Deposition of Magnetic Colloidal Particles on Graphite and Mica

The deposition of colloidal magnetite particles onto graphite and mica surfaces ... aggregates; the mean aggregate diameter is larger on graphite than...
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Langmuir 2006, 22, 5611-5616

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Deposition of Magnetic Colloidal Particles on Graphite and Mica Surfaces Driven by Solvent Evaporation Kevin J. Mutch,† Vasileios Koutsos,‡ and Philip J. Camp*,† School of Chemistry, UniVersity of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, and Institute of Materials and Processes, School of Engineering and Electronics and Centre for Materials Science and Engineering, UniVersity of Edinburgh, Mayfield Road, Edinburgh EH9 3JL, United Kingdom ReceiVed January 16, 2006. In Final Form: March 31, 2006 The deposition of colloidal magnetite particles onto graphite and mica surfaces induced by solvent evaporation is studied using atomic force microscopy. After evaporation under ambient conditions we observe polydisperse beadlike aggregates; the mean aggregate diameter is larger on graphite than on mica. After evaporation at elevated temperatures we observe a variety of effects, including enhanced particle aggregation and spinodal-like deposition patterns. To explain these trends, we propose mechanisms involving the wetting properties of the solvent. We have also made a brief study of the effects of applied magnetic fields on the formation of aggregates. A field applied parallel to the surface enhances aggregation and favors deposition patterns characteristic of hole-nucleation processes. A perpendicular field leads to a reduction in aggregate size and favors a homogeneous distribution of particles on the surface. These effects are explained in terms of the likely orientation of the dipolar particles on the surface.

I. Introduction Ferrofluids are colloidal dispersions of roughly spherical, homogeneously magnetized particles in simple solvents. Commonly, the particles are made from magnetite, iron, or cobalt, and are sterically stabilized by an organic (usually polymeric) coating. Typical particle diameters are tens of nanometers, and so long-range magnetic dipole-dipole interactions can be rather significant as compared to the short-range dispersion interactions between the organic coatings. Dipolar interactions can have profound effects on the structural and dynamical characteristics of colloidal dispersions, not the least of which is a tendency for particles to form chainlike aggregates, stabilized by the lowenergy nose-to-tail conformation. Aggregation is promoted by the application of a magnetic field since the dipole orientations are then strongly aligned, but cluster formation can also be observed in the absence of a field if the dipole-dipole interaction is large enough compared to the thermal energy;1 see refs 2 and 3 for reviews of basic concepts and phenomenology. A thorough understanding of such effects is important not only for informing the design of functional magnetic fluids, but also in gaining insight on analogous network-forming systems such as living polymers, microemulsions, and actin gels.4 Aggregation phenomena in bulk ferrofluids (with and without applied magnetic fields) have therefore been intensively studied using a wide variety of experimental methods including direct optical microscopy,5,6 small-angle neutron scattering,7-10 ferromagnetic resonance,11 * To whom correspondence should be addressed. E-mail: [email protected]. † School of Chemistry. ‡ School of Engineering and Electronics and Centre for Materials Science and Engineering. (1) de Gennes, P. G.; Pincus, P. A. Phys. Kondens. Mater. 1970, 11, 189-198. (2) Teixeira, P. I. C.; Tavares, J. M.; Telo da Gama, M. M. J. Phys.: Condens. Matter 2000, 12, R411-R434. (3) Huke, B.; Lu¨cke, M. Rep. Prog. Phys. 2004, 67, 1731-1768. (4) Zilman A.; Tlusty, T.; Safran, S. A. J. Phys.: Condens. Matter 2003, 15, S57-S64. (5) Hong, C.-Y.; Jang, I. J.; Horng, H. E.; Hsu, C. J.; Yao, Y. D.; Yang, H. C. J. Appl. Phys. 1997, 81, 4275-4277. (6) Hong, C.-Y.; Horng, H. E.; Jang, I. J.; Wu, J. M.; Lee, S. L.; Yeung, W. B.; Yang, H. C. J. Appl. Phys. 1998, 83, 6771-6773. (7) Dubois, E.; Cabuil, V.; Boue´, F.; Perzynski, R. J. Chem. Phys. 1999, 111, 7147-7160.

small-angle X-ray scattering,12 light scattering,8,13,14 nonspecular X-ray reflectivity,15 and photon correlation spectroscopy.16 The study of magnetic particles in confined geometries offers exciting opportunities for directly imaging cluster formation; indeed, some of the earliest images of chainlike clusters in a magnetic colloid were obtained almost 40 years ago using electron microscopy.17 In recent experimental work, transmission electron microscopy (TEM) was used to image single layers of cobalt,18 while cryo-TEM allowed direct visualization of iron and magnetite particles in the matrix of a frozen solvent.19-22 The results are striking, but it must be remembered that cryo-TEM images may not be faithful snapshots of the equilibrium structures in a magnetic fluid. This is significant because some of the primary applications of magnetic colloids (lubricants, seals, magnetic domain detection) involve the fluid phase. In addition, there is a large body of theoretical and simulation results for cluster formation and dynamics in the fluid phase, both in the bulk23-25 and in monolayers.26-29 It would therefore be useful to develop less (8) Shen, L. F.; Stachowiak, A.; Fateen, S.-E. K.; Laibinis, P. E.; Hatton, T. A. Langmuir 2001, 17, 288-299. (9) Aksenov, V. L.; Avdeev, M. V.; Balasoiu, M.; Bica, D.; Rosta, L.; To¨ro¨k, Gy.; Vekas, L. J. Magn. Magn. Mater. 2003, 258-259, 452-455. (10) Wiedenmann, A.; Kammel, M.; Hoell, A. J. Magn. Magn. Mater. 2004, 272-276, 1487-1489. (11) Lacava, L. M.; Lacava, B. M.; Azevedo, R. B.; Lacava, Z. G. M.; Buske, N.; Tronconi, A. L.; Morais, P. C. J. Magn. Magn. Mater. 2001, 225, 79-83. (12) Eberbeck, D.; Bla¨sing, J. J. Appl. Crystallogr. 1999, 32, 273-280. (13) Lui, J.; Lawrence, E. M.; Wu, A.; Ivey, M. L.; Flores, G. A.; Javier, K.; Bibette, J.; Richard, J. Phys. ReV. Lett. 1995, 74, 2828-2831. (14) Licinio, P. J. Magn. Magn. Mater. 2002, 252, 238-240. (15) Takahashi, I.; Tanaka, N.; Doi, S. J. Appl. Crystallogr. 2003, 36, 244248. (16) Bu¨scher, K.; Helm, C. A.; Gross, C.; Glo¨ckl, G.; Romanus, E.; Weitschies, W. Langmuir 2004, 20, 2435-2444. (17) Hess, P. H.; Parker, P. H., Jr. J. Appl. Polym. Sci. 1966, 10, 1915-27. (18) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115-2117. (19) Donselaar, L. N.; Frederik, P. M.; Bomans, P.; Buining, P. A.; Humbel, B. M.; Philipse, A. P. J. Magn. Magn. Mater. 1999, 201, 58-61. (20) Butter, K.; Philipse, A. P.; Vroege, G. J. J. Magn. Magn. Mater. 2002, 252, 1-3. (21) Butter, K.; Bomans, P. H. H.; Frederik, P. M.; Vroege, G. J.; Philipse, A. P. Nat. Mater. 2003, 2, 88-91. (22) Butter, K.; Bomans, P. H.; Frederik, P. M.; Vroege, G. J.; Philipse, A. P. J. Phys.: Condens. Matter 2003, 15, S1451-S1470. (23) Weis, J. J.; Levesque, D. Phys. ReV. Lett. 1993, 71, 2729-2732. (24) Camp, P. J.; Patey, G. N. Phys. ReV. E 2000, 62, 5403-5408.

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disruptive techniques for imaging magnetic particles at, say, solid-fluid interfaces. To this end, atomic force microscopy (AFM) seems a natural choice, although relatively little work on the application of AFM to imaging magnetic nanoparticles has appeared in the literature.11,30-33 For the most part, AFM has been used in particle-sizing studies or in the determination of particle shape and polydispersity, rather than as a technique for characterizing large-scale structures. In this paper, we report the results of an AFM study of magnetite particles deposited onto smooth highly ordered pyrolytic graphite (HOPG) and mica surfaces by solvent evaporation. We show that AFM can be used to characterize the large-scale structures formed by these materials. In particular, we report trends in the particle-aggregate-size distributions, and some observations of qualitatively distinct structures arising from the different mechanisms by which the solvent dewets the surface. Furthermore, we examine the effects of surface heating and applied magnetic fields: we show that in the absence of a magnetic field the solvent evaporation/dewetting is the most significant factor influencing the pattern formation, while in the presence of a magnetic field the magnetic nature of the nanoparticles becomes important and significantly affects the deposition patterns. This paper is organized as follows: In section II we summarize the experimental procedure, and in Sections III and IV we report results for HOPG and mica surfaces, respectively. Magnetic-field effects are considered briefly in section V, and in section VI we present our conclusions.

Mutch et al. adsorption, mica surfaces were coated in poly-L-lysine ostensibly to avoid particle attachment to the tip;32 we did not encounter such problems with the particular ferrofluids used in this study. The samples were diluted as necessary with laboratory-grade n-heptane. In each case a drop of the suspension was deposited on a freshly cleaved surface; the solvent-evaporation time was never more than 1 min. To explore the effects of a parallel (perpendicular) magnetic field, small bar magnets (B ≈ 0.01 T) were placed on either edge (face) of the surface; the magnetic field-dipole interaction energy (µB) is therefore on the order of kBT. We also conducted some experiments in which mica surfaces were heated prior to deposition of the ferrofluid. After drying, the surfaces were examined using a Molecular Imaging PicoSPM atomic force microscope (Molecular Imaging, 4666 S. Ash Ave., Tempe, AZ 85282) operating in tapping mode. The cantilever/tip components were Si3N4 (MikroMasch, Narva mnt 13, 10151 Tallinn, Estonia) with a nominal spring constant k ) 1.75 N m-1 and resonance frequencies in the range of 130-160 kHz. The nominal tip radii are in the region of 10-25 nm, although in some cases the radii could be 50 nm or more. For a spherical tip with radius R, the apparent radius of a surface particle with radius r is 2(Rr)1/2 (this is the horizontal distance between the centers of two hard spheres at contact on a surface). Hence, we expect the particle sizes in our AFM images to be enlarged by factors of between 2.8 and 4.5 for tip radii in the range from 10 to 25 nm, or even as much as 6.3 for a 50 nm radius tip. AFM images were analyzed using SPIP software (Scanning Probe Image Processor, Image Metrology, Diplomvej 376, DK-2800 Lyngby, Denmark).

III. Adsorption on Graphite Surfaces II. Materials and Methods Two commercially available isoparaffin dispersed magnetite (Fe3O4) ferrofluids were studied: ferrofluid A (FFA) with a quoted saturation magnetic flux density B ) 400 G (or saturation magnetization Ms ) B/µ0 ) 3.2 × 104 A m-1, where µ0 is the vacuum magnetic permeability) and a magnetite concentration of ∼20% by mass (Liquids Research Ltd., Bangor, U.K.) and ferrofluid B (FFB) with a quoted saturation magnetic flux density B ) 200 G (Ms ) 1.6 × 104 A m-1) and a magnetite concentration of 3.6 vol % (Ferrofluidics Corp., New Hampshire). In both cases the mean particle diameter, σ, was quoted as being 10 nm, and each particle was assumed to possess a single ferromagnetic domain. Assuming that the domain magnetization is approximately equal to the bulk saturation value (Mbulk ) 4.8 × 105 A m-1), the volume fractions s of the ferromagnetic cores in FFA and FFB are φ ) {Ms/Mbulk s } ) 6.6 × 10-2 and φ ) 3.3 × 10-2, respectively. (The value for FFB is consistent with the quoted magnetite volume fraction, allowing for a thin nonmagnetic layer at the particle surface.) In what follows, we adopt φ as the measure of ferrofluid concentration. The key parameter in determining the significance of dipole-dipole interactions is the ratio of the characteristic dipolar energy to the thermal energy given by λ ) {µ0µ2}/{4πσ3kBT}, where µ ≈ {πσ3Mbulk s }/6 is the magnetic dipole moment, kB is Boltzmann’s constant, and T is the temperature. For 10 nm single-domain magnetite particles at room temperature this parameter is λ ≈ 1.5; simulations show that this corresponds to a weakly dipolar regime.29 The HOPG and mica surfaces were used as supplied (Agar Scientific Ltd., Stansted, U.K.). In a previous AFM study of ferrofluid (25) Murashov, V. V.; Camp, P. J.; Patey, G. N. J. Chem. Phys. 2002, 116, 6731-6737. (26) Weis, J. J. Mol. Phys. 1998, 93, 361-364. (27) Weis, J. J. Mol. Phys. 2002, 100, 579-594. (28) Weis, J.-J. J. Phys.: Condens. Matter 2003, 15, S1471-S1495. (29) Duncan, P. D.; Camp, P. J. J. Chem. Phys. 2004, 121, 11322-11331. (30) Bui, Q. T.; Pankhurst, Q. A.; Zulqarnain, K. IEEE Trans. Magn. 1998, 34, 2117-2119. (31) Ras¸ a, M.; Philipse, A. P. J. Magn. Magn. Mater. 2002, 252, 101-103. (32) Ras¸ a, M.; Kuipers, B. W. M.; Philipse, A. P. J. Colloid Interface Sci. 2002, 250, 303-315. (33) Crisan, O.; Angelakeris, M.; Vouroutzis, N.; Crisan, A. D.; Pavlidou, E.; Kostic, I.; Sobal, N.; Giersig, M.; Flevaris, N. K. J. Magn. Magn. Mater. 2004, 272-276, E1285-E1287.

Initial experiments with FFA and FFB on HOPG surfaces showed that colloid concentrations of 3-7 vol % were so high as to give multilayer coverage with very little observable structure. The stock samples were then diluted to reduce the surface coverage after solvent evaporation. In Figure 1a,b we show results from FFA at φ ) 7.2 × 10-5 and φ ) 3.3 × 10-6, respectively. At φ ) 7.2 × 10-5 the surface coverage was typically around 7080%, and the apparent diameters of the visible objects were in the region of 200-600 nm. The size distribution is shown in Figure 2a: the mean aggregate diameter is 365 ( 168 nm, where the latter number is the standard deviation, not the statistical uncertainty in the mean; the corresponding parameter for the aggregate height is 28 ( 10 nm. The mean aggregate diameter is an order of magnitude larger than the typical parameters for individual colloidal particles, and even taking into account tip convolution effects, the objects must be aggregates of colloidal particles consisting of, at most, a couple of particle layers. Nonetheless, the aggregates in Figure 1a are arranged in an almost close-packed fashion. At φ ) 3.3 × 10-6 FFA forms much better resolved aggregates but now with diameters in the range of 100350 nm. The aggregate-size distributions are shown in Figure 2b: the mean values of the aggregate diameters and heights are 225 ( 54 and 23 ( 5 nm, respectively. In Figure 1b, an almost perfectly straight line of aggregates is visible in the lower half of the image. This was a common observation in deposits of low-concentration ferrofluids on HOPG surfaces. Another example is shown in Figure 1c in which FFB (φ ) 4.6 × 10-6) has formed several parallel lines, with the aggregates randomly spaced along the lines. The formation of lines is attributed to atomic-scale defects such as monolayer terraces, or even trenches, in the HOPG surface which may afford an increased degree of contact with the colloidal aggregates. Indeed, a close inspection of Figure 1c shows that the aggregates are slightly elongated in shape, as if they spread out to increase the contact with the defects. We even produced one or two samples with pairs of

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Figure 1. Images of FFA and FFB on HOPG surfaces: (a) FFA, φ ) 7.2 × 10-5; (b) FFA, φ ) 3.3 × 10-6; (c) FFB, φ ) 4.6 × 10-6; (d) FFA, φ ) 7.6 × 10-6.

Figure 2. Particle diameter distributions for FFA and FFB on HOPG and mica surfaces: (a) FFA (φ ) 7.2 × 10-5) on HOPG (corresponding to Figure 1a); (b) FFA (φ ) 3.3 × 10-6) on graphite (corresponding to Figure 1b); (c) FFB (φ ) 4.6 × 10-6) on mica (corresponding to Figure 3b); FFA (φ ) 1.7 × 10-6) on mica (heat) (corresponding to Figure 3c).

lines at approximately 60° to each other, an example of which was FFA (φ ) 7.6 × 10-6) on graphite as shown in Figure 1d. Presumably, these features are due to terraces or trenches influenced by the underlying hexagonal graphite structure.

IV. Adsorption onto Mica Surfaces The ferrofluids studied in this work are dispersed in nonpolar organic solvents, so it was anticipated that a hydrophilic substrate

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Figure 3. Images of FFA and FFB on mica surfaces: (a) FFA, φ ) 7.6 × 10-6; (b) FFB, φ ) 4.6 × 10-6; (c) FFA (φ ) 1.7 × 10-6) deposited on a heated surface; (d) FFB (φ ) 4.6 × 10-6) deposited on a heated surface.

such as mica might not adsorb the colloidal particles to the same extent as the hydrophobic HOPG surface. Nonetheless, significant surface coverages were observed. In Figure 3a we show FFA (φ ) 7.6 × 10-6) adsorbed on mica. Compared to the graphite surface with the same sample (Figure 1d), the mean aggregate size has decreased considerably. It proved impossible to obtain a reliable aggregate-size distribution for the AFM image in Figure 3a because the aggregates are relatively small and are too close together. We therefore turn to FFB (φ ) 4.6 × 10-6) on mica, which forms better resolved aggregates as shown in Figure 3b. The aggregate-size distributionsshown in Figure 2csis characterized by a mean diameter of 105 ( 31 nm and a mean height of 14 ( 5 nm. Comparison of Figure 3b with that for the same sample on graphite (Figure 1c) shows that the aggregates are very much smaller, and almost certainly flatter, when adsorbed onto mica. Indeed, a mean height of 14 nm would indicate a monolayer, given the quoted particle diameter of the magnetic particles (∼10 nm). The fact that the characteristic aggregate size decreases in changing from a nonpolar to a polar surface may be due to the wetting characteristics of the solvent, and the consequences for the clustering process. For example, FFA is an isoparaffin-based ferrofluid, and we were able to dilute the samples in heptane with no noticeable loss in colloidal stability. Heptane is expected to wet the HOPG surface readily, so when the dispersion spreads over the surface and the solvent begins to evaporate, colloidal particles have the opportunity to migrate

through the solvent layer over large distances and thereby aggregate into large clusters. In the case of mica, however, the dispersion is more likely to form small “beads” because the solvent will not wet the surface so readily. Therefore, one might expect smaller aggregates to condense from the isolated beads as the solvent evaporates. To test this scenario, we carried out experiments in which the mica surfaces were heated to around 320 K on a hot plate prior to the ferrofluid being deposited. Surface tensions generally decrease with increasing temperature,34 so we anticipated that at elevated temperatures the solvent would wet the surface better and hence larger aggregates should have the opportunity to form. On the other hand, increasing the temperature would also increase the solvent evaporation rate, which may favor one particular solvent-dewetting mechanism over another. There are two likely mechanisms for solvent evaporation and particle deposition. The first candidate is the formation of dry spots or holes in the film nucleated by surface defects or heterogeneities (“heterogeneous nucleation”), or as a result of thermal activation (“homogeneous nucleation”).35,36 In this mechanism, the final structure consists of randomly distributed holes on the surface, with the holes (34) Rowlinson, J. S.; Widom, B. Molecular Theory of Capillarity; Dover Publications: Mineola, NY, 2002. (35) Seemann, R.; Herminghaus, S.; Jacobs, K. Phys. ReV. Lett. 2001, 86, 5534-5537. (36) Seemann, R.; Herminghaus, S.; Jacobs, K. J. Phys.: Condens. Matter 2001, 13, 4925-4938.

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Figure 4. Two-dimensional structure factor for the spinodal pattern formed by FFB (φ ) 4.6 × 10-6) deposited on a heated mica surface, as shown in Figure 3d.

being delimited by any initially dispersed particles. In ref 37 the specific example given was of collagen dispersed in water. It has been suggested that this mechanism dominates when there is a very low solvent evaporation rate, in which case the nucleated holes have time to grow and cover the surface before any

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remaining solvent film becomes thinner than about 10 nm. The second candidate mechanism is the spontaneous rupture of the film driven by exponentially growing surface fluctuations as the film thickness is reduced to a critical value (typically 10-100 nm).35,36,38-40 This mechanism has been termed “spinodal dewetting”, and the characteristic length scale of the resulting spinodal structure is dictated by the range and strength of the intermolecular forces. In Figure 3c,d we show images of FFA (φ ) 1.7 × 10-6) and FFB (φ ) 4.6 × 10-6), respectively, on heated mica surfaces. FFA (Figure 3c) exhibits a broad distribution of aggregate sizes, with the largest aggregates having apparent diameters of about 600 nm. The distribution is shown in Figure 2d: the mean diameter and mean height are 125 ( 82 and 14 ( 7 nm, respectively. In stark contrast, FFB (Figure 3d) formed striped structures. The structure shown in Figure 3d is suggestive of the spinodal dewetting mechanism, although it certainly does not provide conclusive evidence. To determine a characteristic length scale for the structure, we computed a two-dimensional structure factor S(k) ∝ h(k) h(-k), where h(k) ) ∫h(r) exp(-ik‚r) dr is a spatial Fourier component of the height profile h(r) and k is a reciprocallattice wave vector derived from the square unit cell of side L ) 6.6 µm shown in Figure 3d. (Periodic boundary conditions are assumed, which introduces errors when k ) |k| ≈ 2π/L ) 0.95

Figure 5. Images of FFA and FFB on mica surfaces with applied magnetic fields: (a) FFA (φ ) 1.7 × 10-6) on mica with the field parallel to the surface; (b) FFA (φ ) 1.7 × 10-6) on mica with the field perpendicular to the surface; (c) FFB (φ ) 4.6 × 10-6) on mica with the field parallel to the surface; (d) FFB (φ ) 4.6 × 10-6) on mica with the field perpendicular to the surface.

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µm-1 but is otherwise reliable for shorter wavelengths, or higher wavevectors.) Contributions with equal k ) |k| were averaged, and are plotted in Figure 4. Included in Figure 4 is a fit to the Lorentzian curve S(k) ) [1 + (k - k0)2/∆k2]-1 with the peak position k0 ) 4.8 ( 0.2 µm-1 and half-width at half-maximum ∆k ) 3.3 ( 0.3 µm-1. The characteristic length scale corresponds to the typical spacing between the ridges in Figure 3d, and is approximately equal to 2π/k0 ) 1.31 ( 0.05 µm. We conclude that higher temperatures favor the formation of larger aggregates and/or spinodal-like dewetting structures. This is further proof of the importance of drying/dewetting effects on the surface deposition of nanoparticles, and shows the importance of the substrate/solvent interactions and the solvent evaporation rate in the absence of an applied magnetic field.

V. Magnetic-Field Effects In Figure 5a,b we show AFM images of ferrofluids adsorbed onto mica surfaces in the presence of (weak) applied magnetic fields. FFA (φ ) 1.7 × 10-6) on mica with a magnetic field applied parallel to the surface (Figure 5a) exhibited an extremely heterogeneous structure, including small aggregates and some chainlike structures of individual colloidal particles. The structure in the upper right of Figure 5a resembles the “Y” defect structure implicated by Tlusty and Safran in driving fluid-fluid phase separation in three-dimensional network-forming systems.41 The particle-field interaction in our experiments is very weak due to the means of applying the field (bar magnets) and the low magnetization of the particles. Nonetheless, it appears that the magnetic field has promoted particle aggregation during deposition, presumably due to a partial alignment of the particle dipole moments in the plane of the surface, giving rise to the lowenergy nose-to-tail configuration. A field applied perpendicular to the plane should therefore diminish the size of the aggregates, in that the particle dipole moments will be partially aligned in a high-energy parallel side-by-side configuration. Figure 5b shows FFA (φ ) 1.7 × 10-6) on mica in the presence of a perpendicular magnetic field; clearly, the distribution of aggregates is very sparse, and the mean aggregate size is small, in qualitative agreement with our expectation. Some similar observations can be made with FFB (φ ) 4.6 × 10-6) on mica with parallel or perpendicular fields applied; parts c and d of Figure 5 show the effects of parallel and perpendicular fields, respectively. With a parallel field (Figure 5c) there are holes in the structure, pointing to the heterogeneous nucleation picture described in section IV.37 In contrast, with a perpendicular field (Figure 5d) there is an almost homogeneous distribution of aggregates on the surface. Although the tendency to form aggregates is stronger than that to form chains of single particles, the magnetic interactions between aggregates must have had some effect on the deposition mechanism; otherwise the structures in Figure 5c,d would be similar.

VI. Conclusions Using atomic force microscopy, we have examined the structures of magnetic colloidal particles deposited on graphite (37) Thiele, U.; Mertig, M.; Pompe, W. Phys. ReV. Lett. 1998, 80, 2869-2872.

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and mica surfaces by solvent evaporation. In all cases the solvent was nonpolar (isoparaffin or n-heptane). Typically, the colloidal particles form aggregates with diameters of 100-400 nm, and heights in the region of 15-30 nm. The polydispersity and organization of the aggregates depends on the initial colloid concentration, and the presence of defects on the surface. On graphite surfaces, the mean aggregate size decreases with decreasing initial colloid concentration. The aggregates are seen to align along defects on the surface, such as monolayer terraces or trenches. On moving to mica surfaces, the aggregate sizes were seen to decrease significantly. This is attributed to the fact that the nonpolar solvent will not be able to wet the polar mica surface as well as it does the nonpolar graphite surface. The effects of heating mica surfaces prior to colloidal deposition were seen to be dramatic. With one ferrofluid sample, the aggregates became more polydisperse and the mean aggregate size increased, as compared to the ambient-temperature situation. This was attributed to the fact that the solvent can wet the surface better with an increase in temperature. With a different ferrofluid sample, the colloid is deposited into a “spinodal” structure, which indicates that the process of the solvent dewetting the surface is driven by surface fluctuations when the liquid film becomes thin. A magnetic field applied in the plane of the surface was shown to promote the formation of aggregates (and in one case, a Y-shaped cluster of individual particles), while the field perpendicular to the plane disfavored the formation of large aggregates. There is also evidence for the solvent-dewetting and consequent particle-deposition processes being affected by the alignment and mutual interactions of the magnetic particles; a parallel field favors patterns characteristic of hole nucleation, while a perpendicular field favors a more homogeneous surface coverage. This study was largely concerned with testing our AFM methodology as applied to the study of magnetic colloids. In future work we will apply the AFM technique to study cluster formation and structure in magnetic colloids adsorbed at a solidliquid interface. To this end we intend to prepare colloidal particles of high-magnetization materialsssuch as cobalt18sso that cluster formation is enhanced. Performing such experiments under liquids may remove some difficulties associated with the instability of the colloidal particles with respect to oxidation. It would also be interesting to carry out large-scale molecular dynamics simulations to shed light on the detailed microscopic mechanisms responsible for different particle-deposition patterns. Acknowledgment. We thank Dr. Vijay Patel (Liquids Research, Bangor, U.K.), Dr. Wolf-Gerrit Fru¨h (Heriot-Watt University), and Ferrofluidics Corp. for gifts of ferrofluid samples. Funding from the University of Edinburgh is gratefully acknowledged. LA060143K (38) Ruckenstein, E.; Jain, R. K. J. Chem. Soc., Faraday Trans. 2 1974, 70, 132-147. (39) Reiter, G.; Sharma, A.; Casoli, A.; David, M.-O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551-2558. (40) Sharma, A.; Khanna, R. J. Chem. Phys. 1999, 110, 4929-4936. (41) Tlusty, T.; Safran, S. A. Science 2000, 290, 1328-1331.