Modeling and simulation of tapered fiber-optic oil concentration sensor using negative dielectrophoresis
Introduction
In recent years, the separation of the oil–water mixture has become a research focus in the serious freshwater resources reduction. The treatment of domestic sewage and industrial effluent are based on the oil–water separation technology such as gravitational separation, centrifugal separation, electrolysis and biological methods [1] to address the growing problems of freshwater. Therefore, it seems to be particularly necessary to monitor the oil content in this process [2]. The low content of micro/nano oil droplet put forward the new challenge to traditional measurement. The dissolved oil, lubricant, disperse oil and float oil exist in the oil–water mixture. Especially, the emulsified oil droplets, whose size is below 10 μm, form a kind of tiny and stable drops in the water [3]. Optical fiber has a great of advantages for many sensing application, including high elastic strain limits, high sensitivity, electromagnetic interference resistance and low cost [4]. However, evanescent field fiber sensor which is also new and popular in the fiber sensor family processes higher sensitivity and deeper penetration than common optical fiber at present.
In addition, the DEP technology is widely used for cellular manipulation [5], liquid movement [6], semiconductor processing [7] and nanorod sensor [8]. Some researchers triumphantly put this technology applied in the sensing fields on later. Dan et al. [9] had immobilized the nanorods which contain poly-3,4-ethylenedioxythiophene/polystyrenesulfonate materials by DEP force. This device was utilized for concentration measurement of acetone, methanol and ethanol by a resistance change in electronic nose vapor sensing systems. Holzkia et al. [10] proposed a fiber-optical evanescent field sensor which depended on the positive DEP force to increase the content of water in order to improve the sensitivity in the lubricating oils.
Thus, this paper design and simulate an emulsified oil concentration sensor which combine the advantage of dielectrophoresis and tapered fiber technology. For this work, an obvious extension of this DEP technology is to introduce the repelling emulsified oils directly into a high sensing by the external non-uniform electric fields. The structure of tapered fiber has so fast time responses with changing refraction index of the medium. However, when the size of oil droplet became much smaller as the continuous purification of oil, the measurement in separation of oil–water is relatively difficult because its content might become low. The oil particles on the surface of the tapered fiber are distracted by the negative DEP. As a consequence, the refractive index of the tapered fiber surface result in the most light obeying the total reflection transmission. Eventually the detection is achieved by the intensity factor ratio of the output light and the input light. The distribution of electric field can be calculated near the electrode. The geometry of the sensor is incorporated in a model that solve the optical wave equations within a tapered fiber with metal-coated film. The output light intensity of light source depends on oil concentration distributions have also been explored. It is expected that the results of this study would provide guidelines to achieve measurement of low content of oil via negative DEP.
Section snippets
Fiber evanescent field
The tapered fiber uses unclad uniform plastic fiber as its material, produce through heating and tapering. Three types of taper geometries are used, including linear, parabolic and exponential. They can change the distribution of longitudinal refractive index and form evanescent field in the cone zone as shown in Fig. 1. Compared with the latter two, linear tapered fiber is usually made and used [11], [12]. This paper studies the oil–water simulation based on linear tapered fiber. On the
Electric field
The value of DEP force have a direct relationship to the electric field strength according to the Eq. (3). There is essential to have the non-uniform electric fields generating an inhomogeneous polarization of a dielectric particle. The electrodes which are shown in Fig. 2(b) could be built, and the boundary conditions are indicated in Fig. 3(a). The electrical double layer adjacent to the surface of the fiber is neglected so the Poisson's equation turn into the Laplace's equation [16] in Eq.
Conclusions
The simulation made with the finite element method described above gave preliminary interesting results. In this design, simulations and preliminary measurements of a tapered fiber for oil–water separation application by negative DEP effect have been presented. The concentration of emulsified particles results in a change of the liquid refractive index around the surface of evanescent field. To improve the sensitivity of the tapered fiber, the introduction of coating metal electrodes is capable
Acknowledgments
This work was supported in part by the National Natural Science Foundation of China under Grant 61273059 and 61203206, and the Fundamental Research Funds for the Central Universities under Grant N130104002, N130604006 and N130404025, and IAPI Fundamental Research Funds under Grant 2013ZCX09.
Sheng Hu was born in Yunnan, China, in 1984. He received his M.A. degrees in the Department of Physics from Liaoning University, China, in 2011. He is now a Ph.D. student of Northeastern University. His research interests are microfluidic biochips, optical tweezers, optoelectronic sensors and its sensing applications. He has authored and co-authored several scientific papers and patents.
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Sheng Hu was born in Yunnan, China, in 1984. He received his M.A. degrees in the Department of Physics from Liaoning University, China, in 2011. He is now a Ph.D. student of Northeastern University. His research interests are microfluidic biochips, optical tweezers, optoelectronic sensors and its sensing applications. He has authored and co-authored several scientific papers and patents.
Yong Zhao received his M.A. and Ph.D. degrees, respectively, in precision instrument & automatic measurement with laser and fiber-optic techniques from the Harbin Institute of Technology, China, in 1998 and 2001. He was awarded a first prize scholarship in 2000 by the China Instrument and Control Society and the Sintered Metal Corporation (SMC) scholarship in Japan. He was a scholarship in Japan. He was a postdoctor in the Department of Electronic Engineering of Tsinghua University from 2001 to 2003, and then worked as an associate professor in the Department of Automation, Tsinghua University of China. In 2006, he was a visiting scholar of University of Illinois in Urbana and Champagne, USA. In 2008, he was awarded as the “New Century Excellent Talents in University” by the Ministry of Education of China. In 2009, he was awarded as the “Liaoning Bai-Qian-Wan Talents” by Liaoning Province. In 2011, he was awarded by the Royal Academy of Engineering as an academic researcher of City University London. Now he is working in Northeastern University as a full professor. As a leader of his research group, his current research interests are the development of fiber-optic sensors and device, fiber Bragg grating sensors, novel sensor materials and principles, slow light and sensor technology, optical measurement technologies. He has authored and co-authored more than 170 scientific papers and conference presentations, 8 patents, and 5 books. He is a member in the Editorial Boards of the international journals of Sensor Letters, Instrumentation Science & Technology, Journal of Sensor Technology, and Advances in Optical Technologies.
Haifeng Hu was born in Liaoning, China, in 1984. He received his Ph.D. degrees in the Institute of Semiconductors, Chinese Academy of Sciences, China, in 2013. He is currently working in the College of Information and Engineering at Northeast University. His research interests are nano-optics, plasmonics, fiber-optic sensors and their applications in biosensing. He has authored and co-authored more than 20 scientific papers, 2 patents and 5 conference presentations.