基于介电润湿效应的微液滴操控研究
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摘要
基于介电润湿(Electrowetting on Dielectric,简称EWOD)的数字微流控芯片具有结构和控制简单,灵敏度和通量高,样品用量小,检测时间短以及自动化和集成化程度高等优点,在细胞操纵、临床检测、DNA分析以及食物和环境监测等生物、医学和化学领域表现出巨大优势并得到了广泛应用。可以预见,随着MEMS技术的发展,数字微流控技术将发展成为当今世界的前沿科技之一。
针对目前所研制的数字微流控芯片功能单一以及微液滴操控灵活性差的不足,本文在研究介电润湿操控微液滴机理的基础上,设计了一种混合结构数字微流控芯片,结合数值仿真和MEMS工艺成功研制出芯片;并设计了外围控制电路,对微液滴进行了操控实验研究。
首先,对微尺度下的微液滴进行受力分析,研究了各种改变表面张力的方法并选择EWOD进行微液滴操控。建立EWOD模型对其原理进行了研究,并利用能量最小化原理对控制微液滴接触角变化的Young-Lippmann方程进行了改进;在EWOD原理基础上,建立了微液滴生成、输运、分离及合并操控模型。
其次,设计了混合结构数字微流控芯片,并推导出单双极板区驱动电极单元尺寸匹配数学关系。建立电动力模型,利用COMSOL Multiphysics有限元仿真软件对芯片中零电极和驱动电极单元的布局和参数进行了仿真优化,得到了一系列用于指导芯片设计的结论;并基于所建立的电动力仿真模型研究了介电润湿的内在机理问题。另外,对所设计的数字微流控芯片进行了电场、流场和温度场耦合仿真,得出微液滴的运动规律,并分析了介电层厚度对流速和温度的影响。
基于理论计算和数值仿真,结合MEMS工艺制定出一套加工数字微流控芯片的工艺流程,将Si_3N_4和SiO_2相结合研制出复合介电层结构芯片。针对微液滴操控的特殊性,采用单片机驱动控制光耦开关设计出微液滴操控电路及操控程序。最后,建立实验平台,对微液滴进行操控实验研究。在不同电压下,通过测量四种介电层结构中去离子水微液滴的动态接触角及其滞后量,验证了本文改进的Young-Lippmann的正确性,并得出最佳介电层组合为Si_3N_4-SiO_2和SiO_2-Si_3N_4-SiO_2层状复合结构。在此基础上,比较了外加电压、开关切换频率以及微液滴体积等因素对微液滴操控的影响;在30-50V电压之间成功实现了对去离子水微液滴的输运、合并及并行操控,通过实验验证了基于EWOD操控微液滴的可行性。
本课题涉及多学科交叉研究与应用,文中所使用的理论、方法和得出的结论对进一步深入研究和设计数字微流控芯片具有一定的启发和借鉴意义。
Electrowetting on dielectric (EWOD) based digital microfluidics(DMF) has shown enormous advantages and potential in biology, medicine, chemistry and so on, and it has been used extensively in all these subjects, such as cell manipulation, clinical tests, DNA analysis, food and environmental monitoring and so on. It can be seen that, with the development of MEMS, digital microfluidics will be an aspect of the science and technology on the leading edge in the twenty-first century.
Aim at the poor functions of DMF developed nowadays and low flexibility in manipulation of droplets, in this paper, a mixed structure DMF was designed based on the research on the theory of EWOD and on the mechanism of droplets manipulation. Combine numerical simulation and MEMS technics, the DMF was successfully developed. Besides,we also designed a control circuit and carried out a series of experimental research on droplets manipulation and control.
Firstly, the forces acting on droplets in micro-scale was analysed, several methods for changing surface tension was compared. Because of its unique advantages, EWOD was selected as driver for droplets manipulation. The model of EWOD was built and the internal mechanism of EWOD was studied. We also improved the mathematical equation of EWOD based on energy minimization principle. After the model of droplets manipulation was founded, we proposed preliminarily methods for the creating, transporting, cutting and merging droplets.
Combine the advantages and disadvantages of single-plate and double-plate DMF, a mixed structure DMF was proposed for the first time, and the mathematical relations for dimensions of electrodes between single region and double region in the designed DMF was deduced. After the governing equations and their boundary conditions were built, we solved for the electric field and calculated the electrodynamic actuation forces acting on the whole surface of droplet by using Maxwell-stress tensor in different kinds of structures of digital microfluidics with the Comsol software package, and the optimal distribution and dimensions for different kinds of electrodes were maked. Based on the electrodynamic simulation model, the internal mechanism of EWOD was also studied by comparing three kinds of simulatation structures, which have the same contact domain but different contact line. Besides, the electrical field, fluidic field and temperature field coupled in the designed DMF were numerically simulated, and based on the coupling simulation results, we generalized the move of droplet to two stage and analysed the influence of dielectric layer thickness on temperature and velocity in the droplet.
Based on the theoretical calculation and numerical simulation results, a process flow for fabricating digital microfluidics was designed by employing MEMS technics. Then a DMF with lamellar composite-structure dielectric layer was successfully developed by combining Si_3N_4 and SiO_2. In addition, a switching hardware circuit and relative program for controlling the manipulation of droplets was successfully designed and debugged based on MSP430F149 microcontroller.
Lastly, an experimental platform including microscopic vision system, computer, dc electrical source, control circuit board and so on was set up for carrying out experimental research on manipulation and control of droplets. It was testified by measuring a series of dynamic contact angle and contact angle saturation under different voltages that SiO_2-Si_3N_4-SiO_2 and Si_3N_4-SiO_2 lamellar composite structure for dielectric layer of DMF was superior to Si_3N_4-SiO_2-Si_3N_4 and SiO_2-Si_3N_4 dielectric layers. The influence of applied voltage, the frequence of switch circuit, the volume of droplets and the space between upper and bottom plate of DMF on manipulation of droplets was compared and analysed by experiments.Transporting, merging aqueous droplets and two aqueous droplets transporting side-by-side were successfully realized, and the feasibility of manipulation and control of droplets based on EWOD was testified.
This paper involves multi-disciplinary intercross investigation and application, the theory, methods and conclusions in this paper can be useful and valuable to a certain extent for further development of DMF and research on manipulation of droplets.
针对目前所研制的数字微流控芯片功能单一以及微液滴操控灵活性差的不足,本文在研究介电润湿操控微液滴机理的基础上,设计了一种混合结构数字微流控芯片,结合数值仿真和MEMS工艺成功研制出芯片;并设计了外围控制电路,对微液滴进行了操控实验研究。
首先,对微尺度下的微液滴进行受力分析,研究了各种改变表面张力的方法并选择EWOD进行微液滴操控。建立EWOD模型对其原理进行了研究,并利用能量最小化原理对控制微液滴接触角变化的Young-Lippmann方程进行了改进;在EWOD原理基础上,建立了微液滴生成、输运、分离及合并操控模型。
其次,设计了混合结构数字微流控芯片,并推导出单双极板区驱动电极单元尺寸匹配数学关系。建立电动力模型,利用COMSOL Multiphysics有限元仿真软件对芯片中零电极和驱动电极单元的布局和参数进行了仿真优化,得到了一系列用于指导芯片设计的结论;并基于所建立的电动力仿真模型研究了介电润湿的内在机理问题。另外,对所设计的数字微流控芯片进行了电场、流场和温度场耦合仿真,得出微液滴的运动规律,并分析了介电层厚度对流速和温度的影响。
基于理论计算和数值仿真,结合MEMS工艺制定出一套加工数字微流控芯片的工艺流程,将Si_3N_4和SiO_2相结合研制出复合介电层结构芯片。针对微液滴操控的特殊性,采用单片机驱动控制光耦开关设计出微液滴操控电路及操控程序。最后,建立实验平台,对微液滴进行操控实验研究。在不同电压下,通过测量四种介电层结构中去离子水微液滴的动态接触角及其滞后量,验证了本文改进的Young-Lippmann的正确性,并得出最佳介电层组合为Si_3N_4-SiO_2和SiO_2-Si_3N_4-SiO_2层状复合结构。在此基础上,比较了外加电压、开关切换频率以及微液滴体积等因素对微液滴操控的影响;在30-50V电压之间成功实现了对去离子水微液滴的输运、合并及并行操控,通过实验验证了基于EWOD操控微液滴的可行性。
本课题涉及多学科交叉研究与应用,文中所使用的理论、方法和得出的结论对进一步深入研究和设计数字微流控芯片具有一定的启发和借鉴意义。
Electrowetting on dielectric (EWOD) based digital microfluidics(DMF) has shown enormous advantages and potential in biology, medicine, chemistry and so on, and it has been used extensively in all these subjects, such as cell manipulation, clinical tests, DNA analysis, food and environmental monitoring and so on. It can be seen that, with the development of MEMS, digital microfluidics will be an aspect of the science and technology on the leading edge in the twenty-first century.
Aim at the poor functions of DMF developed nowadays and low flexibility in manipulation of droplets, in this paper, a mixed structure DMF was designed based on the research on the theory of EWOD and on the mechanism of droplets manipulation. Combine numerical simulation and MEMS technics, the DMF was successfully developed. Besides,we also designed a control circuit and carried out a series of experimental research on droplets manipulation and control.
Firstly, the forces acting on droplets in micro-scale was analysed, several methods for changing surface tension was compared. Because of its unique advantages, EWOD was selected as driver for droplets manipulation. The model of EWOD was built and the internal mechanism of EWOD was studied. We also improved the mathematical equation of EWOD based on energy minimization principle. After the model of droplets manipulation was founded, we proposed preliminarily methods for the creating, transporting, cutting and merging droplets.
Combine the advantages and disadvantages of single-plate and double-plate DMF, a mixed structure DMF was proposed for the first time, and the mathematical relations for dimensions of electrodes between single region and double region in the designed DMF was deduced. After the governing equations and their boundary conditions were built, we solved for the electric field and calculated the electrodynamic actuation forces acting on the whole surface of droplet by using Maxwell-stress tensor in different kinds of structures of digital microfluidics with the Comsol software package, and the optimal distribution and dimensions for different kinds of electrodes were maked. Based on the electrodynamic simulation model, the internal mechanism of EWOD was also studied by comparing three kinds of simulatation structures, which have the same contact domain but different contact line. Besides, the electrical field, fluidic field and temperature field coupled in the designed DMF were numerically simulated, and based on the coupling simulation results, we generalized the move of droplet to two stage and analysed the influence of dielectric layer thickness on temperature and velocity in the droplet.
Based on the theoretical calculation and numerical simulation results, a process flow for fabricating digital microfluidics was designed by employing MEMS technics. Then a DMF with lamellar composite-structure dielectric layer was successfully developed by combining Si_3N_4 and SiO_2. In addition, a switching hardware circuit and relative program for controlling the manipulation of droplets was successfully designed and debugged based on MSP430F149 microcontroller.
Lastly, an experimental platform including microscopic vision system, computer, dc electrical source, control circuit board and so on was set up for carrying out experimental research on manipulation and control of droplets. It was testified by measuring a series of dynamic contact angle and contact angle saturation under different voltages that SiO_2-Si_3N_4-SiO_2 and Si_3N_4-SiO_2 lamellar composite structure for dielectric layer of DMF was superior to Si_3N_4-SiO_2-Si_3N_4 and SiO_2-Si_3N_4 dielectric layers. The influence of applied voltage, the frequence of switch circuit, the volume of droplets and the space between upper and bottom plate of DMF on manipulation of droplets was compared and analysed by experiments.Transporting, merging aqueous droplets and two aqueous droplets transporting side-by-side were successfully realized, and the feasibility of manipulation and control of droplets based on EWOD was testified.
This paper involves multi-disciplinary intercross investigation and application, the theory, methods and conclusions in this paper can be useful and valuable to a certain extent for further development of DMF and research on manipulation of droplets.
引文
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