Songjing Li

Variation in polydispersity in pump- and pressure-driven micro-droplet generators

The polydispersity of droplets produced in a typical T-junction microfluidic channel under both syringe-pump-driven and pressure-driven flow configurations is measured quantitatively. Both flow systems exhibit high-frequency flow fluctuations that result in an intrinsic polydispersity due to the mechanism of droplet generation. In addition to this intrinsic polydispersity, the syringe-pump-driven device also exhibits low-frequency fluctuations due to mechanical oscillations of the pump, which overwhelm the high-frequency flow fluctuations and produce a signficantly heightened level of polydispersity. The quantitative difference in polydispersity between the two configurations and time-resolved measurements of individual droplet sizes are presented in order to enable the design of better flow control systems for droplet production.

Damping properties for vibration suppression in electrohydraulic servo-valve torque motor using magnetic fluid

Aiming to suppress high frequency vibrations of a torque motor in electrohydraulic servo-valves, damping properties of an ester-based Fe3O4 magnetic fluid operating in the squeeze mode are studied in this Letter. The expression of damping forces due to the magnetic fluid on the torque motor is derived and simplified based on the measured magneto-viscosity property. Dynamic characteristics of the torque motor with and without the magnetic fluid are simulated and tested. Damping properties of magnetic fluid for the vibration suppression of a torque motor are verified by the good agreement between the predicted and tested results.

A study of flow-field distribution between the flapper and nozzle in a hydraulic servo-valve

This study is aiming to give understanding of flow field distribution in the pilot stage of a flapper-nozzle hydraulic servo-valve and to study the characteristics between the flow rate and the pressure drop over the nozzle. After introducing the construction and working principle of the hydraulic servo valve, the mathematical models for the flow field simulation based on standard k-∊ model for turbulent flow are built. The simulation results show the velocity flow field distribution at different conditions. The characteristics between the flow rate of the pilot stage and the pressure drop over the nozzle are also given by simulation. The simulation results provide theoretical basis for the design and understanding of flow field distribution in the pilot stage of a flapper-nozzle hydraulic servo-valve.

Mixing indexes considering the combination of mean and dispersion information from intensity images for the performance estimation of micromixing

Herein, micromixing was utilized to achieve chemical reaction, homogenization, emulsification, and in applications of microfluidics. In these applications, efficient mixing is one of the most fundamental and difficult-to-achieve characteristics. To quantitatively represent the mixing performance, nearly all the methods to characterize the micromixing processes in miniaturized devices depend on the images obtained by a microscope coupled with a CCD device or a video camera. The experimental images are generally stored in an RGB or gray-scale format. Intensity information of the micromixing images is most often used to estimate the mixing performance. Reliable quantification of the mixing effects is one of the most important and fundamental issues to study the performances of mixers and to optimize the designs. Thus, mixing indexes are of great significance to quantify the mixing effects. However, mixing indexes merely based on dispersion information cannot always produce reliable results if the variation of the mean intensity with enhanced mixing is neglected. Therefore, mixing indexes that consider the combination of mean and dispersion information from the intensity images in two specific forms were proposed. In addition, two practical criteria were used to evaluate the performances of the quantitative mixing indexes. One is the reliability and the other is repeatability precision. According to the comparisons of different mixing indexes studied herein, mixing indexes that consider the combination of mean and dispersion information can ensure the reliability of the calculated result every time and the repeatability precision was less than ±3.5%. Therefore, it can be concluded that the mixing indexes that consider the combination of mean and dispersion information can more reliably represent the mixing performance.

Characterization of syringe-pump-driven versus pressure-driven microfluidic flows

We compare the dynamic characteristics of syringe-pump-driven and pressure-driven microfluidic flows based on the experimental measurements of the step response of the inlet pressure pi. For syringe-pump-driven flows, periodic fluctuations of pi are observed experimentally, which are induced by the flow-rate fluctuations coming from the mechanical oscillations of the pump motor. For pressure-driven flows, it takes much less time for pi to become steady and no periodic fluctuations are observed. To reduce the deviation between the predicted and the measured pi, a closed-loop control system with feedback of pi is established for pressure-driven flows. Using a PI controller, the deviation can be eliminated, and meanwhile, high stability and accuracy of flow-rate supply can be achieved for pressure-driven flows.

A numerical study of flow field in a flapper-nozzle pilot valve under working condition

It is well known that the flapper-nozzle pilot stage plays a vital role in converting the electric signals into hydraulic outputs between the torque motor and main spool in the electro-hydraulic servo-valve. The characteristics of the flow field greatly influence the working performance of the flapper-nozzle pilot valve. This paper has presented the flow field characteristics in the pilot valve with the flapper under working conditions, which means that the flapper is moving. The velocity of the flapper is imposed based on the dynamic response of the torque motor and spool valve approximately. Meanwhile, the displacement of the flapper is determined using the general design criterion of the servo-valve. During the simulation, the dynamic mesh strategy is utilized to update the moving mesh. Then the simulation results are firstly validated in terms of the mass flow rates of the two nozzles comparing with the theoretical evaluations. The results show that the numerical method is able to predict the flow field of the flapper-nozzle pilot valve under working condition. Finally, the transient velocity field, pressure field and vorticity field are presented. The results prove that the movement of the flapper can result in asymmetric flow field distribution and increase the static pressure and vorticity in the smaller clearance between the flapper and nozzle.

Flow forces acting on particles in the flow in a well and slotted filter

Slotted filters are used for retaining solid particles during the fluid extraction from the well. In the present study a model of slotted filter is considered for hydraulic calculations. The force analysis is done for solid particles (the size is 0.2 (mm)) in a filtration flow in a well with installed slotted filter. Several cases of solid particle motion are observed. The direction of solid particle and the magnitude of net force depend on the flow velocity and fluid production rate. The theoretical analysis shows that types of flow might influence on the slot width.

Test and analysis of self-excited pressure oscillations and noise in a hydraulic jet-pipe servo-valve

Self-excited high frequency pressure oscillations in the flow field of a hydraulic servo-valve are usually the source of high frequency noise. In this paper, by using a piezoelectric dynamic pressure transducer and a microphone, the self-excited high frequency pressure oscillations and noise in a hydraulic jet-pipe servo-valve are measured. The experiments are carried out when the supply pressures of the servo-valve are 11MPa and 20MPa respectively. In order to obtain more precise information, FFT and wavelet analysis are applied to resolve the tested pressure oscillation and noise signals into several basic wave signals. The possible causes of the self-excited high frequency oscillations and noise are predicted according to the analysis results. Based on the results, suggestions are given to reduce the pressure oscillations and noise in hydraulic servo-valves.

Design and performance study on electromagnetic microvalve

An electromagnetic microvalve which can be used in pneumatic pressure control for lab-on-a-chip applications is presented and its structure, working principle, and encapsulation process with valve body fabricated by using polydimethylsiloxane (PDMS) are provided. The numerical simulation of flow filed under several opening degrees of the microvalve is made by software Fluent with UDF Function, and its velocity, pressure, and outlet flow flux under different opening degrees of the electromagnetic microvalve are got. Experimental study of its performance is done under different actuated pressures, frequencies, duty cycles. Influencing factors of the average flow and transient flow under On-Off model and PWM model are analyzed, and its theoretical reason are given. Results show that the designed electromagnetic microvalve has the advantages of fast response time, low-cost, high precision and can be easily integrated with pneumatic microfluidic chips. The results are helpful for the improvement of integration and control accuracy of microfluidic chips.

Closed-Loop Pressure Feedback Control of a Pressure-Driven Microdroplet Generator

To predict the size of droplets formed by pressure-driven flows, the droplet size as a nonlinear function of the pressure ratio is measured experimentally. The mathematical model of the pressure-driven microfluidic device is established, and by varying the volume of a container, comparative and quantitative measurements of the response speed and control accuracy of pressure-driven flows are presented. In particular, a closed-loop control system with feedback of the driven pressure is demonstrated, and the deviation between the measured and the predicted value of the driven pressure can be eliminated by using a PI controller. As a result, by accurately controlling the driven pressure of pressure-driven flows, monodisperse droplets with a desired size can be formed for pressure-driven microdroplet generators.Copyright