg, D. J. Mater. Sci. Eng., R 1996, 17, 57103. (42) Klingenberg, D. J.; Vanswol, F.; Zukoski, C. F. J. Chem. Phys. 1989, 91, 7888-7895. (43) Lin, Z. Q.; Kerle, T.; Russell, T. P.; Schaffer, E.; Steiner, U. Macromolecules 2002, 35, 3971-3976. (44) Chou, S. Y.; Zhuang, L. J. Vac. Sci. Technol., B 1999, 17, 3197-3202. (45) Chou, S. Y.; Zhuang, L.; Guo, L. J. Appl. Phys. Lett. 1999, 75, 10041006. (46) Wu, N.; Pease, L. F.; Russel, W. B. Langmuir 2005, 21, 12290-12302. (47) Wu, N.; Russel, W. B. Ind. Eng. Chem. Res. 2006, 45, 5455-5465. (48) Leach, K. A.; Lin, Z. Q.; Russell, T. P. Macromolecules 2005, 38, 48684873. (49) Frechette, J.; Vanderlick, T. K. Langmuir 2001, 17, 7620-7627. (50) Frechette, J.; Vanderlick, T. K. Langmuir 2005, 21, 985-991. (51) Frechette, J.; Vanderlick, T. K. J. Phys. Chem. B 2005, 109, 4007-4013. (52) Antelmi, D. A.; Connor, J. N.; Horn, R. G. J. Phys. Chem. B 2004, 108, 1030-1037. (53) Connor, J. N.; Horn, R. G. Langmuir 2001, 17, 7194-7197.
Zeng et al.
Figure 1. Experiment setup for applying an electrical field in an SFA.
Experimental Section Applying Electric Fields in SFA Experiments. Atomically smooth 1-4 µm thick sheets of back-silvered muscovite mica (S&J Trading, Glen Oaks, NY) were glued to cylindrical silica disks (radius R ) 2 cm) with UV-cured glue (Norland Optical Adhesive 61) or epoxy (Shell Epon) glue, where the back-silvered 500-550 Å thick silver layer was used as the reflecting layer to produce fringes of equal chromatic order (FECO) in the interferometry measurements but also as an electrode surface (described below). Other metal films of thickness 70-100 Å were also deposited on the mica surfaces using a Temescal VES 2550 E-beam system, where the mica surfaces were first cleaned in an ozone cleaner for 3-5 min before deposition. Ag and Au were chosen for the metal films in this study, where each Au film was deposited over a 15-20 Å prelayer of Cr or Ti for good adhesion and smoothness. After the metal deposition, the surfaces were stored in a desiccator under reduced pressure until used. Prior to use, very thin shielded, flexible wires (Cooner wire, AS631) were carefully connected to the upper metal film (close to the edge of the film) of each surface using a conducting epoxy (Circuitworks, CW2400). Depending on the insulation requirement of the experiments, the conducting epoxy bond could be strengthened and insulated by coating it with a thin layer of cyanoacrylate glue after the conductive glue was thoroughly cured. Depending on the different purposes of experiments, various kinds of sample films can be deposited, cast, or grafted onto metal surfaces by physical or chemical methods, such as self-assembly, spin coating, vapor deposition, and chemical bonding. For example, a surfactant or lipid monolayer can be deposited onto a metal surface by chemical bonding through some specific functional group (such as a thiol group) or by Langmuir-Blodgett (LB) deposition; a uniformly smooth polymer film can be prepared by spin coating a drop of polymer solution or the (pure) polymer melt. The two disk-supported surfaces were then mounted into the SFA chamber in the geometry of crossed cylinders (each cylinder of radius R ) 2 cm) as shown in Figure 1. This geometry locally corresponds to a sphere of radius R on a flat surface or two spheres of radius 2R. It was important to arrange the disks such that the protruding conducting epoxy bonds did not touch each other or the opposite surfaces during experiments. The shielded wires were connected to an electrical feed-through in the SFA chamber wall and then to to a power supply resource outside, as shown in Figures 1 and 2a. A simpler way to add two electrode surfaces to the SFA is to glue the mica sheets to the silica disk in a silver sides out (rather than in) arrangement, as shown in Figure 2b.
New SFA Techniques
Langmuir, Vol. 24, No. 4, 2008 1175
Figure 2. Two ways to add electrode surfaces and apply an electrical field in an SFA. The two surfaces were then brought closer to the desired starting separation or into contact, and then different voltages were applied across the opposing metal surfaces to generate the electric field required. For the typical surface radii of R ≈ 2 cm used in SFA experiments and at surface separations D, where R . D, the electric field in the central region of radius ∼3.5 mm can be approximated by the uniform field between two plane parallel plates, with a discrepancy of less than 5% (Appendix, Supporting Information). Using a beam splitter, the surfaces were imaged simultaneously using both normal optical microscopy and multiple beam interferometry using FECO. The FECO measurements gave the thickness and uniformity of the films (to (1 Å) and the surface contact diameter (to (1 µm). The rheological studies were carried out with an SFA equipped with a friction attachment where the upper surface was attached to a movable friction force-measuring spring or friction device and the lower surface was attached to a movable normal force-measuring spring that could be moved laterally at a uniform velocity using a piezoelectric bimorph slider by applying a triangular voltage signal, as shown in Figure 1.26 The relative sliding distance and sliding velocity of the two surfaces are determined by the amplitude and frequency of the input triangular voltage signal. The resulting friction was measured using strain gauges attached to the friction force-measuring springs on the friction device supporting the upper surface. Optical Visualization by FECO when Applying Electric Fields. For conventional SFA experiments without electrode surfaces, when the two surfaces are in contact and white light is passed normally through them, as shown in Figures 1 and 3a, only certain wavelengths pass through. Therefore, the emerging light beam consists of discrete wavelengths λ0n (n ) 1, 2, 3, ...); if the beam is focused on the entrance slit of a grating spectrometer, then these wavelengths are split up and appear in the exit port as an array of sharp fringes (FECO) as shown in Figure 3a. For “symmetric” systems (i.e., when the two mica films have the same thickness T), if the surfaces are separated by a gap distance D, then D and medium refractive index µ can be determined independently by simultaneous measurements of FECO wavelengths λ0n, λn0 - 1, λn0 - 2, λDn , and λnD- 1 and solving the equation23 tan(2πµD/λDn ) )
2µ j sin(nFnπ∆λn/λDn ) (1 + µ j 2)cos(nFnπ∆λn/λDn ) ( (µ j 2 - 1)
(1)
where µ j ) µmica/µ and µmica and µ are the refractive indices of mica and the medium at λDn and ∆λn ) (λDn - λ0n), respectively. nFn ) λn0 - 1/(λn0 - 1 - λ0n), where n is the interference fringe order and Fn can be considered to be a correction factor due to the phase changes at the mica-silver interface and the dispersion effects; + refers to odd order fringes (odd n, concave fringes in Figure 3a), and - refers to even order fringes (even n, convex fringes in Figure 3a) in eq 1.23 The FECO fringes shown in Figure 3a are only the primary fringes due to the reflections between the two silver-mica interfaces or “mirrors” 1 and 4 in Figure 3a. Because of the reflections at all four interfaces 1-4, six different FECO patterns, superimposed on each
other, could occur, as shown in Figure 3a (although some could be “degenerate”, i.e., identical and therefore superimposed on each other, depending on the thicknesses and refractive indices of the three media):54 primary fringes 1-4, two patterns of secondary FECO fringes 1-2 and 3-4, gap fringe 2-3, and two patterns of tertiary fringes 1-3 and 2-4. Primary fringes mainly probe 2, 3, and the optical distance between 1 and 4, and secondary fringes are sensitive to the topography of the 2-3 interface. Except for the secondary fringes (between 1-2 and 3-4), all of the other fringe patterns move when the separation D changes. In conventional SFA experiments, the Ag mirror thickness is typically 500-550 Å, which yields a high reflectivity of >95%, and only the primary FECO are clearly visible, as shown in Figure 3a. (The secondary fringes between 1-2 and 3-4 can just be seen as weak, broad fringes superimposed on the primary fringes, having twice the spacing, with the bright regions centered on alternate contact fringes). If the mirror layers are thinner than 250 Å or if high reflections also occur at other interfaces, say at interfaces 2 and 3, then both secondary and tertiary fringes are clearly visible in the FECO spectrogram. As described in the Experimental Methods and shown in Figure 2a, one method of charging the surfaces in the SFA is to deposit a 70-100 Å thick Ag or Au layer on the outer mica surfaces. The absorption and reflection of these electrode films cannot be neglected; for example, a 70-100 Å thick silver film has a reflectivity of 20-30%, which makes the secondary and tertiary fringes clearly visible in the FECO pattern, as shown in Figure 3b. Under these conditions (for the geometry shown in Figures 2a and 3b), the general equation for the primary FECO fringes (eq 1) is still valid and can be used to estimate the gap distance D and medium refractive index µ when an electric field is applied across the two surfaces. To avoid the difficulty of the complex fringe analysis due to the appearance of secondary and tertiary fringes, a simpler way to apply a field is to use the two mirror layers as the electrodes. Schematics of the geometry and corresponding FECO images are shown in Figures 2b and 3c. This second method can be further expanded by using the two outer conducting surfaces as substrates for ultrathin films, which can be asymmetric, giving rise to a “two-layer interferometer”, as shown in Figure 4. Such a configuration can be very useful for studying the adhesion (rather than cohesion) and friction forces between different surfaces or materials (mediums A and B in Figure 4, left panel). Equation 1 is no longer valid for this geometry. An approximate equation relating the film thicknesses, TA and TB, and refractive indices, µA and µB, for this type of two-layer interferometer was previously derived23 tan(4πµBTB/λn) )
2µ˜ sin(4πµATA/λn) (µ˜ - 1) - (1 + µ˜ 2)cos(4πµATA/λn) 2
(2)
(54) Heuberger, M.; Luengo, G.; Israelachvili, J. Langmuir 1997, 13, 38393848.
1176 Langmuir, Vol. 24, No. 4, 2008
Zeng et al.
Figure 3. Symmetrical interferometers and corresponding FECO fringe images. (a) Three-layer interferometer in conventional SFA experiments. (b) Three-layer interferometer with two thin metal films as the electrode surfaces. (c) One-layer interferometer using the reflecting mirror surfaces as the electrodes. where µ˜ ) µA/µB and λn is the wavelength of the nth-order bright fringe. Equation 2 is approximate because it neglects the phase changes at the A/silver and B/silver interfaces. In the simple case of only one layer (medium A) between the two surfaces, TB ) 0 and TA is related to λn by 4πµATA/λn ) 2nπ or nλn (n - 1)λn - 1 TA ) ) ) ... for any n 2µA 2µA
(3)
From eq 3, it is easy to derive the thickness of medium A and the fringe order of any fringe from the (measured) wavelengths of any two adjacent fringes, say, λn - 1 and λn, as given by eqs 4 and 5. TA )
λn - 1λn 2µA(λn - 1 - λn)
n)
λn - 1 λn - 1 - λn
If the gap distance between the two mirror surface shifts from TA to T′A, then the distance shift ∆TA is given by eq 6, where λnTA- 1, λTn A and λnTA-′ 1, λTn A′ are the wavelengths of any two adjacent fringes before and after the distance change, respectively. ∆TA ) T′A - TA )
(
TA′ T A′ λnTA- 1λTn A 1 λn - 1 λn 2µA λTA′ - λTA′ λTA - λTA n-1 n n-1 n
(6)
Another simple way to estimate the distance shift ∆TA is to measure the change in the chord length 2rλ (Figure 4, right panel) at some fixed wavelength λ, for example, at one of the calibration wavelengths such as the green Hg line at λ ) 5460.75 Å. Using the chord theorem for a circle7 and assuming that the surfaces have a locally welldefined (i.e., constant) radius of curvature R, ∆TA is given by
(4) ∆TA ≈ (5)
)
r′λ2 - rλ2 2R
(7)
where 2rλ and 2r′λ are the chord distances before and after the distance change. 2rλ and 2r′λ can be easily obtained from the FECO pattern
New SFA Techniques
Langmuir, Vol. 24, No. 4, 2008 1177
Figure 4. Asymmetrical two-layer interferometer and the corresponding FECO fringe image using the mirror surfaces as the electrodes. (Figure 4, right panel) after calibrating the lateral magnification of the surfaces by the microscope spectrometer system. Equation 7 also assumes that TA , R, which is generally the case in SFA experiments where R is typically of order 1 cm and TA is less than 1 µm. As mentioned, phase changes φA and φB at the medium A/silver and medium B/silver interfaces, respectively, are neglected in the above analysis. When these are included, eq 2 becomes55,56
A multilayer matrix method is required for the detailed analysis of more complicated asymmetric cases, which can be found else where.55-58 Besides silver, other materials such as Au, Ti, and Cr can also be used as the mirror and electrode films in the geometries shown in Figures 2b, 3c, and 4. One common problem with these latter materials is their low transmittance (high absorbance) relative to Ag, which results in dimmer FECO patterns. Additional Experimental Factors and Considerations. We have found eqs 1-8 to be useful for determining surface separations and film thickness changes when studying the effects of electric fields on adhesion, rheology, and friction in both symmetric and asymmetric systems (two contacting films of the same or different materials). In some of these studies, instead of the atomically smooth mica surfaces used in conventional SFA experiments, the electrode surfaces are the supporting substrates for the confined fluid and/or solid films across which the electric field is applied. The smoothness of the electrode surfaces, which can be a critical factor in some studies, is determined by the growth of the thin metal films during, for example, E-beam deposition, which is a nonequilibrium process. The final topography of the deposited film is usually determined by a combination of many factors, including the initial (lattice) structure of the substrate, the deposition temperature, the thickness of the deposited film, the deposition rate, the diffusion of the deposited atoms on the surface, and the atom-surface and atom-atom interaction energies. In our studies, the depositions of metal films were performed using a Temescal VES 2550 E-beam system, and optimum operating conditions to get uniformly smooth films were found to be a base pressure of (3-5) × 10-7 Torr, an operation pressure of (5-8) × 10-7 Torr, an electric field of 10 kV, and a rate of deposition of 0.1-0.3 Å/s. The deposition rate was found to have a particularly significant effect on the smoothness of the deposited films. When the deposition rate is 0.1-0.3 Å/s, both gold and silver surfaces have very low root-mean-square (rms) roughness values: 3 Å for Au (10 nm) and 5 Å for Ag (55 nm), as shown in Figure 5a,b, respectively. Higher rms roughnesses were obtained at higher evaporation rates as follows: 5-8 Å for a Au film of 10 nm and 8-12 Å for a Ag film of 55 nm at a deposition rate of 0.8-1.0 Å/s over a scan size of 5 × 5 µm2. For silver surfaces, small holes and
bumps could even appear at higher evaporation rates, as shown in Figure 5c where a 55 nm thick silver film deposited at an evaporation rate of 1.3 ( 0.1 Å/s produced a film with an rms roughness of 15 Å over a scan size of 5 × 5 µm2 . The rms roughness of a silver film can be as large as 20-60 Å if the evaporation rate is higher than 1.5 Å/s and is not kept constant during the deposition. Theoretically, there is not much difference in applying an electric field (or surface potential difference) in the SFA using the two configurations shown in Figure 2a,b. However, the two configurations do have some different experimental or practical features and limitations: (1) For the configuration shown in Figure 2a, there are secondary FECO fringes (vertical lines in Figure 3b) that do not move when the surface separation changes; this could cause difficulties when tracking large changes in the surface profile or separation if the primary FECO fringes move into the wavelength range where the secondary fringes are located; (2) for the configuration shown in Figure 2b, as the separation of the two electrode surfaces decreases, the fringe spacing increases; if the number of fringes in the wavelength field of view is less than 1, then one may have to shift the field of view to a lower or higher wavelength range to track the surface separation. However, in most cases, there is no difference in applying an electric field using either of the configurations of Figure 2a,b. The configuration of Figure 2b was chosen for the two experiments described in this article because of its simplicity, and there should be no difference if the experiments are performed under the same conditions using the configuration of Figure 2a. Materials, Surface and Sample Preparation, and Experimental Procedures. Two different systems with an applied electrical field were studied using the SFA: (1) electrorheology of colloid suspensions and (2) electric field-induced polymer thin film patterning. The electrorheological (ER) fluid in our first study was a suspension of micrometer-sized (1-5 µm diameter) spherical surfactant-modified zeolite particles (obtained from Dongguan Haixing Chemical Co., Ltd, China) with high conductivity (Cp ) 1 × 10-4 S/m) and a high dielectric constant (p ) 30 000 at 100 Hz) at a volume fraction of 12%, suspended in silicone oil with a viscosity of 50 mPa s at 20 °C.59-61 The properties of zeolite particles and a suspension have been reported previously.59,61 The suspension was stable over the experimental time scale, with visible sedimentation occurring only over the course of weeks or months. During experiments, two silver surfaces were mounted into the surface SFA chamber in the configuration of Figure 2b and brought to a desired separation in air (about 40 µm), and the separation distance was calculated using eq 5 (with a medium refractive index of 1.00). The colloidal suspension was then injected into the gap between the surfaces, and the surface separation remained effectively unchanged because the Laplace pressure force of the liquid bridge pulling the two surfaces together, of order F ≈ 4πRγ ≈ 4π × 0.02 × 0.021 ≈ 5 mN, was negligible compared to the restoring force of the stiff cantilever springs (stiffness >106 N m-1) holding the surfaces in place.7 Thus, the spring deformation (change in surface separation) due to the liquid bridge was on the order of 5 mN/106 Nm-1 ≈ 5 nm (i.e., a
(55) Clarkson, M. T. J. Phys. D: Appl. Phys. 1989, 22, 475-482. (56) Horn, R. G.; Smith, D. T. Appl. Opt. 1991, 30, 59-65. (57) Levins, J. M.; Vanderlick, T. K. Langmuir 1994, 10, 2389-2394. (58) Horn, R. G.; Smith, D. T. Science 1992, 256, 362-364.
(59) Tian, Y.; Meng, Y. G.; Wen, S. Z. J. Appl. Phys. 2001, 90, 493-496. (60) Tian, Y.; Meng, Y. G.; Wen, S. Z. Appl. Phys. Lett. 2006, 88, 094106. (61) Tian, Y.; Zeng, H. B.; Anderson, T.; Zhao, B. X.; Mcguiggan, P. M.; Israelachvili, J. Phys. ReV. E 2007, 75, 011409.
(
)
4πµBTB tan ) λn + φ B
(
2µ˜ sin
) (
4πµATA λ n + φA
)
4πµATA (µ˜ - 1) - (1 + µ˜ )cos λn + φA 2
2
(8)
1178 Langmuir, Vol. 24, No. 4, 2008
Zeng et al.
Figure 5. AFM images of (a) a gold surface (rms roughness of 3 Å) deposited on a mica surface (with a predeposited 16 Å thick layer of Cr) at an evaporation rate of 0.1-0.2 Å/s; (b) a silver surface (rms roughness 5 Å) deposited at an evaporation rate of 0.1-0.2 Å/s; and (c) a silver surface (rms roughness 15 Å) deposited at an evaporation rate of 1.3 Å/s. few nanometers) and therefore very much smaller than the surface separation of D ≈ 40 µm. Different voltages V (0-400 V) were then applied across the two silver surfaces, and the shear forces and effective viscosities were measured as a function of the applied voltage V and (calculated) maximum electric field strength Emax ) V/D.
A low-molecular-weight polystyrene melt, Mw ) 800 Da, Mw/Mn e 1.3 (referred to as PS 800), with ηbulk ≈ 2800 Pa s, was chosen for the study of electric field-induced polymer film patterning. The PS 800 was purchased from Pressure Chemical Co. The bulk shear viscosity ηbulk of the polymer was measured using an ARES-controlled strain rheometer from Rheometrics Scientific in cone-plate geometry
New SFA Techniques
Langmuir, Vol. 24, No. 4, 2008 1179
with 25 mm diameter plates and a 0.1 rad cone angle at the experimental temperature of 24.0 ( 0.2 °C. The polymer film was prepared by spin coating a droplet of polymer solution in toluene on the silver substrate deposited on a mica sheet glued onto a silica disk and then very slowly dried under reduced pressure overnight to remove the solvent. The two surfaces (polymer-coated lower surface and silveredcoated upper surface) were mounted into the surface SFA chamber in the configuration of Figure 2b. A voltage of 250 V was then applied across the electrode surfaces for more than 20 h. Both the FECO and top-view microscopic images were recorded simultaneously for later analysis.
Results and Discussion Electrorheology of Colloid Suspensions. Since the pioneering work of Winslow in the late 1940s,62 the mechanism and application of electrorheological (ER) fluids, which are usually composed of small particles suspended in nonconducting fluids, have been studied in great detail and have attracted continually increasing interest.41 Most studies of ER fluids have focused on their macro- or microscale properties because of the experimental difficulties of studying their nanoscale behavior. Similar studies in the submicroscopic regime63,64 (e.g., on nanoparticles) can now be made using the SFA technique in combination with an electric field. In a typical SFA experiment (Experimental Section), the effective shear viscosity of the colloid suspension confined between the two surfaces before and after applying an electric field at room temperature (∼23 °C) was obtained from a standard SFA rheological shear experiment26 by measuring the shear force F| at any shear velocity V|. The effective shear viscosity ηeff was estimated using eq 926
ηeff ≈
5F| 2R 16πRV| ln D
( )
for R . D
Figure 6. Shear forces and effective shear viscosity vs applied voltage and the maximum electric field strength (Emax ) V/D) for a 40 µm thick film of 1-5 µm diameter zeolite particles suspended in silicon oil.
(9)
where R ≈ 2 cm is the local radius of curvature and D is the closest distance of separation as defined in Figure 2.26 In a typical experiment, D ≈ 40 µm and V| ≈ 12 µm/s. Without an electric field, the shear force of the zeolite particles/silicone oil suspension was F| ) 16.5 µΝ, giving an effective shear viscosity of ηeff ≈ 1.0 Pa s from eq 9, which is the same as the bulk viscosity of the colloid suspension. Different voltages V were then applied across the two silver surfaces. The surface separation hardly changed on applying an electric field because of the semirigid spring used here to support the lower surface (stiffness >106 N m-1). The spring deformation (change in surface separation) due to the electrostatic attraction of the two surfaces was on the order of 10 mN/106 N m-1 ≈ 10 nm and therefore very much smaller than the surface separation of D ≈ 40 µm. The shear forces and effective viscosities were measured as a function of the applied voltage V and (calculated) maximum electric field strength Emax ) V/D. The results are shown in Figure 6. With increasing V and E, the shear force and effective viscosity of the suspension increase dramatically; for example, at V ) 60 V (Emax ) 1.5 × 106 V/m), we measured F| ) 1.49 mN and ηeff ) 89.1 Pa s, about 90 times larger than the values in zero field. The rheological changes of the system are due to jamming of the colloidal particles brought about by the field-induced enhancement of the attractive interparticle forces.41 Some typical (62) Winslow, W. M. J. Appl. Phys. 1949, 20, 1137-1140. (63) Drummond, C.; Alcantar, N.; Israelachvili, J.; Tenne, R.; Golan, Y. AdV. Funct. Mater. 2001, 11, 348-354. (64) Cohen, I.; Mason, T. G.; Weitz, D. A. Phys. ReV. Lett. 2004, 93, 046001.
Figure 7. (a, b) Top-view microscope images of zeolite particles suspended in silicone oil sheared between two silver surfaces with a minimum gap distance of 40 µm in the SFA. Particle volume fraction, 12%; particle diameter, 1-5 µm; bulk viscosity of silicone oil at room temperature, 50 mPa s. (c, d) Side-view microscope images of the suspension in a separate experiment (not in the SFA). (Left panels) Before applying an electric field. (Right panels) After applying a field.
real-time, top-view microscopic images of the colloid suspension before and after applying an electric field in the SFA are shown in Figure 7a,b. After switching on the electric field, the colloidal particles in the suspension first associate into single filaments very rapidly (in milliseconds, the so-called fast mode). These filaments then associate or aggregate into bundles or columns more slowly (taking from hundreds of milliseconds to several seconds, the so-called slow mode).41,61 The former process is caused by the strong short-range dipole-dipole pair interactions excited by the applied field, and the latter process is driven by the weaker long-range forces between the filaments associated with the increased viscous drag forces of the suspension.65 The aggregation time of the particles and filaments depends mainly on the applied electric field, the viscosity of the solvent, and the particle volume fraction in the suspension. An estimate of the characteristic aggregation time is given by41,42
ts )
16η0 0cβ2E2
(10)
1180 Langmuir, Vol. 24, No. 4, 2008
where η0 is the viscosity of the suspending fluid, 0 is the dielectric permittivity of vacuum, c is the relative dielectric permittivity of the suspending fluid, β is the polarizability of the particles given by β ) (Cp - Cf)/(Cp + 2Cf) where β ≈ 1 in this study because the conductivity of the silicone oil, Cf = 1 × 10-12 S/m, is much smaller than that of the particles, Cp ) 1 × 10-4 S/m, and E is the strength of the electric field. Equation 10 ignores the inertia and Brownian motion of the particles.41,42 Inserting η0 ) 50 mPa s, c ) 2.5, β ) 1, and E ) (1.0-6.0) × 106 V/m into eq 10 gives ts ) 1-36 ms. In our study, the aggregation process was also visualized by a direct side-view microscope observation (i.e., not in the SFA) using the same colloidal suspension under similar electric field strength conditions. The side-view microscopic images of the suspension with and without an electric field are shown in Figure 7c,d. The oriented texture along the direction of the electric field shown in Figure 7d includes both bundles and filaments (mostly bundles because the filaments associate into bundles in the E field). The time required for the particles to form filaments and bundles (shown in Figure 7d) was several hundred milliseconds (the total time of two processessslow and fast modes), which is larger than the above theoretically predicted characteristic times for ts but falls in the range (of hundreds of milliseconds to several seconds) found in previous experimental studies.41 The time for the colloids to aggregate in an SFA experiment was consistent with the microscope observations under the same E-field strength. More detailed studies on the aggregation time of the fast and slow processes have been reported elsewhere.41,61 Electric Field-Induced Patterns in Thin Polymer Films. External field-induced self-organized surface patterns have attracted much attention during the past decade because of their importance in fundamental research and potential applications in biotechnological, polymer-engineered and MEMS systems.29-35,46 Among these, the surface deformations induced in fluids by external electric fields have been studied for more than 100 years, and the recent discovery of electric field-induced patterning of thin polymer melt films has attracted particular interest.33-35,46,47 Theoretical and experimental work has been done to help understand the linear and nonlinear evolution of electric field-induced patterning.33-35,46,47 Understanding all aspects of the real-time dynamic evolution of surface patterning instability due to an applied electric field remains a challenging problem. By using an SFA with an electric field, such processes can be easily studied using the optical multiple-beam interferometry FECO technique with simultaneous top-view microscopy. During the experiment, the two surfaces (the polymer-coated surface as the bottom surface and the bare silvered surface as the top surface) were mounted into the surface SFA chamber in the configuration of Figure 2b, as described in the Experimental Section. The initial polymer film thickness of Tp,0 ) 162 ( 2 nm was measured by FECO and confirmed using an ellipsometer, and the air gap distance was set at TA,0 ) 7 µm. A voltage of 250 V was then applied across the two electrode surfaces. Both the FECO and top-view microscopic images were recorded simultaneously; typical images are shown in Figure 8. At first, both the FECO fringes and top microscopic images revealed a smooth and featureless surface of the polymer film (Figure 8a). With increasing the time, three- to six-sided structures or cells appeared on the polymer surface, as shown in Figure 8b-e. These structures continued to distort (Figure 8f-h), finally ordering as roughly circular or six-sided (hexagonal), closepacked cells, as shown in Figure 8i-l. The field-induced pattern (65) Halsey, T. C.; Martin, J. E.; Adolf, D. Phys. ReV. Lett. 1992, 68, 15191522.
Zeng et al.
was observed to propagate radially out from the center (the region of closest approach of the curved surfaces and hence of highest field strength) to the outer regions shown in Figure 8h-j. The appearance of the patterned structures on the polymer surface is due to the competition between the electrostatic field pressure and the capillary (Laplace) pressure at the polymer/air interface.32,35,46 The electrostatic field enhances the growth of the ridges, whereas the (Laplace) pressure opposes the growth. When the electrostatic pressure overcomes the opposing curvaturedependent capillary pressure, patterns or cells of specific wavelengths are generated and amplified as polymer flows into the ridges of the cells from their centers (the troughs). Different patterns have been reported to form on polymer surfaces depending on the topography of the top electrode surface (e.g., whether patterned or unpatterned).46 Recent nonlinear dynamic studies by Wu et al.46 have shown that hexagonal patterns grow the fastest under an unpatterned mask (electrode surface). The hexagonal structures observed in our experiment (Figure 8) are consistent with these previous studies. Linearized stability analysis was used to model the surface patterns as a sinusoidal perturbation with wave number q, amplitude u, and growth rate . The topography of the polymer/ air interface Tp(x, t) is given by eq 11, where x is the spatial coordinate, t is the time, and Tp,0 is the initial polymer thickenss.35
Tp(x, t) ) Tp,0 + ueiqx+(t/τ)
(11)
∆L ) 2µpTp + 2µATA ) nλtrough n
(12)
∆L′ ) 2µp(Tp + ∆Tp) + 2µA(TA - ∆Tp) ) nλpeak (13) n In our study, the amplitude of the perturbation u was estimated from the real-time FECO fringes by considering the optical path difference ∆L, ∆L′ between the troughs and peaks, as given by eqs 12 and 13, where µp and µA are the refractive indices of the polymer and air, Tp and TA are the polymer and air gap thicknesses, and λtrough are the wavelengths of the same nth-order λpeak n n interference fringe of the closest peak and trough of the fieldinduced pattern, and n ≈ λn - 1/(λn - 1 - λn) as defined by eq 5. The amplitude of the wave pattern is then given by eq 14
u ) (∆Tp)max )
n(λpeak - λtrough ) n n 2(µp - µA)
(14)
from which the exact shape of the pattern can be determined. The development of the peak amplitude with time is plotted on both a semilog scale (open symbols) and a linear scale (solid symbols) in Figure 9. In the early stage of pattern evolution, the peak amplitude increases roughly linearly with time when plotted on a semilog scale, implying an exponential increase in the peak amplitude with time (viz., u ∝ t/τ) with a characteristic growth time in the early stage of the evolution stage of τ ≈ (1.5 ( 0.1) × 104 s. According to the linear analysis model, the characteristic growth time is predicted to be given by eq 15, where γ is the polymer surface energy, η is the viscosity, Tp,0 is the initial polymer thickness, V is the applied voltage, D is the gap between the two electrodes given by eq 16, 0 is the dielectric permittivity of free space (vacuum), and p is the relative dielectric constant of the polymer32,35
τ)
3γη T3p,0V4(0p)2(p
- 1)
[pD - (p - 1)Tp,0]6
4
(15)
New SFA Techniques
Langmuir, Vol. 24, No. 4, 2008 1181
Figure 8. (a-k) Online FECO and top-view microscope images during the evolution of an electric field-induced instability of a thin PS 800 film in the SFA. (l) Frozen image of the patterned surface, frozen using liquid nitrogen, at the end of the experiment. (This image was taken using a Nikon OPTIPHOT 200 microscope.)
where
D ) T p + TA
(16)
Substituting γ ≈ 0.035 N/m, η ≈ 2800 Pa s, V ) 250 V, Tp,0 ) 162 nm, D ≈ 7.16 µm, p ≈ 2.55, and 0 ) 8.854 × 10-12
C2/J m into eq 15 gives τ ≈ 1.9 × 105 s, which is about 1 order of magnitude larger than the experiment value of ∼1.5 × 104 s quoted in the previous paragraph. The possible reason for the difference is that in our experiment the ratio of the initial polymer film thickness Tp,0 to the (total) distance between the two electrode surfaces is Tp,0/(Tp,0 + TA,0) ≈ 0.023, which is very small and
1182 Langmuir, Vol. 24, No. 4, 2008
Zeng et al.
Ep )
Figure 9. Evolution of electric field-induced patterns with time showing the growth of the peak amplitude, u, with time, t, plotted in two ways (linear on the right axis and semilogarithmic on the left axis).
could make the real electric field-induced instability evolution behavior depart from the linear approximation, although exponential growth is still observed. As shown in Figure 9, with increasing time, the growth of the peaks accelerates (when viewed on a linear scale), but they still grow roughly linearly with time when plotted on a semilog plot, implying an exponential increase in the peak amplitude with time, u ∝ t/τ, but with a smaller characteristic growth time of τ ) (0.6 ( 0.1) × 104 s. The reason for the accelerating growth is that the air gap becomes smaller as the peaks grow, which strengthens the electric field, thereby accelerating the growth. At t ≈ 5 × 104 s, the peak amplitude starts to exceed the initial polymer thickness, and the growth and pattern of the peaks become more intricate. This stage can be seen in the FECO and top-view microscopic images of Figure 8f-h, showing that the original semiordered pattern begins to collapse or distort. Later (for t > 7 × 104 s, shown in Figure 8i-l), the distorted pattern becomes ordered again but rounded instead of hexagonal and still in a hexagonal lattice. The time evolution of the surface structures and nonlinear growth of the peak heights are consistent with recent nonlinear dynamic analyses,33,34,46,47 which concluded that a hexagonal pattern is preferred when the top (mask) surface is not patterned, and the hexagonal patterns can become distorted or even destroyed in the nonlinear regime. On the basis of a linear analysis, the wavelength of the fastest growing wavelike pattern is predicted to be given by eq 17, where Ep is the electric field strength in the polymer film, given by eq 18.32,35
λ ) 2π
x
γV Ep-3/2 0p(p - 1)2
(17)
V pD - (p - 1)Tp
(18)
Substituting the experimental values into these two equations, the electric field strength in the initial polymer film is about 1.4 × 107 V/m, and the wavelength of the fastest growing fluctuation is predicted to be about 47 µm (maximum wavelength). Experimentally, the mean distance (or wavelength) of the waves was easily measurable from both the FECO fringes and top-view microscopic images, yielding a value of λmeas ) 16 ( 1 µm, which is about 3 times smaller than the theoretical value. Possible reasons for this discrepancy could be due to the use of eq 17, which (1) predicts the maximum and fastest growing pattern wavelength and (2) is a linear approximation. In our system, the ratio of the initial polymer film thickness Tp,0 to the (total) distance between the two electrode surfaces is Tp,0/(Tp,0 + TA,0) ≈ 0.023, which is much smaller than in previous studies on thicker films (ratio >0.2) for which the linear approximation was shown to apply.35,44-46
Conclusions We have shown how electric fields can be applied in surface forces apparatus experiments to successfully measure their effects on surface and viscous forces and pattern formation. Two different ways to include surface electrodes in the SFA were described, including the optical methods for calculating the film thicknesses and surface separation. Two types of experiments were performed to illustrate two applications of the new technique: (1) a measurement of electrical field effects on the rheology (modified viscosity) of a suspension of zeolite particles in silicone oil and (2) an on-line dynamic study of the electric field-induced patterning of a thin liquid polymer film. Under the influence of an electric field, the shear forces and shear viscosity of the colloid suspension were found to be 2 orders of magnitude larger than the values without the field. The on-line dynamic study showed that the amplitude of the field-induced quasi-hexagonal pattern grows roughly exponentially with time in the early stage of growth and nonexponentially and more intricately in the late stage when the height of the patterns exceeds the initial polymer film thicknesssresults that are consistent with previous linear and nonlinear theories, respectively. By adding electrode surfaces to the SFA, detailed nanoscale studies on the effects of electric fields on fundamental surface interactionssboth long-ranged and short-ranged (adhesion and friction) forces and both in air and in a liquid mediumsshould be possible. Acknowledgment. This work was supported by ONR award no. N00014-05-1-0540. Y.T. thanks Tsinghua University for a Huaxin Distinguished Scientist Scholarship. Supporting Information Available: Estimation of the electric field strength and electrostatic force between two curved conducting surfaces in SFA experiments. This material is available free of charge via the Internet at http://pubs.acs.org. LA7017242
