In a T-junction microdroplet generator, as monodisperse droplets are formed for the droplet digital polymerase chain reaction (PCR), a mathematical model which can describe the nonlinear relation of droplet generation under pressure-driven microfluidic flows is established. For various viscosity of the fluids, the relation between the droplet length and the driving-pressure ratio is measured at a low capillary number (Ca < 0.01) of droplet generation. It can be observed that the droplet length varies nonlinearly with the driving-pressure ratio for a specific geometry of the T-junction. In particular, the relation between the droplet length and the driving-pressure ratio is tested experimentally for different geometries of the T-junctions. At low capillary numbers, the nonlinear relation between the droplet length and the driving-pressure ratio is basically determined by the geometrical parameters of the T-junction. Most importantly, a closed-loop control droplet microfluidic system is demonstrated, and the size of the droplets can be controlled more accurately by pressure-driven flows. The control precision of the droplet size is quantitatively studied for a wide range of typical conditions of droplet generation. In particular, the relative standard deviation of the droplet size for various flow conditions is less than 2%, which is of great importance for improving the detection accuracy of nucleic acid based on the droplet digital PCR. (c) 2021 Elsevier Ltd. All rights reserved.

For droplet microfluidics, the electrical-detection method which can precisely detect the size of monodisperse droplets is demonstrated in this paper. In a Flow-focusing microdroplet generator, three pairs of the microelectrodes are allocated along the microchannel, and during the passing-by process of each droplet, both the length, the velocity and the production speed of the droplets can be obtained from the experimental measurements of the time-varying capacitance between each pair of the microelectrodes. Particularly, for different geometries of the Flow-focusing microchannel, the method of the electrical-detection is validated experimentally over a wide range of the typical conditions of monodisperse droplet production. In addition, the droplet size measured by the electrical-detection method is compared with that by the method of image processing, and the detection precision of the electrical-detection method is verified experimentally. Most importantly, by calculating the root-mean-square value of the droplet lengths for three pairs of the microelectrodes, the detection precision of the droplet size can be increased drastically.

In a T-junction microdroplet generator, the relation between the droplet length and the flow-rate ratio of the immiscible fluids is studied experimentally at both low and high capillary numbers. In particular, different geometries of the T-junctions are designed for monodisperse droplet formation, and the droplet length as a function of the flow-rate ratio is measured for various viscosity of the fluids. It is observed that there is a linear relation between the droplet length and the flow-rate ratio at a low capillary number(Ca⩽0.1), while the droplet length is varying nonlinearly with the flow-rate ratio at a high capillary number(0.1⩽Ca⩽1.0). More importantly, by using the feedback control of the droplet size, a closed-loop control droplet microfluidic system is demonstrated. Especially, by choosing specific control method, good agreements are shown between the predicted and the measured droplet size for a wide range of the capillary numbers of monodisperse droplet production.

Most current Internet of Things systems face many problems like privacy leakage, information island and low application value, because they are designed from the perspective of Internet of Things service providers. However, the practicality and privacy security of Internet of Things services are more important from the perspective of Internet of Things system users. Considering that there aren’t too many Internet of Things devices to use and manage for ordinary people and service, a more service-oriented architecture of the lightweight private IOT remote system is proposed in this paper. To verify this system, verifications like remote sensors, remote actuators, and a micromixing application are developed and illustrated. Owing to its distributed server architecture, this system has high flexibility, practicability, parallelism, and low coupling. This novel system architecture provides a new perspective to develop practical valuable private Internet of Things services.

High-molecular-weight polymeric nanoparticles are critical to increasing the loading efficacy and tuning the release profile of targeted molecules for medical diagnosis, imaging, and therapeutics. Although a number of microfluidic approaches have attained reproducible nanoparticle synthesis, it is still challenging to fabricate nanoparticles from high-molecular-weight polymers in a size and structure-controlled manner. In this work, an acoustofluidic platform is developed to synthesize size-tunable, high-molecular-weight (>45 kDa) poly(lactic-co-glycolic acid)-b-poly(ethylene glycol) (PLGA-PEG) nanoparticles without polymer aggregation by exploiting the characteristics of complete and ultrafast mixing. Moreover, the acoustofluidic approach achieves two features that have not been achieved by existing microfluidic approaches: (1) multi-step (≥2) sequential nanoprecipitation in a single device, and (2) synthesis of core-shell structured PLGA-PEG/lipid nanoparticles with high molecular weights. The developed platform expands microfluidic potential in nanomaterial synthesis, where high-molecular-weight polymers, multiple reagents, or sequential nanoprecipitations are needed.

The pressure-driven device is designed and the flow rates of the microfluidic systems can be supplied by the pressure-driven flows, which can significantly reduce the flow-rate fluctuations coming from the pump source. For pressure-driven flows, the flow rates of the fluids can be predicted by measuring the pressure drop along a polytetrafluoroethylene (PTFE) tubing. Especially, by varying the geometrical parameters of the PTFE tubing, the predicted flow rates of the fluids are compared with the experimental measurements, and the testing precision of the pressure-driven flows can be obtained. Meanwhile, the dynamic characteristics of the open-loop and closed-loop control pressure-driven device are comparatively studied. Particularly, a proportional and integral (PI) controller is integrated with the closed-loop control pressure-driven device, and the effects of the parameters of the PI controller on the dynamic characteristics of the pressure-driven devices are mainly discussed. Most importantly, by improving the dynamic characteristics of the pressure-driven devices, precise measurement and control of the pressure-driven flows can be achieved for microfluidic systems.

Advances in lab-on-a-chip technologies are driven by the pursuit of programmable microscale bioreactors or fluidic processors that mimic electronic functionality, scalability, and convenience. However, few fluidic mechanisms allow for basic logic operations on rewritable fluidic paths due to cross-contamination, which leads to random interference between "fluidic bits" or droplets. Here, we introduce a mechanism that allows for contact-free gating of individual droplets based on the scalable features of acoustic streaming vortices (ASVs). By shifting the hydrodynamic equilibrium positions inside interconnected ASVs with multitonal electrical signals, different functions such as controlling the routing and gating of droplets on rewritable fluidic paths are demonstrated with minimal biochemical cross-contamination. Electrical control of this ASV-based mechanism allows for unidirectional routing and active gating behaviors, which can potentially be scaled to functional fluidic processors that can regulate the flow of droplets in a manner similar to the current in transistor arrays.

To precisely control the size of droplets produced in a flow-focusing microdroplet generator, the pressure-driven device is designed to control the flow rates of the fluids during the experiments of droplet generation. While monodisperse droplets are produced by the pressure-driven microfluidic flows, the nonlinear relation between the droplet length and the driving-pressure ratio of the two phases can be observed for the open-loop control of droplet generation. In particular, by using the method of the closed-loop control of droplet generation, good agreements are shown between the tested size of droplets and the predicted droplet size. Consequently, based on the closed-loop control of droplet generation under pressure-driven flows, the effects of the nonlinearity of the flow-focusing microchannel on the control accuracy of the droplet size is drastically reduced, which can significantly increase the control precision of the droplet size in the flow-focusing microdroplet generator.

Based on droplet microfluidics, we demonstrate a new method that can quantitatively measure the somatic cell count (SCC) of fat-free milk.

We demonstrate an efficient method that can precisely measure the viscosity of fluids based on droplet microfluidics. For our design of the droplet microfluidic viscometer, the volume of the fluid sample required for testing the fluid viscosity is on the order of nanoliters. In particular, a T-junction microdroplet generator is designed for the production of monodisperse droplets, and the flow rates of the continuous and dispersed phases are controlled by the pressure-driven microfluidic device. By giving a specified viscosity of the dispersed phase, the viscosity of the continuous phase can be measured, while considering the linear relation between the droplet length and the flow-rate ratio of the two phases, the linear relation between the droplet length and the viscosity ratio of the two phases can be obtained. For our design of the T-junction microdroplet generator, the viscosity ratio of the two phases can be predicted by testing the length of droplets formed in the microchannel, and therefore, the fluid viscosity of the continuous phase can be calculated. More importantly, the comparison between the measured and the given viscosity of the continuous phase is provided for three different geometries of the T-junctions, and consequently, the testing precision of the fluid viscosity can be validated experimentally.

Acoustic-based techniques can manipulate particles in a label-free, contact-free, and biocompatible manner. However, most previous work in acoustic manipulation has been constrained by axisymmetric patterns of pressure nodes and antinodes. Acoustic holography is an emerging technique that offers the potential to generate arbitrary pressure distributions which can be applied to particle manipulation with higher degrees of freedom. However, since current acoustic holography techniques rely on acoustic radiation forces, which decrease dramatically when the target particle size decreases, they have difficulty manipulating particles in the micro/nanoscale. Here, we introduce a holography technique that leverages both an arbitrary acoustic field and controllable fluid motion to offer an effective approach for manipulating micro/nano particles. Our approach, termed acoustofluidic holography (AFH), can manipulate a variety of materials, including cells, polymers, and metals, across sizes ranging from hundreds of micrometers to tens of nanometers.

Droplet microfluidics has become an indispensable tool for biomedical research and lab-on-a-chip applications owing to its unprecedented throughput, precision, and cost-effectiveness. Although droplets can be generated and screened in a high-throughput manner, the inability to label the inordinate amounts of droplets hinders identifying the individual droplets after generation. Herein, we demonstrate an acoustofluidic platform that enables on-demand, real-time dispensing, and deterministic coding of droplets based on their volumes. By dynamically splitting the aqueous flow using an oil jet triggered by focused traveling surface acoustic waves, a sequence of droplets with deterministic volumes can be continuously dispensed at a throughput of 100 Hz. These sequences encode barcoding information through the combination of various droplet lengths. As a proof-of-concept, we encoded droplet sequences into end-to-end packages (e.g., a series of 50 droplets), which consisted of an address barcode with binary volumetric combinations and a sample package with consistent volumes for hosting analytes. This acoustofluidics-based, deterministic droplet coding technique enables the tagging of droplets with high capacity and high error-tolerance, and can potentially benefit various applications involving single cell phenotyping and multiplexed screening.

In a T-junction microdroplet generator, a mathematical model which can describe the linear relation between the droplet length and the flow-rate ratio for different geometries of the T-junctions is established. For different viscosity of the fluids, the droplet length as a function of the flow-rate ratio is measured experimentally. We observe that the droplet length is a linear function of the flow-rate ratio for different Capillary numbers, while the droplet length varies nonlinearly with the flow-rate ratio at a high Capillary number. Particularly, two geometries of the T-junction microchannels are designed for droplet formation, and good agreements are found between the predicted and the measured droplet length for low Capillary numbers. More importantly, the linear model of droplet formation is only determined by the geometry of a T-junction and independent of the viscosity of the fluids for high Capillary numbers. As a result, our linear model can be experimentally validated, and the size of the droplets can be precisely predicted for different geometries of the T-junctions.

Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve distinct functions on demand. Here a programmable acoustic metasurface that contains an array of tunable subwavelength unit cells to break the limitation and realize versatile two-dimensional wave manipulation functions is reported. Each unit cell of the metasurface is composed of a straight channel and five shunted Helmholtz resonators, whose effective mass can be tuned by a robust fluidic system. The phase and amplitude of acoustic waves transmitting through each unit cell can be modulated dynamically and continuously. Based on such mechanism, the metasurface is able to achieve versatile wave manipulation functions, by engineering the phase and amplitude of transmission waves in the subwavelength scale. Through acoustic field scanning experiments, multiple wave manipulation functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow by using one design of metasurface are visually demonstrated. This work extends the metasurface research and holds great potential for a wide range of applications including acoustic imaging, communication, levitation, and tweezers.

In a flow-focusing microdroplet generator, by changing the flow rates of the two immiscible fluids, production speed can be increased from tens to thousands of droplets per second. However, because of the nonlinearity of the flow-focusing microdroplet generator, the production speed of droplets is difficult to quantitatively study for the typical flow-focusing geometry. In this paper, we demonstrate an efficient method that can precisely predict the droplet production speed for a wide range of fluid flow rates. While monodisperse droplets are formed in the flow-focusing microchannel, droplet spacing as a function of time was measured experimentally. We discovered that droplet spacing changes periodically with time during each process of droplet generation. By comparing the frequency of droplet spacing fluctuations with the droplet production speed, precise predictions of droplet production speed can be obtained for different flow conditions in the flow-focusing microdroplet generator.

Over the past several decades, a litany of acoustofluidic devices have been developed which purport to have significant advantages over traditional benchtop analytical tools. These acoustofluidic devices are frequently labeled as "labs-on-chips"; however, many do an insufficient job of limiting their dependence on the lab. Often, acoustofluidic devices still require skilled operators and complex external equipment. In an effort to address these shortcomings, we developed a low-cost, expandable, and multifunctional system for controlling acoustofluidic devices in the audible to low ultrasonic frequency range (31 Hz to 65 kHz). The system was designed around the readily available Arduino prototyping platform because of its user-friendly coding environment and expansive network of open source material; these factors enabled us to create a system capable of generating high voltage oscillatory signals and controlling microscale flows in acoustofluidic devices. Utilizing the established open source system, we achieved a series of acoustofluidic applications involving the manipulation of fluids and biological objects in a portable fashion. In particular, we used our open source acoustofluidic devices to achieve active rotation of cells and microorganisms, and operation of an acoustofluidic mixing device which has previously shown potential for viscous sample preparation, in a portable fashion. Additionally, using low frequency flexural waves and our portable system, we achieved acoustofluidic separation of particles based on size. It is our hope that the open source platform presented here can pave the way for future acoustofluidic devices to be used at the point-of-care, as well as simplify the operation of these devices to enable resource limited users to leverage the benefits of acoustofluidics in their work.

Platelet separation is a crucial step for both blood donation and treatment of essential thrombocytosis. Here we present an acoustofluidic device that is capable of performing high-throughput, biocompatible platelet separation using sound waves. The device is entirely made of plastic material, which renders the device disposable and more suitable for clinical use. We used this device to process undiluted human whole blood, and we demonstrate a sample throughput of 20 mL min-1, a platelet recovery rate of 87.3%, and a red/white blood cell removal rate of 88.9%. We preserved better platelet function and integrity for isolated platelets than those which are isolated using established methods. Our device features advantages such as rapid fabrication, high throughput, and biocompatibility, so it is a promising alternative to existing platelet separation approaches.

To quantitatively study the pressure fluctuations induced by the process of droplet formation in a T-junction microdroplet generator, the mathematical model which can accurately predict the amplitude and the frequency of the pressure fluctuations is demonstrated. Different geometries of the T-junction microchannels are designed for the experiments of droplet formation, and the time-varying pressure difference between the upstream and downstream of each droplet is measured during the droplet production progress. From both theoretical and experimental study, it can be observed the pressure difference is a periodic function of time for the whole process of droplet generation. In particular, the frequency of the pressure fluctuations coincides with the production rate of droplets, and the amplitude of the pressure fluctuations varies with the geometrical parameters of the T-junctions. Additionally, good agreements are shown between the theoretical calculations and the experimental measurements of the magnitude of the pressure fluctuations. Therefore, our mathematical model is validated experimentally, and the magnitude of the pressure fluctuations can be drastically reduced by optimization of the geometrical parameters of the T-junction.