Article pubs.acs.org/Langmuir
Magnetic Field Aligned Assembly of Nonmagnetic Composite Dumbbells in Nanoparticle-Based Aqueous Ferrofluid Hayato Takahashi, Daisuke Nagao,* Kanako Watanabe, Haruyuki Ishii, and Mikio Konno* Department of Chemical Engineering, Tohoku University, 6-6-07 Aoba, Aramaki-aza Aoba-ku, Sendai, 980-8579 Japan S Supporting Information *
ABSTRACT: Monodisperse, nonmagnetic, asymmetrical composite dumbbells in a suspension of magnetic nanoparticles (ferrofluid) were aligned by application of an external magnetic field to the ferrofluid. The asymmetrical composite dumbbells were prepared by two-step soap-free emulsion polymerization consisting of the first polymerization to coat spherical silica cores with cross-linked poly(methyl methacrylate) (PMMA) shell and the second polymerization to protrude a polystyrene (PSt) lobe from the core−shell particles. A chain structure of nonmagnetic dumbbells oriented to the applied magnetic field was observed at nanoparticle content of 2.0 vol % and field strengths higher than 1.0 mT. A similar chain structure of the dumbbells was observed under application of alternating electric field at strengths higher than 50 V/mm. Parallel and orthogonally combined applications of the electric and magnetic fields were also conducted to examine independence of the electric and magnetic applications as operational factors in the dumbbell assembling. Dumbbell chains stiffer than those in a single application of external field were formed in the parallel combined application of electric and magnetic fields. The orthogonal combination of the different applied fields could form a magnetically aligned chain structure of the nonmagnetic dumbbells oriented to the electric field. The present work experimentally indicated that the employment of inverse magnetorheological effect for nonmagnetic, anisotropic particles can be a useful method for the simultaneous controls over the orientation and the positon of anisotropic particles in their assembling.
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INTRODUCTION A variety of anisotropic particles with different shapes have been synthesized because of their intrinsic properties that are not observed for isotropic particles.1−4 Control over assembled structures of the anisotropic particles5−7 is an important issue to develop practical “bottom-up” processes for application of the intrinsic properties to optical, electronic, and magnetic devices. Since anisotropic particles exhibit phase behavior richer than spherical particles,8,9 external fields such as electric and magnetic fields have been applied to assist the orientation and the position of anisotropic particles in the fabrication of assembled structures.10−14,27,30,31 Dipole−dipole interaction induced among particles under the external fields can be tuned by a number of particle factors such as particle dimension, shape, composition, and permittivity/magnetic permeability of the particles. The dipole−dipole interaction can also be varied with electric/magnetic properties of the continuous media dispersing the particles.15 An interesting example of tuning the dipole−dipole interaction by the medium properties is inverse ferrofluids in which nonmagnetic particles were dispersed in a mixture of magnetic nanoparticles and a liquid medium.25,26 The nonmagnetic particles dispersed in the ferrofuild exhibit a diamagnetic response to induce moments antiparallel to an applied magnetic field.29 The diamagnetic response under the magnetic field causes attractive or repulsive particle−particle © XXXX American Chemical Society
interactions depending on the relative permeability between nonmagnetic particles and medium and the relative positions between the particles. Inverse magnetorheological effect was directly observed with an optical microscope to confirm that a chain structure of micron-sized, monodisperse polystyrene spheres dispersed in a kerosene-based ferrofluid was formed parallel to a magnetic field applied to the ferrofuild.16 The inverse effect was also employed to form linearly assembled structure of cells in a ferrofuild.17 for developing a new method of cell manipulation. A variety of assembled structures composed of bidisperse or tridisperse spheres were recently demonstrated in an aqueous ferrofluid under an applied magnetic field.18,24 The present work applied the inverse magnetorheological effect to a ferrofuild in which anisotropic, nonmagnetic particles were dispersed. Monodisperse asymmetrical dumbbells were used as anisotropic building blocks dispersed in the ferrofluid of magnetic nanoparticles. The purpose of the present work is to find a new method for aligned assembled structure of anisotropic, nonmagnetic particles oriented to a specific direction. Since synthetic processes for the nonmagnetic particles do not need incorporation of magnetic components Received: February 26, 2015 Revised: April 28, 2015
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DOI: 10.1021/acs.langmuir.5b00737 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Nonmagnetic dumbbells and magnetic nanoparticles used in the assembling process. TEM images of dumbbells with low and high magnifications and a corresponding SEM image are shown in (a)−(c), respectively. TEM image of magnetic nanoparticles and their size distribution measured are presented in (d) and (e), respectively.
of new bottom-up processes to create self-assembled architectures composed of anisotropic building blocks.
into the particles and they are generally simpler than those for the magnetic particles, nonmagnetic particles with low polydispersity can be synthesized more easily than magnetic particles. The superiority of nonmagnetic particles expects wellordered assembled structure of the anisotropic particles. Another advantage of employing nonmagnetic, anisotropic particles as building blocks is no incorporation of lightabsorbing materials into assembled anisotropic particles, because magnetic nanoparticles (i.e., magnetite nanoparticles in the present work) in the assembled architecture can be selectively dissolved with an acidic solution. No incorporation of light-absorbing materials in the architecture of anisotropic particles offers their high potentials of optical applications. Therefore, we first prepared nonmagnetic dumbbells composed of silica, poly(methyl methacrylate) (PMMA) and polystyrene (PSt) by two-step soap-free emulsion polymerization.19 An external magnetic field was applied to the nonmagnetic dumbbells to study the controllability over their orientation and assembled structure in the ferrofluid. Then, an external electric field was applied to explore another operational factor for controls over the orientation of nonmagnetic dumbbells in the fluid. Finally, combined applications of electric and magnetic fields23 were carried out to examine independence of the two different applications as operational factors in the dumbbell assembling process. The present work proposes for the first time that the inverse magnetorheological effect is a useful phenomenon for nonmagnetic, anisotropic particles to be assembled simultaneously controlling their orientation and location. No incorporation of highly dielectric materials is required for anisotropic building blocks in the present method whereas asymmetric incorporation of titania cores into building blocks of dumbbells was required in our previous21 to induce interaction between specific parts of the dumbbells. The high applicability to nonmagnetic, anisotropic particles is essential for development
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EXPERIMENTAL SECTION
Materials. Tetraethylorthosilicate (TEOS, 95%), aqueous ammonia solution (25%), ethanol (99.5%), methyl methacrylate (MMA, 98%), p-styrenesulfonate (NaSS), potassium persulfate (KPS, 95%), styrene (St, 99%), sodium chloride (99.5%), iron(III) chloride (FeCl3, 95%), and hydrochloric acid (HCl, 2 M) were purchased from Wako Pure Chemical Industries (Osaka, Japan). Methacryloxypropyltrimethoxysilane (MPTMS) and iron(II) chloride (FeCl2, 99.9%) were obtained from Shin-Etsu Chemical (Tokyo, Japan) and Kojundo Chemical Laboratory (Saitama, Japan), respectively. The inhibitors of MMA and St were removed before use by inhibitor removal columns. The other chemicals were used as received. Particle Synthesis. Asymmetrical composite dumbbells were prepared by two-step soap-free emulsion polymerization with silica particles used as spherical cores.19 The silica particles with an average size of 890 nm and a coefficient of variation (CV) of 1.3% were synthesized in Stöber method. The silica particles were coated with cross-linked PMMA by the following polymerization. A suspension of the silica cores and MPTMS were added to water that was bubbled with nitrogen for 30 min, and the mixed suspension was stirred at 35 °C. After 30 min stirring, MMA and NaSS were added to the mixture and further stirred for 2 h. The polymerization of MMA and NaSS was conducted at 65 °C for 2 h with KPS initiator. The concentrations of silica cores, MPTMS, MMA, NaSS, and KPS were 0.15 vol %, 2.0 mM, 0.2 M, 1.0 mM, and 2.0 mM, respectively. The PMMA-coated silica cores were washed three times in iterative centrifuge processes and redispersed into deionized water. Protrusion of a PSt lobe from the PMMA shell was performed by the following polymerization of St. A suspension of the silica−PMMA core−shell particles and NaCl were bubbled with nitrogen for 30 min, and St monomer was added to the mixture. After 2 h stirring, the polymerization of St was conducted with KPS initiator at 65 °C for 7 h and more. The concentrations of core−shell particles, styrene, NaCl, and KPS were 0.11 vol %, 0.2 M, 1.0 mM, and 2.0 mM, respectively. The particles formed were washed by iterative centrifuges and B
DOI: 10.1021/acs.langmuir.5b00737 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. Nonmagnetic dumbbells observed in the ferrofluid (upper row) and in water (lower row). Concentrations of dumbbells and magnetic nanoparticles in the ferrofluid were 0.1 and 2.0 vol %, respectively. Snapshots were taken under no magnetic fields and 3−120 s after initiation of magnetic field application at 1.0 mT. redispersed into deionized water. The composite dumbbells obtained were observed with field emission transmission electron microscopy (FE-TEM; Hitachi, HD-2700). Magnetic nanoparticles to be used for ferrofluid preparation were synthesized with a modified Massart method.20 Briefly, a hydrochloric solution dissolving Fe2+ and Fe3+ at a molar ration of 1:2 was mixed with NH3 solution and stirred for 1 h. The black suspension formed was centrifuged and redispersed to water. Then, an ethanolic solution dissolving a silane coupling agent of MPTMS was added to the redispersed suspension. The MPTMS molecules were used as a steric stabilizer to improve colloidal stability of the magnetic nanoparticles. The suspension was reacted for 1 h under stirring, and the combined process of centrifuge and redispersion was carried out again to remove MPTMS molecules not to be introduced onto the nanoparticle surface. The colloidal stability of the MPTMS-modified nanoparticles was confirmed by measurement with dynamic light scattering as shown in Supporting Information Figure S1, showing that no aggregates larger than 1 μm were dispersed in the ferrofluid. Particle Assembling. The nonmagnetic composite dumbbells in the ferrofluid under external fields were observed in a capillary cell (0.1 × 1 mm rectangular cross section, VITRO COM) with a digital microscope (Keyence, VHX-2000). To introduce electrodes into the capillary cell, two 50 μm diameter copper wires (99.99%, NIRACO) were threaded through along the side capillary walls. The capillary was filled with the aqueous ferrofluid of nonmagnetic composite dumbbells, and the ends of capillary were sealed with glue. The AC field was applied by connecting the copper wires to a function generator (GWINTEK, SFG-2004) and amplifier (NF Circuit Design Bloc, HSA4011). The electric field strength (peak to peak) was measured with a digital oscilloscope (GWINTEK, GDS-1062A). An application of external magnetic field was conducted with a ferrite magnet. Strength of a magnetic field applied to the capillary cell was measured with a tesla meter (KANETEC, TM-701) before the observation of nonmagnetic dumbbells. The concentrations of dumbbells and magnetic nanoparticles in the ferrofluid were set to 0.1 vol % and 2.0 vol %, respectively, to acquire clear images of their assembled structure with the microscope. Since some images of dumbbells were not clear sufficiently to identify the orientation of dumbbells on paper, schematic drawings for dumbbells in a specific area on an optical microscope screen are presented in the Supporting Information (see Figures S3−S7).
1a. The PSt lobes in the dumbbells had sizes almost the same as those for silica−PMMA core−shell parts jointed to the PSt lobe, although the surface of PSt lobes was smoother than the core−shell parts as shown in Figure 1b and c. Figure 1d shows the magnetic nanoparticles prepared in the modified Massart method. The nanoparticles had an average size of 10 nm, much smaller than the sizes of nonmagnetic dumbbells (see Figure 1e), and they exhibited a saturation magnetization of 65 emu/g. The inverse magnetorheological effect was confirmed by comparing dumbbell motions in the presence and absence of magnetic nanoparticles. The snapshots of the nonmagnetic dumbbells dispersed in the ferrofluid and in water at a magnetic field strength of 1.0 mT are shown in the upper and lower rows of Figure 2, respectively. The concentrations of dumbbells and magnetic nanoparticles for the upper snapshots were 0.1 and 2.0 vol %, respectively. The location of the silica spheres in the asymmetric dumbbells could be identified with the optical microscope images due to difference in the refractive indices. An example of optical image for distinction between the PSt lobe and the PMMA-coated silica was presented in Figure S2 of the Supporting Information. The lobe of PSt with a refractive index higher than PMMA exhibit high contrast in water on the microscope observation. Also, the PSt lobe seemed yellowish in the optical microscope because they formed in polymerization with potassium persulfate initiator. In the upper row of Figure 2 (also see Figure S3), the long axes of nonmagnetic dumbbells in the ferrofluid were aligned to the applied magnetic field and they were assembled to form a chain structure within a minute after initiation of the field application (see Movie 1 (a5b00737_si_002.avi) in the Supporting Information), whereas random dispersion of the dumbbells was maintained in the absence of magnetic nanoparticles under the applied magnetic field as shown in the lower row of Figure 2. Observation of dumbbells at magnetic nanoparticle concentrations lower than 1.0 vol % was similar to the one shown in the lower snapshots of Figure 2. The interaction force between two nonmagnetic spheres in the presence of magnetic nanoparticles is attractive when a nonmagnetic sphere is located next to another sphere with the center-to-center line parallel to the field.15 Since the dipole moments of the nonmagnetic spheres induced by application of magnetic field is of the first order with respect to the volume of spheres,16 a main reason for the chain structure of dumbbells in Figure 2 is attractive dipolar interaction
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RESULTS AND DISCUSSION Figure 1 shows nonmagnetic composite dumbbells and magnetic nanoparticles used for the observation of inverse magnetorheological effect. Monodisperse dumbbell particles incorporating a silica core could be prepared as shown in Figure C
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Figure 3. Nonmagnetic dumbbells observed in the ferrofluid 1 min after initiation of magnetic field application at different strengths of 1.0 (a), 2.5 (b), and 5.0 (c) mT. See Figure 2 for the concentrations of particles.
Figure 4. Nonmagnetic dumbbells observed in the ferrofluid 1 min after initiation of electric field application at different strengths of 25 (a), 50 (b) and 100 (c) V/mm. See Figure 2 for the concentrations of particles.
Figure 5. Chain structures of nonmagnetic dumbbells formed in the ferrofluid under the parallel applications of electric and magnetic fields. Electric field at 25 (b), 50 (c) and 100 (d) V/mm was applied 1 min after initiation of the magnetic field application at 2.5 mT. Snapshots were taken 0 (a) and 1 (b−d) min after initiation of the additional electric fields. See Figure 2 for the concentrations of particles.
Figure 6. Chain structures of nonmagnetic dumbbells formed in the ferrofluid under the orthogonal applications of electric and magnetic fields. Electric field at 25 (b), 50 (c) and 100 (d) V/mm was applied 1 min after initiation of the magnetic field application at 2.5 mT. The snapshots were taken 0 (a) and 1 (b−d) min after initiation of the additional electric fields. See Figure 2 for the concentrations of particles.
ferrofluid. Figure 4 shows snapshots of the dumbbells observed 1 min after initiation of 1 MHz electric field applications at 25, 50, and 100 V/mm (also see Figure S5). The applications at field strengths of 50 and 100 V/mm could form field-oriented chain structures32 similar to those observed in Figure 3b. It seemed that dumbbell chains observed at 100 V/mm were stiffer than those at 50 V/mm 28 (compare Movie 2 (la5b00737_si_003.avi) for 50 V/mm and Movie 3 (la5b00737_si_004.avi) for 100 V/mm). The responses of dumbbells to the applied electric field in Figure 4 were, however, slightly weakened by the addition of magnetic nanoparticles to the aqueous medium probably because permittivity and viscosity of the medium were decreased and increased, respectively, by the nanoparticle addition. According to the observations presented above, the two different fields of magnetic and electric fields were parallel
strongly induced between dumbbells with long axes oriented to the field. The chain structure formed in the ferrofluid was also observed at magnetic field strengths higher than 1.0 mT. Figure 3 shows snapshots of the dumbbells observed 1 min after initiation of field applications at different strengths of 1.0, 2.5, and 5.0 mT. Long-axis oriented dumbbell chain structure slightly longer than that formed at 1.0 mT was observed at the high strengths of 2.5 and 5.0 mT (also see Figure S4), showing that the application of magnetic field can be an operational factor for orientation of the dumbbells in their assembling process. The high magnetic field of 5.0 mT, however, caused partial aggregations of the dumbbells in the ferrofluid as shown in Figure 3c. Controllability on chain formation with an external electric field was also examined for the nonmagnetic dumbbells in the D
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Langmuir applied to the dumbbells dispersed in the ferrofluid. Figure 5 shows snapshots of the dumbbells observed 1 min after an electric field application that were initiated 1 min after application of 2.5 mT magnetic field (also see Figure S6). On the applications of 1 MHz electric field, different field strengths of 25, 50, and 100 V/mm were chosen to examine the combined application to the chain structure. The parallel application of electric field could promote the chain formation extending to the direction of the combined fields. Reversibility of extending chain formation was confirmed by a simple test in which switching off the electric field made it back to the chain structure observed without the electric field. The dumbbell motions suggested that the additional application of electric field could change the assembled structure of dumbbells without weakening the effect of applied magnetic field on structuring dumbbells. Based on the independence of the two different applications, the electric field was orthogonally applied to the dumbbells under the application of magnetic field. Figure 6 shows snapshots of the dumbbells observed 1 min after an electric field orthogonally applied to the 2.5 mT magnetic field. The field strengths, the same as in Figure 5, were applied to the ferrofluid. A unique chain structure of dumbbells, where a magnetically aligned chain structure of the dumbbells oriented to the electric field was formed, was observed at electric field strength of 50 V/mm (see Figure S7 and Movie 4 (la5b00737_si_005.avi) in the Supporting Information). Creation of the unique structure of dumbbells indicated that the combined application of electric and magnetic fields can be a practical method for simultaneous control over both the assembled structure and the orientation of dumbbells dispersed in the ferrofluid. To explore application conditions for the field-extended chain of anisotropic particles orthogonally oriented to the field, external fields at some other strengths and frequencies were applied to the dumbbells. The applications of electric and magnetic fields indicated the importance of the balance between the interparticle force and the torque to the dumbbells. When the electric field was applied to the dumbbells under a magnetic field applied in Figure 6c, the torque was induced in the dumbbells depending on the strength of electric field. As shown in Figure S8, a strong interparticle force to the dumbbells under a strong magnetic field suppressed chaining of the dumbbells oriented to the electric field, whereas a strong electric filed newly formed electrically extended chain of dumbbells oriented to the electric field. Based on the combined applications of electric and magnetic fields, it can be concluded that the magnetically extended chain of dumbbells orthogonally oriented to the magnetic field was the case where the torque was dominated by the electric field and the interparticle force was induced by the magnetic field. The codomination of the electric and magnetic fields was caused by balanced application of the external fields. On the application of 1 kHz electric field, there were no balanced applications of electric and magnetic fields as shown in Figure S9. We have already reported that local incorporations of fieldresponsive materials such as titania and magnetite into anisotropic particles are an effective way for simultaneous controls over the orientation and the assembled structures of anisotropic particles.21,22 The present work proposed that the employment of inverse magnetorheological effect without the local incorporation can be another choice for the simultaneous controls in the assembling process of anisotropic particles.
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CONCLUSION
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ASSOCIATED CONTENT
Applications of electric and/or magnetic fields to nonmagnetic dumbbells dispersed in the ferrofluid were conducted to examine applicability of the inverse magnetorheological effect to control the assembled structure of the dumbbells. The application of magnetic field to the ferrofluid could align the long axes of dumbbells parallel to the magnetic field under no application of electric field. The additional application of electric field orthogonal to the magnetic field was effective for orientation of the dumbbells perpendicular to the dumbbell chain. The orthogonally combined application of electric and magnetic fields could biaxially direct the nonmagnetic dumbbells of the chain structure formed in the ferrofluid.
S Supporting Information *
Movie on dumbbell chains formed under an applied magnetic field at 1.0 mT. Other movies on unique dumbbell chain structures formed by the orthogonal application of magnetic and electric fields. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b00737.
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AUTHOR INFORMATION
Corresponding Authors
*Tel: +81-22-795-7239. Fax: +81-22-795-7241. E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was mainly supported by the Ministry of Education, Culture, Sports, Science and Technology (JSPS KAKENHI Grant Numbers 25600001 and 26286019) and also partially supported by the Advanced Low Carbon Technology Research and Development Program Grant from Japan Science and Technology (JST) Agency.
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REFERENCES
(1) Busbee, B. D.; Obare, S. O.; Murphy, C. J. An Improved Synthesis of High-Aspect-Ratio Gold Nanorods. Adv. Mater. 2003, 15, 414−416. (2) Pastoriza-Santos, I.; Pérez-Juste, J.; Liz-Marzán, L. M. SilicaCoating and Hydrophobation of CTAB-Stabilized Gold Nanorods. Chem. Mater. 2006, 18, 2465−2467. (3) Rossi, L.; Sacanna, S.; Irvine, W. T. M.; Chaikin, P. M.; Pine, D. J.; Philipse, A. P. Cubic Crystals from Cubic Colloids. Soft Matter 2011, 7, 4139−4142. (4) Sugimoto, T.; Sakata, K. Preparation of Monodisperse Pseudocubic α-Fe203 Particles from Condensed Ferric Hydroxide Gel. J. Colloid Interface Sci. 1992, 152, 587−590. (5) Meijer, J.-M.; Hagemans, F.; Rossi, L.; Byelov, D. V.; Castillo, S. I. R.; Snigirev, A.; Snigireva, I.; Philipse, A. P.; Petukhov, A. V. SelfAssembly of Colloidal Cubes via Vertical Deposition. Langmuir 2012, 28, 7631−7638. (6) Tierno, P. Recent Advances in Anisotropic Magnetic Colloids: Realization, Assembly and Applications. Phys. Chem. Chem. Phys. 2014, 16, 23515−23528. (7) van Anders, G.; Klotsa, D.; Ahmed, N. K.; Engel, M.; Glotzera, S. C. Understanding Shape Entropy through Local Dense Packing. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, E4812−E4821. E
DOI: 10.1021/acs.langmuir.5b00737 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir (8) Ni, R.; Gantapara, A. P.; de Graaf, J.; van Roij, R.; Dijkstra, M. Phase Diagram of Colloidal Hard Superballs: from Cubes via Spheres to octahedral. Soft Matter 2012, 8, 8826−8834. (9) Marechal, M.; Cuetos, A.; Martínez-Haya, B.; Dijkstra, M. Phase Behavior of Hard Colloidal Platelets using Free Energy Calculations. J. Chem. Phys. 2011, 134, 094501. (10) Ding, T.; Song, K.; Clays, K.; Tung, C.-H. Fabrication of 3D Photonic Crystals of Ellipsoids: Convective Self-Assembly in Magnetic Field. Adv. Mater. 2009, 21, 1936−1940. (11) Kuijk, A.; van Blaaderen, A.; Imhof, A. Synthesis of Monodisperse, Rodlike Silica Colloids with Tunable Aspect Ratio. J. Am. Chem. Soc. 2011, 133, 2346−2349. (12) van Blaaderen, A.; Dijkstra, M.; van Roij, R.; Imhof, A.; Kamp, M.; Kwaadgras, B. W.; Vissers, T.; Liu, B. Manipulating the Self Assembly of Colloids in Electric Fields. Eur. Phys. J. 2013, 222, 2895− 2909. (13) Forster, J. D.; Park, J.-G.; Mittal, M.; Noh, H.; Schreck, C. F.; O’Hern, C. S.; Cao, H.; Furst, E. M.; Dufresne, E. R. Assembly of Optical-Scale Dumbbells into Dense Photonic Crystals. ACS Nano 2011, 5, 6695−6700. (14) Lee, S. H.; Liddell, C. M. Anisotropic Magnetic Colloids: A Strategy to Form Complex Structures Using Nonspherical Building Blocks. Small 2009, 5, 1957−1962. (15) Rodríguez-Arco, L.; López-López, M. T.; Zubarev, A. Y.; Gdul, K.; Durán, J. D. G. Inverse Magnetorheological Fluids. Soft Matter 2014, 10, 6256−6265. (16) Skjeltorp, A. T. One- anti Two-Dimensional Crystallization of Magnetic Holes. Phys. Rev. Lett. 1983, 51, 2306−2309. (17) Krebs, M. D.; Erb, R. M.; Yellen, B. B.; Samanta, B.; Bajaj, A.; Rotello, V. M.; Alsberg, E. Formation of Ordered Cellular Structures in Suspension via Label-Free Negative Magnetophoresis. Nano Lett. 2009, 9, 1812−1817. (18) Yang, Y.; Gao, L.; Lopez, G. P.; Yellen, B. B. Tunable Assembly of Colloidal Crystal Alloys Using Magnetic Nanoparticle Fluids. ACS Nano 2013, 7, 2705−2716. (19) Nagao, D.; Hashimoto, M.; Hayasaka, K.; Konno, M. Synthesis of Anisotropic Polymer Particles with Soap-Free Emulsion Polymerization in the Presence of a Reactive Silane Coupling Agent. Macromol. Rapid Commun. 2008, 29, 1484−1488. (20) Massart, R. Preparation of Aqueous Magnetic Liquids in Alkaline and Acidic Media. IEEE Trans. Magn. 1981, MAG-17, 1247− 1248. (21) Nagao, D.; Sugimoto, M.; Okada, A.; Ishii, H.; Konno, M.; Imhof, A.; van Blaaderen, A. Directed Orientation of Asymmetric Composite Dumbbells by Electric Field Induced Assembly. Langmuir 2012, 28, 6546−6550. (22) Nishi, M.; Nagao, D.; Ishii, H.; Konno, M. Magnetoresponsive, Anisotropic Composite Particles Reversibly Changing Their Chain Lengths by a Combined External Field. Soft Matter 2012, 8, 11152− 11155. (23) Liu, J.; Mao, Y.; Ge, J. Electric Field Tuning of Magnetically Assembled Photonic Crystals. J. Mater. Chem. C 2013, 1, 6129−6135. (24) Khalil1, K. S.; Sagastegui, A.; Li, Y.; Tahir, M. A.; Socolar, J. E. S.; Wiley, B. J.; Yellen, B. B. Binary Colloidal Structures Assembled through Ising Interactions. Nat. Commun. 2012, 3, 794. (25) He, L.; Hu, Y.; Kim, H.; Ge, J.; Kwon, S.; Yin, Y. Magnetic Assembly of Nonmagnetic Particles into Photonic Crystal Structures. Nano Lett. 2010, 10, 4708−4714. (26) Popplewell, J.; Rosensweig, R. E.; Siller, J. K. Magnetorheology of Ferrofluid Composites. J. Magn. Magn. Mater. 1995, 149, 53−56. (27) Vutukuri, H. R.; Smallenburg, F.; Badaire, S.; Imhof, A.; Dijkstra, M.; van Blaaderen, A. An Experimental and Simulation Study on the Self-Assembly of Colloidal Cubes in External Electric Fields. Soft Matter 2014, 10, 9110−9119. (28) Vutukuri, H. R.; Demirçrs, A. F.; Peng, B.; van Oostrum, P. D. J.; Imhof, A.; van Blaaderen, A. Colloidal Analogues of Charged and Uncharged Polymer Chains with Tunable Stiffness. Angew. Chem., Int. Ed. 2012, 51, 11249−11253.
(29) Erb, R. M.; Son, H. S.; Samanta, B.; Rotello, V. M.; Yellen, B. B. Magnetic Assembly of Colloidal Superstructures with Multipole Symmetry. Nature 2009, 457, 999−1002. (30) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzán, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591− 3605. (31) Dugyala, V. R.; Daware, S. V.; Basavaraj, M. G. Shape Anisotropic Colloids: Synthesis, Packing Behavior, Evaporation Driven Assembly, and Their Application in Emulsion Stabilization. Soft Matter 2013, 9, 6711−6725. (32) Mittal, M.; Furst, E. M. Electric Field-Directed Convective Assembly of Ellipsoidal Colloidal Particles to Create Optically and Mechanically Anisotropic Thin Films. Adv. Funct. Mater. 2009, 19, 3271−3278.
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