Microdroplet Generation Research Articles

High Precision Piezoelectric Micro Droplet Control and Generation System

To analyze and design an intelligent system that can achieve the generation and control of micro droplets. This article is based on the ANSYS finite element analysis and calculation software to design a fluid piezoelectric actuator with a circular oscillator, establish an overall structural model, and provide the design principle and driving structure of the piezoelectric actuator. And different control methods and driving performance were explored. We studied the simulation method of calculating the pressure at the nozzle through the variation of signal excitation amplitude and the control basis of micro droplets. Through the excitation signal, the movement of the fluid can be controlled, and the driving control effect of the fluid has been verified through simulation calculations.

Development of an ILC controller for the Droplet-on-Demand generation system

The droplet-on-demand systems have been increasingly attracting attention from many research groups. The droplet-on- demand system is a promising platform for many biochemical applications, especially for drug delivery applications, where the matter quantity requires very high accuracy, in microliter- scale or even in nanoliter-scale, for enhancing the efficiency, reducing the toxicity as well as the adverse effects of medicines. In order to generate microdroplets, microfluidic technology is emerged as one of the most effective methods. In this paper, we propose a droplet-on-demand generation system based on the Y-junction microfluidic structure with the generated droplet sizes are adjusted by iterative learning control (ILC) technique. The ILC controller can modify the current control input based on the error information measured during earlier experimental procedures, resulting in the desired droplet size in the proposed Y-junction microfluidic channel. The effectiveness of the ILC method was validated by experiments. The obtained results show that the proposed system can generate the required droplet size after 6 to 7 iterations. The results also prove the potential of integrating the ILC technique in the microdroplet generation system. The propose system can be extended to apply in the drug delivery systems for patients in hospitals or chemical mixing systems.

Acoustically Activated Nozzle for Microdroplet Generation and Dispensing

Acoustically Activated Nozzle for Microdroplet Generation and Dispensing

Experimental study on the droplet dynamics after impacting an inclined superhydrophobic surface

Super-hydrophobic surface has been widely studied, related to surface self-cleaning and liquid transport. In this study, effects of the inclined angle and impact velocity on the dynamic characteristics of droplet impact are experimentally investigated, using high-speed photography. When the spreading break-up does not occur, the effect of the inclined angle on the maximum spreading diameter is mainly determined by the competition between the decrease in the inertial force component normal to the surface and the increase in the gravity and inertial force component parallel to the surface. A formula has been developed to predict the maximum spreading factor under different inclined angles. The increasing inclined angle promotes the complete rebound. When the impact velocity is constant, there exists a critical inclined angle, at which the rebounding time is minimum. Furthermore, the critical inclined angle increases with the impact velocity. The increasing inclined angle can affect droplet ejection and potentially hinder its occurrence. Droplet ejection is classified into five stages based on the change in the ratio of the ejected to initial droplet diameter versus Weber number normal to the surface. For stage 5, an empirical formula that predicts the diameter of ejected droplets under different inclined angles is given out. This study can provide a novel idea for the generation and transport of microdroplets of directed size, as well as the recycling of the remaining liquid film after droplet ejection.

Deposition of ultra-thin coatings by a nature-inspired Spray-on-Screen technology

Nanometre-thick, ultrathin coatings applied over a large area are of paramount importance for various application fields such as biomedicine, space and automotive, organic electronics, memory devices, or energy storage devices. So far wet chemical deposition as a cost-effective, scalable, and versatile method can only be used for thicker deposits. Here the formation of uniform ultra-thin coatings with thicknesses below 15 nm using a nature-inspired, roll-to-roll compatible Spray-on-Screen (SoS) technology is reported. For this, the finite micro-droplet generation of Ultrasonic Spray Coating (USSC) is combined with the coating formation from a screen printing mesh. Hydrophobic micro-threads of the mesh, resembling the micro-hair on the legs of water striders, produce millidroplets from micro droplets, and when applying an external pressure to the mesh, dynamic wetting is enforced. The proposed technology is applicable for a wide variety of substrates and applications. It is shown by theory and experiment that ultra-thin coatings below 5 nm homogeneous over a large area can be deposited without the use of extended ink formulation or high substrate temperatures during or after deposition. This simple yet effective technique enables the deposition of ultra-thin films on any substrates, and is very promising to fabricate the organic, inorganic electronics devices and batteries cost effectively.

Microdroplet PCR in Microfluidic Chip Based on Constant Pressure Regulation.

A device and method for the constant pressure regulation of microdroplet PCR in microfluidic chips are developed to optimize for the microdroplet movement, fragmentation, and bubble generation in microfluidic chips. In the developed device, an air source device is adopted to regulate the pressure in the chip, such that microdroplet generation and PCR amplification without bubbles can be achieved. In 3 min, the sample in 20 μL will be distributed into nearly 50,000 water-in-oil droplets exhibiting a diameter of about 87 μm, and the microdroplet will be subjected to a close arrangement in the chip without air bubbles. The device and chip are adopted to quantitatively detect human genes. As indicated by the experimental results, a good linear relationship exists between the detection signal and DNA concentration ranging from 101 to 105 copies/μL (R2 = 0.999). The microdroplet PCR devices based on constant pressure regulation chips exhibit a wide variety of advantages (e.g., achieving high pollution resistance, microdroplet fragmentation and integration avoidance, reducing human interference, and standardizing results). Thus, microdroplet PCR devices based on constant pressure regulation chips have promising applications for nucleic acid quantification.

Super-resolution imaging based on cleanable, controllable, and convenient microdroplet generation

Technology uses optical tweezers and microdroplets as an alternative approach to high-resolution imaging.

A novel array-type microdroplet parallel-generation device.

In this study, the innovative design of a new array microdroplet parallel-generation device is proposed based on the principle of fluid inertial force using a capillary glass needle. The entire device used an electromagnetic actuator as the power source. It was designed as a 9-channel parallel array of glass needles. All glass needles feed independently, allowing different solutions to be sprayed simultaneously while effectively avoiding cross-contamination. We achieved non-contact parallel precision dispensing of nanoliter-sized microdroplet arrays using a relatively simple method. In this study, we first investigated the homogeneity of the generated droplet arrays and the stability of the device over long periods of operation. Then, the influence of the driving-voltage amplitude of the electromagnet and nozzle diameter on microdroplet generation was analyzed. Finally, a prediction model for the droplet size was developed using regression analysis to investigate the on-demand generation of droplets. In summary, the device designed in this study had a novel design, low cost, and modular assembly. It has excellent potential for applications in high precision and low-volume microdroplet-array generation.

Light-induced microdroplet suspension and directional self-driving.

In this Letter, we show stable suspension and directional manipulation of microdroplets on a liquid surface employing simple-mode fiber with a Gaussian beam at 1480-nm wavelength using the photothermal effect. The intensity of the light field generated by the single-mode fiber is used to generate droplets of different numbers and sizes. In addition, the effect of the heat generated at different heights from the liquid surface is discussed through numerical simulation. In this work, the optical fiber is not only free to move at any angle, solving the difficulty that a certain working distance is needed to generate microdroplets on free space, it can also allow the continuous generation and directional manipulation of multiple microdroplets, which is of tremendous scientific relevance and application value in promoting the development and cross-fertilization of life sciences and other interdisciplinary fields.

Numerical Research on the Effects of Process Parameters on Microdroplet Jetting Characteristics by Piezoelectric Printhead

The precision and consistency of the microdroplet jetting procedure are crucial for the casting sand mold’s performance during binder injection. The generation and jetting of microdroplets in piezoelectric printheads were examined in this study in relation to changes in specific jetting process parameters. Using finite element analysis and a simplified physical model of a microdroplet jetting device, an electromechanically coupled model of a microdroplet jetting device was created in order to study the characteristics of microdroplet jetting. A volume-of-fluid model was also created in order to study the microdroplet jetting process and perform repeatability tests. The effects of altering nozzle radius, actuation pulse width, intake velocity, and fluid viscosity on microdroplet jetting properties were then investigated using the models. We were able to control the development of satellite droplets thanks to the knowledge we gained about how each process parameter affected droplet status. This study demonstrates how the radius of the nozzle and the pulse width of the piezoelectric actuation signal have a significant impact on the jetting properties of piezoelectric printheads and the production of microdroplets. The quantitative correlations between process factors and jetting characteristics can be used to optimize microdroplet production and reduce droplet size. Finally, this study will help create control systems for microdroplet jetting operations and enhance the precision of 3D printed casting sand molds.

Monodisperse Micro-Droplet Generation in Microfluidic Channel with Asymmetric Cross-Sectional Shape.

Micro-droplets are widely used in the fields of chemical and biological research, such as drug delivery, material synthesis, point-of-care diagnostics, and digital PCR. Droplet-based microfluidics has many advantages, such as small reagent consumption, fast reaction time, and independent control of each droplet. Therefore, various micro-droplet generation methods have been proposed, including T-junction breakup, capillary flow-focusing, planar flow-focusing, step emulsification, and high aspect (height-to-width) ratio confinement. In this study, we propose a microfluidic device for generating monodisperse micro-droplets, the microfluidic channel of which has an asymmetric cross-sectional shape and high hypotenuse-to-width ratio (HTWR). It was fabricated using basic MEMS processes, such as photolithography, anisotropic wet etching of Si, and polydimethylsiloxane (PDMS) molding. Due to the geometric similarity of a Si channel and a PDMS mold, both of which were created through the anisotropic etching process of a single crystal Si, the microfluidic channel with the asymmetric cross-sectional shape and high HTWR was easily realized. The effects of HTWR of channels on the size and uniformity of generated micro-droplets were investigated. The monodisperse micro-droplets were generated as the HTWR of the asymmetric channel was over 3.5. In addition, it was found that the flow direction of the oil solution (continuous phase) affected the size of micro-droplets due to the asymmetric channel structures. Two kinds of monodisperse droplets with different sizes were successfully generated for a wider range of flow rates using the asymmetric channel structure in the developed microfluidic device.

Pinch-off droplet generator using microscale gigahertz acoustics.

The generation and dispensing of microdroplets is a vital process in various fields such as biomedicine, medical diagnosis and chemistry. However, most methods still require the structures of nozzles or microchannels to assist droplet generation, which leads to limitations on system flexibility and restrictions on the size range of the generated droplets. In this paper, we propose a nozzle-free acoustic-based method for generating droplets using a gigahertz (GHz) bulk acoustic wave (BAW). Unlike most of the acoustofluidic approaches, the proposed method produces the droplet by pinching-off the liquid column generated by the acoustic body force at the oil-water interface. Benefitting from the focused acoustic energy and small footprint of the device, four orders of magnitude (ranging from 2 μm to 1800 μm) of droplet size could be produced by controlling the working time and power of the device. We also demonstrated cell encapsulation in the droplet and a high cell viability was achieved. The proposed acoustic-based droplet generation method exhibits capacity for generating droplets with a wide size range, versatility toward different viscosities, as well as biocompatibility for handling viable samples, which shows potential in miniaturization and scalability.

Highly parallel, wash-free, and ultrasensitive centrifugal droplet digital protein detection in sub-microliter blood.

The ability to efficiently detect low-abundance protein biomarkers in tiny blood samples is a significant challenge in clinical and laboratory settings. Currently, high-sensitivity approaches require specialized instrumentation, involve multiple washing steps, and lack the ability to parallelize, preventing their widespread implementation. Herein, we developed a parallelized, wash-free, and ultrasensitive centrifugal droplet digital protein detection (CDPro) technology that achieves a femtomolar limit of detection (LoD) of target proteins with sub-microliters of plasma. The CDPro combines two techniques, namely a centrifugal microdroplet generation device and a digital immuno-PCR assay. Miniaturized centrifugal devices enable emulsification of hundreds of samples within 3 minutes using a common centrifuge. The bead-free digital immuno-PCR assay not only eliminates the need for multistep washing, but also possesses ultra-high detection sensitivity and accuracy. We characterized the performance of CDPro using recombinant interleukins (IL-3 and IL-6) as example targets and reported a LoD of 0.0128 pg mL-1. We also quantified IL-6 from 7 human clinical blood samples using the CDPro with only 0.5 μL plasma, which showed excellent agreement with an existing clinical protein diagnostic system with 25 μL plasma from those samples (R2 = 0.98).

Examining Microdroplet Characteristics of Electrospray Electric Precipitation for Direct and Indirect Voltage Application Methods

Dust and fine dust exist in various forms and can lead to various problems such as dust explosion and air pollution. To reduce them, recently, research on electrospray electric precipitation, among various dust collection methods, has been actively conducted. In this study, comparative analysis experiments were performed on microdroplet generation characteristics for direct and indirect charging methods of electrospray electric precipitation. A visualization device was fabricated to generate microdroplets, and microdroplets were photographed with a high-magnification camera according to each charging method. The size and number of microdroplets were then analyzed via ImageJ software.

Pre-programmed microdroplet generator to control wide-ranging chemical concentrations

Controlling the chemical concentration in droplets is an essential function in numerous applications of biology, chemistry, and engineering. However, it is difficult to perform an automated control of wide-ranging chemical concentrations accurately without any high-precision off-chip controllers. In this study, we present a preprogrammed microdroplet generator that controls the chemical concentration of droplets by more than three orders of magnitude. The device used constant water-head pressure as the sole driving force and regulated the concentration in a periodic and stepwise manner while keeping the droplet size constant. For this process, an autonomous microfluidic circuit controlled the flow rate ratio of two solutions, and an external mechanical capacitor tuned the step size of the control pressure over a wide range of 0.31–237 Pa. The utility of the device was demonstrated using a DNA-mediated homogeneous binding assay.

Concept and electromechanical-coupling modeling of a torsional vibration excitation method

The sandwiched type torsional piezoelectric actuators have widely been employed in the fields of ultrasonic cutting, ultrasonic drilling, ultrasonic welding, ultrasonic wire drawing, ultrasonic spraying, and microdroplet generation, due to their unique vibration form. However, the circular piezoelectric ceramic plate polarized in the circumferential direction holds a complicated manufacturing process, a long processing period, and a high price. It can only be used for the excitation of the torsional vibration of circular rods, and cannot be used for square cross-section beams, severely restricting its applications. In order to solve the above problems, a novel excitation method is proposed for generating torsional vibration in the square cross-section beam in this study, which uses the d15 vibration mode of rectangular piezoelectric ceramic plates to produce the torsional vibration, and a novel transfer matrix model of square torsional piezoelectric element is created. Then a case study is conducted in this paper to confirm the proposed excitation method and the developed model, in which two novel sandwiched type torsional piezoelectric actuators are designed. Moreover, electromechanical coupling models of these two torsional piezoelectric actuators are developed using the transfer matrix method, in order to reveal their dynamics behaviors. Finally, the finite element method and the experimental investigations are adopted to analyze and test the frequency response characteristics and vibration shapes of the proposed piezoelectric actuators. Both simulation analysis and experimental tests verify the effectiveness of the proposed excitation method and the correctness of the developed electromechanical coupling dynamic models. The proposed excitation method provides a new idea for the design of sandwiched type torsional piezoelectric actuators, and the developed theoretical model lays a theoretical foundation for the further research of torsional piezoelectric actuators.

Micro-injection Molded Droplet Generation System for Digital PCR Application.

Sensitive, effective, and quantitative analysis of infectious pathogens is an important task for the prevention of human health threats. Herein, we present an advanced approach to producing gene-encapsulated microdroplets for quantitative analysis using a micropatterned metal mold and injection molding technique with an automatically operated system. An injection molded microdroplet generation device was successfully fabricated with a minimum channel width of 30 μm and optimized to produce 100 μm diameter droplets. The optimized microchannel design and flow rate also enable the production of stable numbers of microdroplets (~ 16,000 droplets). To verify the applicability of our device and system to droplet-based digital PCR analysis, Escherichia coli (E. coli) O157:H7 was selected as a model bacterial pathogen, and the stx2 gene was amplified in the microdroplets. The generated microdroplets exhibit both chemical and mechanical stability, and our results are similar to those obtained by a commercially available method. Accordingly, the usefulness of the microdroplet generative device and system is confirmed as a simple, fast, and reliable tool for the quantitative molecular analysis of infectious diseases.

A microfluidic droplet system for ultra-monodisperse droplet generation: A universal approach

Despite the importance of droplet monodispersity, a universal methodology for high monodispersity droplet generation does not exist yet. We have recently demonstrated that unlike the conventional method of droplet generation, applying an identical pressure from a single source makes the microfluidic droplet system immune to the external fluctuations that originate from the imperfection of the flow source. In this work, we show that our method is universal and applicable to other common microfluidic devices and flow sources. We applied this method to flow-focusing and coflow devices that are commonly used for high-frequency microdroplet generation. In addition to the pressure pump, we used a syringe pump to show that our method is applicable to flow rate controllable systems as well. We compared the monodispersity of droplets formed by the conventional methods and the novel method explained in this work.

Generation of monodisperse micro-droplets within the stable narrowing jetting regime: effects of viscosity and interfacial tension

Generation of monodisperse micro-droplets within the stable narrowing jetting regime: effects of viscosity and interfacial tension

PZT actuator-controlled high-frequency microdroplet generation: Reducing the restrictions of channel size, fluid viscosity, and flow rate

For passive generation of microdroplets, the droplet volume correlates with the flow rate and the generation frequency, and is further influenced by the channel size and fluid viscosity. To improve the control flexibility, we study the active droplet generation using a cost-effective piezo disk actuator. In this method, the droplet generation frequency is synchronized with, and hence can be directly set by, the piezo actuation. Both the flow rates of the discrete and continuous fluids can be independently controlled, leading to linear tunability of the droplet volume and spacing. The restrictions of the channel size and fluid viscosity are also reduced. Furthermore, this is achieved at high frequencies up to several thousand Hertz, and at reduced continuous-fluid/discrete-fluid flow rate ratios. Using the same method, flexible modulation on the size and morphology of double-emulsion microdroplets is also demonstrated.