Uniform Microdroplets Research Articles

A smart roof that transforms raindrops into agricultural spraying.

We present a smart roof that makes fragmented droplets from the impact of raindrops on superhydrophobic meshes and utilizes the droplets for agricultural spraying. This facile method transforms raindrops or waterdrops into uniform microdroplets, which can both reduce crop lodging induced by heavy rainfall, and realize uniform spraying of pesticides.

Sensitive, rapid detection of NCOA4-m6A towards precisely quantifying radiation dosage on a Cas13a-Microdroplet platform

Precise quantification of low-dose ionizing radiation is of great significance in protecting people from damage caused by clinical radiotherapy or environmental radiation. Traditional techniques for detecting radiation, however, remain extreme challenges to achieve high sensitivity and speed in quantifying radiation dosage. In this work, we report a Cas13a-Microdroplet platform that enables sensitive detection of ultra-low doses of radiation (0.5 Gy vs. 1 Gy traditional) within 1 h. The micro-platform adopts an ideal, specific radiation-sensitive marker, m6A on NCOA4 gene (NCOA4-m6A) that was first reported in our recent work. Microfluidics of the platform generate uniform microdroplets that encapsulate a CRISPR/Cas13a detection system and NCOA4-m6A target from the whole RNA extraction, achieving 10-fold enhancement in sensitivity and significantly reduced limit of detection (LOD). Systematic mouse models and clinical patient samples demonstrated its superior sensitivity and LOD (0.5 Gy) than traditional qPCR, which show wide potentials in radiation tracking and damage protection.

Machine vision–based automatic planning for microdroplet coating trajectory on irregularly shaped regions of metal surfaces

Metal surface coating technology plays a crucial role in various aspects of metal engineering such as enhancing metal oxidation resistance, corrosion prevention, insulation, decoration, and surface performance. This paper presents a machine vision–based automatic planning algorithm for microdroplet coating trajectory on irregularly shaped regions of metal surfaces, aiming to achieve uniform coating on different surface shapes and sizes. First, the process of microdroplet shape changes on metal surfaces is analyzed, the wetting circle diameter of a fully spread microdroplet is estimated, and the distance between two neighboring microdroplets is confirmed. Then, color pattern image processing (region contour extraction, feature point extraction, deburring, and equidistant offset algorithms) and template matching technology are developed to extract coating trajectory contours and accurately locate the workpieces on the workbench. To ensure a smooth and continuous coating trajectory, rational cubic Bézier curves are employed to interpolate the contour feature points, forming C1 continuous curves. Finally, taking into consideration the distance between two neighboring microdroplets, the continuous curves are equidistant offset and equichord discretized to determine the microdroplet locations. The experimental results demonstrate that the proposed algorithm successfully achieves arbitrary workpiece positioning, automatic planning of coating trajectory and uniform microdroplet coating on different surface shapes and sizes.

A Customizable and Low-Cost Ultraviolet Exposure System for Photolithography.

For microfluidic device fabrication in the research, industry, and commercial areas, the curing and transfer of patterns on photoresist relies on ultraviolet (UV) light. Often, this step is performed by commercial mask aligner or UV lamp exposure systems; however, these machines are often expensive, large, and inaccessible. To find an alternative solution, we present an inexpensive, customizable, and lightweight UV exposure system that is user-friendly and readily available for a homemade cleanroom. We fabricated a portable UV exposure system that costs under $200. The wafer holder's adjustable height enabled for the selection of the appropriate curing distance, demonstrating our system's ability to be easily tailored for different applications. The high light uniformity across a 4" diameter wafer holder (light intensity error ~2.9%) was achieved by adding a light diffusing film to the apparatus. These values are comparable to the light uniformity across a 5" diameter wafer holder from a commercial mask aligner (ABM 3000HR Mask Aligner), that has a light intensity error of ~4.0%. We demonstrated the ability to perform photolithography with high quality by fabricating microfluidic devices and generating uniform microdroplets. We achieved comparable quality to the wafer patterns, microfluidic devices, and droplets made from the ABM 3000HR Mask Aligner.

An unconventional vertical fluidic-controlled wearable platform for synchronously detecting sweat rate and electrolyte concentration

Epidermal microfluidic devices with long microchannels have been developed for continuous sweat analysis, which are crucial to assess personal hydration status and underlying health conditions. However, the flow resistance in long channels and the ionic concentration variation significantly affect the accuracy of both the sweat rate and electrolyte concentration measurements. Herein, we present a novel fluidic-controlled wearable platform for synchronously dropwise-detecting the sweat rate and total electrolyte concentration. The unconventional platform consisting of a vertically shortened channel, a pair of embedded electrodes and an absorption layer, is designed to minimize the flow resistance and transform sweat fluidics into uniform micro-droplets for chronological and dropwise detection. Real-time sweat conductance is decoupled from a square-wave-like curve, where the sweat rate and electrolyte concentration can be derived from the interval time and peak value, respectively. Flexible and wearable band devices are demonstrated to show their potential application for hydration status assessment during exercises.

Recent developments of droplets-based microfluidics for bacterial analysis

Droplet-based microfluidics enables the generation of uniform microdroplets at picoliter or nanoliter scale with high frequency (∼kHz) under precise control. The droplets can function as bioreactors for versatile chemical/biological study and analysis. Taking advantage of the discrete compartment with a confined volume, (1) isolation and manipulation of a single cell, (2) improvement of in-droplet effective concentrations, (3) elimination of heterogeneous population effects, (4) diminution of contamination risks can be achieved, making it a powerful tool for rapid, sensitive, and high-throughput detection and analysis of bacteria, even for rare or unculturable strains in conventional methods. This mini-review will focus on the generation and manipulation of micro-droplets and bacteria detection and analysis carried out by droplet-based microfluidics. Finally, applications with high potential of droplet-based bacteria analysis are briefly introduced. Due to the advantages of rapid, sensitive, high throughput, and compatibility with rare and unculturable bacteria in conventional methods, droplet-based microfluidics has tremendous potential of providing novel solutions for biological medicine, microbiological engineering, environmental ecology, etc.

Controllable and high-throughput preparation of microdroplet using an ultra-high speed rotating packed bed

Microdroplets and their dispersion, with a large specific surface area and a short diffusion distance, have been applied in various unit operations and reaction processes. However, it is still a challenge to control the size and size distribution of microdroplets, especially for high-throughput generation. In this work, a novel ultra-high speed rotating packed bed (UHS-RPB) was invented, in which rotating foam packing with a speed of 4000–12000 r·min−1 provides microfluidic channels to disperse liquid into microdroplets with high throughput. Then generated microdroplets can be directly dispersed into a continuous falling film for obtaining a mixture of microdroplet dispersion. In this UHS-RPB, the effects of rotational speed, liquid initial velocity, liquid viscosity, liquid surface tension and packing pore size on the average size (d32) and size distribution of microdroplets were systematically investigated. Results showed that the UHS-RPB could produce microdroplets with a d32 of 25–63 μm at a liquid flow rate of 1025 L·h−1, and the size distribution of the microdroplets accords well with Rosin–Rammler distribution model. In addition, a correlation was established for the prediction of d32, and the predicted d32 was in good agreement with the experimental data with a deviation within ± 15%. These results demonstrated that UHS-RPB could be a promising candidate for controllable preparation of uniform microdroplets.

Experimental Analysis of a Pneumatic Drop-on-Demand (DOD) Injection Technology for 3D Printing Using a Gallium-Indium Alloy

Many liquid metals have a high boiling point, strong electrical conductivity, high thermal conductivity, and non-toxic properties, which make them ideal targets for applications in different fields such as optics, microcircuits, electronic switches, micro-electromechanical System (MEMS) devices and 3D printing manufacturing. However, owing to the generally high surface tension of these liquids, achieving uniform micro-droplets is often a challenge due to the inherent difficulties in controlling their size and shape. In this study, a gallium indium alloy (GaIn24.5) has been used in combination with a pneumatic drop-on-demand (DOD) injection technology to carry out a series of experiments. The micro-droplet forming process has been explored for different pressure and pulse width conditions. Uniform metal droplets (diameter 1080 μm) have been obtained with a 1.5 kPa jet pressure, 100 ms pulse width, and 50% duty ratio. The standard deviation of the measured metal droplets diameter has been found to be approximately 20 μm.

Bioinspired Tip-Guidance Liquid Jetting and Droplet Emission at a Rotary Disk via a Surface Energy Gradient.

Effective droplet emission is of fundamental importance for practical application, such as agricultural sprays to painting, atomization, emulsification, and catalytic action. Highly viscous liquids are commonly used, such as printing inks, which hinder the ejection at the nozzle. A big challenge faced by people is how to obtain stable and controllable liquid droplets in a wide range of viscosities. Inspired by the rotation shaking of droplets on fiber clusters and the rotary spraying disk technique, here, we demonstrate uniform microdroplet (1-2000 mPa·s) generation in a tip-guided way that replaces the commonly confined nozzle by a double-layer spinning "sandwich" multitip disk (SSMD). A surface energy gradient induced by the margin structure of the alternating gas wedge and solid tip guides liquid to move along the solid tip, which is ejected at the end of the tip, forming a ring of droplet clusters. SSMD improves the effective droplet-jet process to 7/10 of the whole drainage process and enhances the efficiency with a production drop volume of ∼3.19 × 107 μL/h and production droplet numbers of ∼3.3 × 104 per second. Droplets can be fine-tuned between 0.1 and 1.0 mm via the tip structure, liquid property, and spinning angular velocity with a narrow size distribution. This facile tip-guided design could inspire the possibility of energy-efficient droplet production techniques in various fluid applications, such as spraying and printing. It may further improve other fluid systems that serve as a crucial component for high-speed droplet manipulation, liquid transport, and water vapor capturing.

STUDY OF PROCESS CONTROL ON PIEZOELECTRIC DROP-ON-DEMAND EJECTION

The size and uniformity of micro-droplets are key factors influencing the quality of the molded part by micro-droplet ejecting additive manufacturing technology. In this paper, a piezo-actuated micro-droplet ejection device for generating uniform micro-droplets is studied. The piezoelectric material drives the flexible diaphragm to vibrate and pushes the liquid out of the nozzle and produces micro-droplets. The amplitude of the diaphragm under different control parameters and its influence on the size and uniformity of the generated micro-droplets are investigated by numerical simulation and experiment. The results indicate that the amplitude of the diaphragm is affected by the driving voltage and the piezoelectric frequency and the experimental value of the diaphragm's center point amplitude is less than that of the theoretical calculation value which is mainly influenced by the piezoelectric frequency. The amplitude of the diaphragm will change the pressure inside the nozzle, which leads to the varied sizes of the micro-droplet. When the driving voltage is constant, the diaphragm has the maximum amplitude for the piezoelectric frequency at 10 Hz. As the amplitude of the diaphragm increases, droplets can be generated when the liquid velocity at the orifice and the length of the liquid column increase to a critical value. As the amplitude of the diaphragm continues to increase and the length of the liquid column at the orifice is beyond to a critical value, a satellite micro-droplet is formed. When the amplitude range of the membrane is between 30 $\mu $m and 42 $\mu $m, micro-droplets can be stably formed whose uniformity and size meet the demand well, and the minimum generating micro-droplets size is 339.8 $\mu $m. The maximum change rate of droplet diameter and adjacent two droplets are 0.29% and 2.67% respectively. Results are benefit to promote the uniformity of micro-droplets and will provide a reference for the development of piezoelectric droplet ejection devices.

Producing solder droplets using piezoelectric membrane-piston-based jetting technology

This paper proposes piezoelectric membrane-piston-based jetting technology, an improved drop-on-demand (DOD) jetting technology for generating micro-droplets. The main components of the experimental apparatus are a nozzle head, a cooling chamber, a vibration bar, a computer system, a temperature controller and a three-dimensional (3D) platform. Based on the proposed working principle, a physical model was constructed and used to conduct a theoretical analysis of the jetting process. This theoretical analysis clearly separated the critical experimental parameters from the other parameters. The critical parameters were found to be the pulse length (t) and voltage value (U) of the electrical pulse signal and the temperature (T) of the chamber. With the commercial tin-lead alloy Sn63Pb37 as the raw material, the experimental apparatus was used to produce micro-droplets. Fifty micro-droplets were randomly selected, their diameters were measured, and the average value and corresponding standard deviations were calculated. The results showed that the micro-droplets fabricated under the same experimental conditions displayed only a small standard deviation in diameter. The diameter of the micro-droplets increased with the pulse length (t) and voltage value (U) of the electrical pulse signal and with the temperature (T) of the chamber. The results also indicated that the values of these three critical experimental parameters should be within an appropriate range. Larger values resulted in the production of satellite droplets, and smaller values did not permit the steady micro-droplets. As a demonstration, a micro-droplet array was fabricated. The diameters of all micro-droplets in the array were approximately 85 μm. The results indicate that the proposed membrane-piston-based jetting technology can produce micro-droplets on demand, and serve as an alternative method for the generation of uniform micro-droplets.

The development of polycaprolactone (PCL) microcarriers with an emulsification module by ultrasonic spraying

The biodegradable microcarriers of polycaprolactone (PCL) have been broadly used in the embolism for treating localized tumor. Size controllability is essential in producing such microcarriers. There is difficulty in obtaining uniform particle size or lack of production efficiency with the conventional processes. In this work an ultrasonic spraying module, which is prevalent in dispensing uniform microdroplets in the coating applications, was incorporated in diffusing the PCL solution into microdroplets of fine and uniform size during the emulsion-solvent evaporation process. By carefully adjusting the production parameters, a median particle size of 50 μm with diameters ranging from 10 to 100 μm was achieved. The primary production parameters included the power supply of the spraying module, the concentration and feeding rate of the PCL solution, and the concentration and the stirring speed of the polyvinyl alcohol solution. A trial production of doxorubicin-loaded PCL microcarriers, used in breast cancer curing, was also carried out satisfactorily. The produced particle size ranged from 10 to 150 μm.

Novel carboxymethyl chitosan-graphene oxide hybrid particles for drug delivery.

We describe an electrostatic droplet generation method to prepare a novel carboxymethyl chitosan-graphene oxide hybrid particles for delivery purpose. Under an adjustable electrostatic field, graphene oxide and carboxymethyl chitosan mixed solution was sprayed as uniform micro-droplets, which were solidified as particles in CaCl2 solution. Such hybrid particles are wished to have excellent stability in saline solution, and better delivery properties than pristine carboxymethyl chitosan particles. The effects of micro-droplets generation conditions on particles formation were systematically investigated. At conditions of 40 mg/ml of carboxymethyl chitosan, 2 mg/ml of graphene oxide, 3 ml/h of feed speed, electrostatic field parameters was 9 kV and 20 cm, uniformly sized carboxymethyl chitosan-graphene oxide particles in the diameter range of 250-300 μm was successfully prepared. In NaCl saline, these particles could maintain stable for at least a week, while pristine carboxymethyl chitosan particles quickly collapsed within an hour. The results of loading experiments showed that carboxymethyl chitosan-graphene oxide particles could effectively adsorb gatifloxacin, ofloxacin, bovine serum albumin, lysozyme, doxorubicin hydrochloride. And gatifloxacin was chosen as a model drug to study the exact effect of graphene oxide content on the loading and release properties. In 40:2 group, the highest loading capacity of 0.45 ± 0.19 mg/mg was achieved, and also a good sustained release was available. Above all, we believed that carboxymethyl chitosan-graphene oxide particles as a versatile carrier, has great potential in Medicine and Pharmacy.

Cell-induced flow-focusing instability in gelatin methacrylate microdroplet generation.

Photo-crosslinkable gelatin methacrylate (GelMa) microspheres are applicable to deliver cells or drugs in biological or biomedical applications. To fabricate GelMa microdroplets, a flow focusing technique with advantages of size control and rapid production was used in a T-junction microfluidic device. Instability played an important role in promoting microdroplet uniformity. 5 wt. % GelMa prepolymer solution mixed with cells affected cell-induced instability. At low flow rate ratio of GelMa to mineral oil below 0.200, stability was maintained regardless of GelMa concentration (5 and 8 wt. %) and cell presence, which led to uniform microdroplet generation. In contrast, instability at high flow rate ratio above 0.200 was worsened by cell presence and unstable jetting length, resulting in the generation of non-uniform cell-laden microdroplets. Therefore, the effect of cell-induced instability on microdroplet generation was minimized at a low flow rate ratio.

Numerical Simulation of Jetting Instability in Flow Focusing Microfluidics

In this paper, jetting behavior of two immiscible liquids, water as the outer liquid and silicone oil as the inner liquid in typical flow focusing microchannels were numerically studied using VOF method. At low capillary number, uniform microdroplets were obtained by the absolute instability. With the increasing of fluid flow ratio, the jet is thinner and tends to break up further away the cross junction. The results showed that the flow rate ratio is the main factor that influences the microdroplet sizes, while the frequency of microdroplets formation can be controlled mainly by the surface tension when it is in the jetting regime.

Integrated lipase-catalyzed isoamyl acetate synthesis in a miniaturized system with enzyme and ionic liquid recycle

Abstract Isoamyl acetate synthesis employing aqueous Candida antarctica lipase B (CaLB) solution was performed in a two-phase solvent system comprising hydrophilic ionic liquid 1-butyl-3-methylpyridinium dicyanamide and n-heptane. An X-junction glass microfluidic chip was used to obtain uniform microdroplets of n-heptane within a continuous phase of ionic liquid with dissolved enzyme and reactants, namely, isoamyl alcohol and acetic anhydride. A developed flow pattern resulted in a very large specific interfacial area for the reaction with amphiphilic CaLB and simultaneous extraction of isoamyl acetate in n-heptane. Biotransformation was performed within a continuously operated microfluidic system consisting of an X-junction chip and a silanized tube of a submillimeter diameter, which was further integrated with a microseparator based on a hydrophobic membrane. Efficient separation of n-heptane with product from ionic liquid phase containing enzyme and remaining substrates and products was achieved, enabling reuse of CaLB together with ionic liquid phase. More than 80% of productivity was preserved in each of the eight consecutive biotransformations within the integrated microfluidic system with reused lipase B in the ionic liquid phase. Subsequent ionic liquid regeneration accomplished by vacuum distillation enabled efficient reuse of this solvent in esterification with fresh enzyme.

Liquids microprinting through a novel film-free femtosecond laser based technique

Laser-induced forward transfer (LIFT) is a high resolution microprinting technique in which small amounts of material are transferred from a previously prepared donor thin film to a receptor substrate. The application of LIFT to liquid donor films allows depositing complex and fragile materials in solution or suspension without compromising the integrity of the deposited material. However, the main drawback of LIFT is the preparation of the donor material in thin film form, being difficult to obtain reproducible thin films with thickness uniformity and good stability. In this work we present a laser microprinting technique that is able to overcome the drawbacks associated with the preparation of the liquid film, allowing the deposition of well-defined uniform microdroplets with high reproducibility and resolution. The droplet transfer mechanism relies on the highly localized absorption of strongly focused femtosecond laser pulses underneath the free surface of the liquid contained in a reservoir. An analysis of the influence of laser pulse energy on the morphology of the printed droplets is carried out, revealing a clear correlation between the printed droplet dimensions and the laser pulse energy. Such correlation is interpreted in terms of the dynamics of the liquid displaced by a laser-generated cavitation bubble close to the free surface of the liquid. Finally, the feasibility of the technique for the production of miniaturized biosensors is tested.

Novel laser printing technique for miniaturized biosensors preparation

A new laser backward transfer technique for printing transparent liquids which avoids the drawbacks associated with the preparation of the liquids in thin film form has been developed. The principle of operation consists in strongly focusing a short laser pulse underneath the free surface of the liquid. Subsurface absorption of the laser radiation results in the generation of a bubble and the propulsion of liquid away the surface, leading to material deposition on a substrate facing that surface. The substrate, being traversed by the laser beam, must be transparent to the laser radiation. It has been demonstrated that there exists a focusing depth range which allows the deposition of well-defined, uniform microdroplets on the substrate. In addition, it has been shown that very high degrees of reproducibility and resolution can be achieved. Finally, the preparation of microarrays of different biomolecules (proteins and DNA) has revealed that the technique is feasible for miniaturized biosensors production.

Microfluidic controlling monodisperse microdroplet for 5-fluorouracil loaded genipin-gelatin microcapsules

This paper demonstrates a proof-of-concept approach for producing genipin-gelatin microcapsules of precisely controlled and monodisperse size distributions by the microfluidic channels. We have demonstrated that one could control the size of emulsions from 130 μm to 580 μm in diameter (with a variation of less than 5%) by altering the relative sheath/sample flow rate ratio. In addition, Results show that the encapsulation and in vitro release of a model drug, 5-fluorouracil, to enhance the effect of controlled release. We demonstrated that the appropriate particle size for different release patterns is predictable, enabling better application of genipin-gelatin microcapsules as a drug carrier. The proposed microfluidic chip is capable of generating relatively uniform micro-droplets with well controllable diameter, and it has the added advantages of being a simple, low cost, and high throughput process.

Microfluidic emulsification and sorting assisted preparation of monodisperse chitosan microparticles

A microfluidic device for generating monodisperse chitosan microparticles and separating the desired particle from smaller particles created as a byproduct of this process was described. The purpose of this study is to separate the satellite droplets from their parent droplets to enhance the size uniformity of the desired microparticles. A double T-junction design was first employed to control the emulsification and the separation, respectively. The results show that the size and gap of the parent droplets are tunable by adjusting the water and oil flow rates. A separation ratio of the satellite droplets of more than 99% was observed. The proposed microfluidic chip is capable of generating relatively uniform micro-droplets with well controllable diameter, and it has the added advantages of being a simple, low cost, and high throughput process. In the future this apparatus can be used to fabricate size-controlled monodisperse microparticles to act as drug carriers for biotechnology and biomedicine applications.