| The droplet microfluidic system is one of the most important microfluidic systems,which involves many disciplines such as fluid mechanics,biology,medicine,chemistry and system control.In particular,how to generate mondisperse droplets and precisely control the size of the droplets,is currently the most important and difficult research point for droplet microfluidic systems,and high precision and monodispersity of droplet generation is quite important for the applications of droplet microfluidic systems in interdisciplinary study.In this paper,the electrical-detection closed-loop control droplet microfluidic system is reported,where the flow rates of the system are supplied by the pressure-driven device and the droplet size can be measured by the method of electrical detection.With feedback of the droplet size during droplet generation,the closed-loop control of the droplet size can be achieved.In this paper,the method of electrical detection and the dynamic characteristics of the droplet microfluidic closed-loop control system are studied,which are quite important and meaningful for improving the control precision of the droplet size,and reducing the polydispersity of the droplets.By analyzing the process of droplet formation in a T-junction microchannel,the mathematical models of the linear relation between the droplet length and the flow-rate ratio is established.Meanwhile,by comparing the transient process of droplet generation with the dynamic response of the microfluidic system,the mathematical model of the droplet generation process can be established.In addition,both the flow-rate disturbance of the pump source and the dynamic process of droplet generation can contribute to the variation of the droplet size,which is associated with the polydispersity of droplets.The experiments of droplet generation are performed in the T-junction microchannels with different geometrical parameters,and the droplet length as a function of the flow-rate ratio is measured quantitatively.For low capillary numbers,it is observed that the droplet length varies linearly with the flow-rate ratio.Based on the mathematical model of droplet generation process,the pressure fluctuations which are induced by the process of droplet generation can be quantified.In particular,the effects of the pressure fluctuations on the control precision of droplet size are discussed in details,as a result,the dynamic characteristics of the microfluidic closed-loop control system can be obtained.The method of electrical detection is used to measure the droplet size,and by calculating the root-mean-square value of the measurements for three pairs of microelectrodes,the detection precision of droplet size can be improved.Meanwhile,by integrating the method of electrical detection with the droplet microfluidic system,high control precison of the droplet size can be achieved.The methematical model of the method of electrical detection is established,and while the droplets are passing through the microelectrodes,the time-varying capacitance of the microelectrodes can be simulated,which are compared with the experimental measurements for differernt lengths of droplets.The detection precision of the droplet size is mainly discussed in this paper,especially,the detection precision can be affected by the variations of the droplet size,which are induced by the process of the droplet generation.By allocating three pairs of the microelectrodes along the T-junction microchannel,the droplet size can be measured by different microelectrodes simultaneously,and the detection precision of the droplet size for different microelectrodes can be obtained.From the experimental results,by increasing the distance between the microelectrodes and the T-junction and taking the root-mean-square value of the droplet size measured by different microelectrodes,the effects of the process of droplet generation on the detection precision of the droplet size are greatly reduced.As a result,the droplet size can be measured more accurately by the method of electrical detection.For the electrical-detection closed-loop control droplet microfluidic system reported in this paper,a PI controller is used to improve the performance of the system and increase its control precision.Based on the mathematical model,the dynamic characteristics of the closed-loop control system are simulated.More importantly,the method of electrical detection,the dynamic characteristics of the pressure-driven device and the closed-loop control of the droplet size are studied experimentally.Based on the relation between the production rates of droplets and the detection speed of the droplet size,the effects of the production rates on the dynamic characteristics of the closed-loop control system can be described.Moreover,by integrating a PI controller into the closed-loop control system and properly choosing the PI coefficients,the variation of the droplet size induced by the external disburbance of pump source can be reduced significantly.The electrical-detection closed-loop control droplet microfluidic system is established,and its dynamic characteristics are measured experimentally.Additionally,by choosing differernt coefficients of the PI controller,the dynamic response speed and control precision of the droplet size are comparatively discussed.From both simulation and experimental studies,high precision of the droplet-size control can be achieved for the electrical-detection closed-loop control droplet microfluidic system,which can provide both theoretical and experimental guidance for the applications of droplet microfluidic systems. |
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