Enhanced magnetic-field-induced optical properties of nanostructured magnetic fluids by doping nematic liquid crystals
© Wang et al.; licensee Springer. 2012
Received: 5 February 2012
Accepted: 21 April 2012
Published: 15 May 2012
Ferronematic materials composed of 4-cyano-4′-pentylbiphenyl nematic liquid crystal and oil-based Fe3O4 magnetic fluid were prepared using ultrasonic agitation. The birefringence (Δn) and figure of merit of optical properties (Q = Δn/α, where α is the extinction coefficient) of pure magnetic fluids and the as-prepared ferronematic materials were examined and compared. The figure of merit of optical properties weighs the birefringence and extinction of the materials and is more appropriate to evaluate their optical properties. Similar magnetic-field- and magnetic-particle-concentration-dependent properties of birefringence and figure of merit of optical properties were obtained for the pure magnetic fluids and the ferronematic materials. For the ferronematic materials, the values of Q increase with the volume fractions of nematic liquid crystal under certain fixed field strength and are larger than those of their corresponding pure magnetic fluids at high field region. In addition, the enhancement of Q value increases monotonously with the magnetic field and becomes remarkable when the applied magnetic field is beyond 50 mT. The maximum relative enhanced value of QR exceeds 6.8% in our experiments. The results of this work may conduce to extend the pragmatic applications of nanostructured magnetic fluids in optical field.
KeywordsMagnetic fluids Nematic liquid crystals Ferronematic materials Birefringence Figure of merit of optical properties
Magnetic fluid (MF) is a kind of stable colloidal suspension of small magnetic particles with typical sizes of 10 nm. The liquid carrier may be either an aqueous or a nonpolar solvent. The nanoparticles are coated with a surfactant layer, such as oleic acid and polymer, which will prevent agglomeration by overcoming the van der Waals attractive forces between the particles. The Brownian motion can keep the nanoparticles from settling under gravity . MF was firstly invented in the mid-1960s, and then the study and applications of MF have been extended to multidisciplinary sciences, such as chemistry, fluid mechanics, and magnetics. Recently, with the fast development of MF, it has been broadly used for dynamic sealing, shock absorbers, audio loudspeaker coolant, and biomedical sciences [2, 3]. Because of the dramatic development of optical communication and integrated optics, the optical properties of MF have attracted a great deal of attention from researchers since the late part of the twentieth century, which include tunable refractive index , birefringence , Faraday effect , optical transmittance [7, 8], optical scattering [9, 10], and so on. Most of these are based on the fluid behavior and magnetism of the MF. Until now, several potential applications of MF to optical devices have been proposed, such as MF optical switches , MF gratings , MF light modulation , MF optical fiber modulator , and MF optical limiting . Recently, some experimental investigations about the magneto-optical effects of binary, multiple-phase, ionic, and doped MFs show that these kinds of MFs can present some unique optical properties [16–19].
Nematic liquid crystal has attracted a great deal of attention from researchers owing to its optical birefringence and scattering properties, which can be controlled by the external stimuli, such as electrical, magnetic fields, and shear stresses . Usually, a strong magnetic field is needed to study the magnetic-field-induced optical properties of liquid crystals. This is due to the small anisotropic diamagnetic susceptibility (Δχ) of the liquid crystal . To lower the applied magnetic field, liquid crystal doped with ferromagnetic grains (the corresponding mixture is denoted as ferronematic material) has been proposed [21, 22]. Burylov and Raikher have reported that magneto-optical response of a liquid crystalline system can be enhanced by uniformly doping with ferromagnetic grains and the orientational state of the ferronematic material can be fully controlled by a rather weak magnetic field (much less than 10 mT) . Raikher and Stepanov have discussed a transient birefringence response of a nematic liquid crystal doped with single-domain ferromagnetic particles and have derived a set of coupled macroscopic equations describing the evolution of the director texture and the magnetization distribution within the ferronematic material during the change of the external field . It has been disclosed that many properties of liquid crystal can be improved and enhanced by doping with ferromagnetic grains, which are favorable for practical applications [25–28].
Considering the unique properties of MFs and ferronematic materials, doping MFs with nematic liquid crystal is attractive and needs further in-depth investigation. To the best of our knowledge, few experiments about the magnetic-field-induced birefringence Δn and figure of merit of optical properties Q (Q = Δn/α, where α is the extinction coefficient) of this composite have been done. This work will experimentally investigate the magnetic-field- and concentration-dependent Δn and Q of the as-prepared ferronematic materials. The results of this work may be helpful to better understand the optical properties of nematic liquid crystal-doped MFs (ferronematic materials) and extend the applications of MFs in optical field.
Preparation of ferronematic materials
Pure MF samples with different concentrations of magnetic particles
Volume ratio of pure MF to liquid paraffin
Volume fraction of magnetic particle
Concentrations of samples a-5CB, b-5CB, c-5CB, and d-5CB
Volume ratio of sample a, b, c, or d to 5CB
Volume fraction of magnetic particle
Volume fraction of 5CB
Concentrations of samples MF-5CB, MF-5CB(2), MF-5CB(3), and MF-5CB(4)
Volume fraction of magnetic particle
Volume fraction of 5CB
Results and discussion
Figure 2 also indicates that samples with high volume fraction of magnetic particle have high birefringence under the same field strength. Low concentration of magnetic particles will lead to weak interaction between magnetic particles under the fixed field. The average distance between the magnetic particles within the samples will then become relatively large. Therefore, the number of chains per unit area will decrease as the concentration of magnetic particles decreases, which contributes to the low degree of structure anisotropy and then small value of birefringence of the sample under low field.
The experimental results for the corresponding ferronematic samples (samples a-5CB, b-5CB, c-5CB, and d-5CB) are also shown in Figure 2, which are very similar to those of the pure MF samples. Table 2 shows that all the ferronematic samples have the same volume fraction of 5CB (25.00%) but have different magnetic particle volume fractions, so the variation in birefringence between different ferronematic samples is attributed to the change of magnetic particle concentration. Besides, the birefringence of the ferronematic samples is weaker than those of the corresponding pure MF samples. This is probably assigned to the decrease of magnetic particle concentration through doping 5CB.
To quantify the enhanced optical properties, the relative enhancement of the Q value defined as as a function of magnetic field is calculated and plotted in Figure 5. Herein, QMF and QMF+5CB are the Q values at a certain magnetic field for the pure MF samples and their corresponding ferronematic material samples, respectively. Figure 5 shows that QR increases with the applied magnetic field and tends to saturate at high field. When the externally magnetic field B is less than 50 mT, the QR is slight, while QR becomes notable when B is larger than 50 mT. Comparing with the Q value of the pure MF samples, the relative enhancement of Q value of the corresponding ferronematic samples (QR) can reach about 6.8%.
Figure 6 also indicates that the value of Q increases gradually with the applied magnetic field and tends to saturate at high field. Moreover, the higher the volume fraction of 5CB is, the higher the value of Q is under certain fixed field strength. Therefore, the value of Q for the ferronematic materials can be tuned by adjusting the volume fraction of 5CB.
The optical properties (birefringence and figure of merit of optical properties) of the pure MF and ferronematic thin films under externally applied magnetic field are investigated. The pure MF and the ferronematic samples show the similar magnetic-field-dependent properties of birefringence and figure of merit of optical properties, which increase with magnetic induction and tend to saturate at high field. Both types of samples with high volume fraction of magnetic particle have high birefringence. Besides, the ferronematic material with high volume fraction of 5CB has a relatively high value of Q under a certain fixed field strength. The experimental results reveal that the magnetic particle concentration is crucial to the birefringence of the samples and that 5CB concentration has no influence on the birefringence of the samples. However, the 5CB concentration is crucial to the figure of merit of optical properties of the ferronematic samples, and magnetic particle concentration has no influence on the figure of merit of optical properties of the ferronematic samples. The Q values of the ferronematic materials are larger than those of their counterparts (pure MFs). The maximum relative increase in Q value is around 6.8% for our experimental samples.
SP is an associate professor at the College of Science, University of Shanghai for Science and Technology. His research interests focus on advanced photonic materials and magneto-optics. XW, HJ, and GY are pursuing their master's degrees.
4-cyano-4′-pentylbiphenyl (C18H19N) nematic liquid crystal
This research was supported by the Innovation Program of Shanghai Municipal Education Commission (11YZ120) and the National Natural Science Foundation of China (10704048).
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