Microdroplet Formation Research Articles

Correlation of the kinetics of aggregation and inactivation of L-glutamate dehyrogenase during drying and particle formation of a levitated microdroplet

ABSTRACTMeasurement of single droplet drying in the acoustic levitator has been used to detect a correlation between the kinetics of aggregation and inactivation of the model protein l-glutamate dehydrogenase (GDH) during drying and particle formation. The droplet drying was performed at one set of conditions: drying gas temperature = 60°C; drying gas relative humidity = 1%, and drying gas velocity = 0.5 L/min. A centrifugation technique was used to remove insoluble aggregates from the rehydrated microparticles removed from the levitator chamber. Size exclusion chromatography could then be used to quantify the contents of nonaggregated native protein. During the constant-rate drying period up to the critical point of drying, there was retention of nonaggregated native structure. Only after the critical point of drying has been reached, a rapid decline in nonaggregated native content was observed owing to formation of insoluble aggregates. No sign of a soluble trimer of GDH was found. The kinetics of this behavior correlates closely with the kinetics of inactivation of the same enzyme in levitated microdroplets from the literature, i.e., retention of activity up to the critical point and rapid loss of activity after the critical point. The novelty of this work is that such a correlation between aggregation and inactivation can be detected during drying and particle formation on both sides of the critical point of drying.

Formation of Uniform Water Microdroplets on Wrinkled Graphene for Ultrafast Humidity Sensing.

Portable humidity sensors with ultrafast responses fabricated in wearable devices have promising application prospects in disease diagnostics, health status monitoring, and personal healthcare data collecting. However, prolonged exposures to high-humidity environments usually cause device degradation or failure due to excessive water adsorbed on the sensor surface. In the present work, a graphene film based humidity sensor with a hydrophobic surface and uniformly distributed ring-like wrinkles is designed and fabricated that exhibits excellent performance in breath sensing. The wrinkled morphology of the graphene sensor is able to effectively prevent the aggregation of water microdroplets and thus maximize the evaporation rate. The as-fabricated sensor responds to and recovers from humidity in 12.5 ms, the fastest response of humidity sensors reported so far, yet in a very stable manner. The sensor is fabricated into a mask and successfully applied to monitoring sudden changes in respiratory rate and depth, such as breathing disorder or arrest, as well as subtle changes in humidity level caused by talking, cough and skin evaporation. The sensor can potentially enable long-term daily monitoring of breath and skin evaporation with its ultrafast response and high sensitivity, as well as excellent stability in high-humidity environments.

Formation of microdroplets in Newtonian and shear thinning fluids flowing in coaxial capillaries: Numerical modeling

The peculiarities of the Newtonian droplets formation in a Newtonian or non-Newtonian fluid from coaxial capillaries are studied by means of numerical modeling. As a non-Newtonian medium, a shear thinning fluid is considered whose viscosity decreases with the increase of the shear rate according to the Carreau–Yasuda rheological model. It is shown that in the case of a Newtonian continuous fluid, the calculated sizes of the droplets decrease with an increase of the ratio of the flow rates in the outer and inner capillaries which is in a good agreement with experimental data. On the other hand, for the shear thinning continuous medium, droplet sizes become significantly larger than those for the Newtonian fluid. Besides, in the latter case, the droplet sizes weakly depend on the ratio of flow rates of the continuous and dispersed fluids.

Towards microprocessor-based control of droplet parameters for endoscopic laryngeal adductor reflex triggering

Abstract The so-called Laryngeal Adductor Reflex (LAR) protects the respiratory tract from particle intrusion by quickly approximating the vocal folds to close the free glottal space. An impaired LAR may be associated with an increased risk of aspiration and other adverse conditions. To evaluate the integrity of the LAR, we recently developed an endoscopic prototype for LAR triggering by shooting accelerated droplets onto a predefined laryngeal target region. We now modified the existing droplet-dispensing system to adapt the fluid system pressure as well as the valve opening time to user-chosen values autonomously. This has been accomplished using a microcontroller board connected to a pressure sensor and a mechatronic syringe pump. For performance validation, we designed a measurement setup capable of tracking the droplet along a vertical trajectory. In addition to the experimental setup, the influence of parameters such as system pressure and valve opening time on the micro-droplet formation is presented. Further development will enable the physician to adjust the droplet momentum by setting a single input value on the microcontroller-based setup, thus further increasing usability of the diagnostic device.

Three-dimensional simulation of microdroplet formation in a co-flowing immiscible fluid system using front tracking method

Liquid-jet breakup in a co-flow system can occur in two regimes: dripping and jetting. A finite difference/front-tracking method is used to simulate the jet breakup. This method has rarely been used in cases where breakup or coalescence occurs during flow simulation. This is due to the lack of a general algorithm for topology change of the interface mesh. In this paper, three-dimensional liquid-jet breakup has been studied in dripping and jetting regimes. A new algorithm has been developed that is capable of changing the topology of the interface mesh. The dripping and jetting regimes are simulated numerically and a new transition criterion between two regimes is defined that is in agreement with existing studies. Also, the drop size formed after breakup in jetting regime is consistent with experimental data. The effect of the capillary number, the Weber number, the viscosity ratio and the density ratio on the size of drops formed is investigated. It is found that the drop size decreases when the Weber number is raised. The same trend is observed when the capillary number increases. The drops increase in size when the velocity ratio of the co-flowing medium increases (the velocity ratio is the ratio of the velocity of inner fluid to that of the surrounding medium). Increasing the density ratio also increases the drop size. The rate of increase in drop size is large at unity density ratio and it decays as the density ratio increases. Also, increasing the viscosity ratio reduces the drop size. Finally, the breakup of liquid jet exiting from an elliptic nozzle is investigated and its behavior is compared with a circular jet.

Cytocompatible cell encapsulation via hydrogel photopolymerization in microfluidic emulsion droplets

Encapsulating cells within biocompatible materials is a widely pursued and promising element of tissue engineering and cell-based therapies. Recently, extensive interest in microfluidic-enabled cell encapsulation has emerged as a strategy to structure hydrogels and establish custom cellular microenvironments. In particular, it has been shown that the microfluidic-enabled photoencapsulation of cells within PEG diacrylate (PEGDA)-based microparticles can be performed cytocompatibly within gas-permeable, nitrogen-jacketed polydimethylsiloxane microfluidic devices, which mitigate the oxygen inhibition of radical chain growth photopolymerization. Compared to bulk polymerization, in which cells are suspended in a static hydrogel-forming solution during gelation, encapsulating cells via microfluidic processing exposes cells to a host of potentially deleterious stresses such as fluidic shear rate, transient oxygen depletion, elevated pressures, and UV exposure. In this work, we systematically examine the effects of these factors on the viability of cells that have been microfluidically photoencapsulated in PEGDA. It was found that the fluidic shear rate during microdroplet formation did not have a direct effect on cell viability, but the flow rate ratio of oil to aqueous solution would impart harmful effects to cells when a critical threshold was exceeded. The effects of UV exposure time and intensity on cells, however, are more complex, as they contribute unequally to the cumulative rate of peroxy radical generation, which is strongly correlated with cell viability. A reaction-diffusion model has been developed to calculate the cumulative peroxy radical concentration over a range of UV light intensity and radiation times, which was used to gain further quantitative understanding of experimental results. Conclusions drawn from this work provide a comprehensive guide to mitigate the physical and biochemical damage imparted to cells during microfluidic photoencapsulation and expands the potential for this technique.

Single fibre model composite: Interfacial shear strength measurements between reactive polyamide-6 and cellulosic or glass fibres by microdroplet pullout test

This work aims to study the effect of reactive molding on polyamide-6 (PA6) based micro composites with either glass or high tenacity viscose as fibre reinforcements. For this purpose, microdroplet pullout test was found to be well suited to determine the interfacial shear strength. For pull-out testing, samples are prepared with a common method implying melting polyamide-6 and an innovative method which allows the microdroplets formation by direct wetting in the reactive mixture. Conditions to obtain microdroplet in this new method are close to those of real size composite molding. First, this work focus on the verification of the new sample preparation method using 1H and 13C NMR to determine conversion rate and verify the presence of polyamide-6 on wetted fibres bundles. Contact angle are calculated to confirm the NMR analysis conclusions. Both of these analyses confirm that PA6 is effectively formed on fibres when wetting with the reactive mixture. Finally, interfacial shear strength is compared for both glass and high tenacity viscose with these two types of preparation procedures. Obtained values for glass fibres are τ = 25 ± 6 MPa with PA6-melting and τ = 20 ± 3 MPa with reactive mixture wetting. Reactive mixture wetting also allows to avoid high tenacity viscose degradation during PA6 formation, leading to the determination of the interfacial shear strength τ = 12 ± 3 MPa.

Self-assembled levitating clusters of water droplets: pattern-formation and stability

Water forms ordered hexagonally symmetric structures (snow crystals) in its solid state, however not as liquid. Typically, mists and clouds are composed of randomly moving small droplets lacking any ordered structure. Self-organized hexagonally patterned microdroplet clusters over locally heated water surfaces have been recently observed. However, many aspects of the phenomenon are far from being well understood including what determines droplets size, arrangement, and the distance between them. Here we show that the Voronoi entropy of the cluster tends to decrease indicating to their self-organization, while coupling of thermal effects and mechanical forces controls the stability of the clusters. We explain the balance of the long-range attraction and repulsion forces which stabilizes the cluster patterns and established the range of parameters, for which the clusters are stable. The cluster is a dissipative structure similar to self-organized Rayleigh–Bénard convective cells. Microdroplet formation plays a role in a variety effects from mist and clouds to aerosols. We anticipate that the discovery of the droplet cluster phenomenon and its explanation will provide new insights on the fundamental physical and chemical processes such as microdroplet role in reaction catalysis in nature as well as new tools for aerosol analysis and microfluidic applications.

Microdebond test development and interfacial shear strength evaluation of basalt and glass fibre reinforced polypropylene composites

Interfacial adhesion of basalt and glass fibre reinforced polypropylene composites was studied using microdebond testing technique. A focus was put on a simple approach of applying extruded thermoplastic films as a matrix material for microdroplet formation. The ability of different viscosity and thickness polypropylene films to form symmetrical droplets under a temperature range of 200–240℃ was evaluated. Emphasis was put on polypropylene matrix chemistry, silane fibre surface treatment and testing loading rate impact on interfacial performance change in polypropylene-basalt fibre and polypropylene-glass fibre microcomposites. It was found that it was possible to obtain high symmetrical droplet yield out of polypropylene films of melt flow rate 50 and 125 g/10 min and 55–85 µm thickness at 240℃. The presence of maleic anhydride grafted polypropylene coupling agent increased the interfacial shear strength significantly. Microcomposites with glass fibre had higher interfacial shear strength in comparison with the used basalt fibre, mainly due to the difference in their sizing. Various silane-based fibre surface coatings did not result in significant interfacial adhesion changes. Polypropylene-glass fibre microcomposite interfacial shear strength at 0.5, 3.0 and 10.0 mm min–1 loading rates had similar values with high maximum pull-out force scatter at 0.5 and 3.0 mm min–1 loading rates and low scatter at 10.0 mm min–1.

Laser-Induced Single Microdroplet Formation and Simultaneous Water-to-Single Microdroplet Extraction/Detection in Aqueous 1-Butanol Solutions

Abstract Focused 1064-nm laser beam irradiation to an aqueous 1-butanol (BuOH) solution (7.1–7.4 wt % in H2O) resulted in formation of a single picoliter-volume BuOH droplet. Since water (H2O) absorbs 1064-nm laser light, an aqueous BuOH solution at the laser beam focus is heated via photo-thermal effects and this leads to thermal phase separation of the solution, producing a single BuOH microdroplet. In the presence of a fluorescent dye (10−5–10−7 mol/dm3) in an aqueous BuOH solution, the dye was extracted from the surrounding water phase to the BuOH droplet produced by laser irradiation as demonstrated by in situ fluorescence and Raman microspectroscopies. The present laser-induced water-to-single microdroplet extraction/detection was also extended successfully to that under pressure-driven and electroosmotic flow conditions in microflow devices. In both cases, the single BuOH microdroplets produced by 1064-nm laser irradiation were optically trapped against flow of the solution. Under electroosmotic flow conditions, highly sensitive detection of a fluorescent Al3+-chelate complex injected to an electrophoresis capillary tube was also achieved successfully by single BuOH microdroplet formation and simultaneous extraction of the Al3+ chelate to the droplet by 1064-nm laser irradiation.

Droplet Microfluidics for the Production of Microparticles and Nanoparticles

Droplet microfluidics technology is recently a highly interesting platform in material fabrication. Droplets can precisely monitor and control entire material fabrication processes and are superior to conventional bulk techniques. Droplet production is controlled by regulating the channel geometry and flow rates of each fluid. The micro-scale size of droplets results in rapid heat and mass-transfer rates. When used as templates, droplets can be used to develop reproducible and scalable microparticles with tailored sizes, shapes and morphologies, which are difficult to obtain using traditional bulk methods. This technology can revolutionize material processing and application platforms. Generally, microparticle preparation methods involve three steps: (1) the formation of micro-droplets using a microfluidics generator; (2) shaping the droplets in micro-channels; and (3) solidifying the droplets to form microparticles. This review discusses the production of microparticles produced by droplet microfluidics according to their morphological categories, which generally determine their physicochemical properties and applications.

Optical nose based on porous silicon photonic crystal infiltrated with ionic liquids

A photonic-nose for the detection and discrimination of volatile organic compounds (VOCs) was constructed. Each sensing element on the photonic sensor array was formed by infiltrating a specific type of ionic liquid (IL) into the pore channel of a patterned porous silicon (PSi) chip. Upon exposure to VOC, the density of IL dramatically decreased due to the nano-confinement effect. As a result, the IL located in pore channel expanded its volume and protrude out of the pore channel, leading to the formation of microdroplets on the PSi surface. These VOC-stimulated microdroplets could scatter the light reflected from the PSi rugate filter, thereby producing an optical response to VOC. The intensity of the optical response produced by IL/PSi sensor mainly depends on the size and shape of microdroplets, which is related to the concentration of VOC and the physi-chemical propertied of ILs. For ethanol vapor, the optical response has linear relationship with its relative vapor pressure within 0–60%. The LOD of the IL/PSi sensor for ethanol detection is calculated to be 1.3 ppm. It takes around 30 s to reach a full optical response, while the time for recovery is less than 1 min. In addition, the sensor displayed good stability and reproducibility. Owing to the different molecular interaction between IL and VOC, the ILs/PSi sensor array can generate a unique cross-reactive “fingerprint” in response to a specific type of VOC analyte. With the assistance of image technologies and principle components analysis (PCA), rapid discrimination of VOC analyte could be achieved based on the pattern recognition of photonic sensor array. The technology established in this work allows monitoring in-door air pollution in a visualized way.

Modeling on Microdroplet Formation for Cell Printing Based on Alternating Viscous-Inertial Force Jetting

Drop-on-demand (DOD) microdroplet jetting technology has diverse applications ranging from additive manufacturing (AM) and the integrated circuit (IC) industry to cell printing. An engineering model of droplet formation can provide insights for optimizing the process and ensuring its controllability and reproducibility. This paper reports a development of an engineering model on the fluid outflow and microdroplet formation based on alternating viscous-inertial force jetting (AVIFJ). The model provides a fundamental understanding on the mechanism of droplet formation driven by the alternating viscous force and inetial force. Furthermore, the model studies the fluid acceleration, velocity, and displacement under the conditions of a uniform cylindrical nozzle and a nonuniform cylindrical nozzle. In conjunction with an energy-based criterion for droplet formation, the model is applied to predict the formability of single microdroplets and the volume and velocity of formed microdroplets. A series of experiments was conducted to validate the developed model. The results show that the model predictions agree well with the experimental results. Specifically, comparing the model prediction and experimental results, the maximum difference of drop diameter is 4 μm, and the maximum difference of drop velocity is 0.3 m/s. These results suggest that the developed theoretical model will provide guidance to the subsequent cell printing applications.

Chemical Wet Etching of an Optical Fiber Using a Hydrogen Fluoride-Free Solution for a Saturable Absorber Based on the Evanescent Field Interaction

We propose a chemical wet etching technique that does not make use of toxic Hydrofluoric solution to produce cladding-etched optical fiber intended for use as a platform for a fiberized saturable absorber. The wet etching technique is based on the use of a mixed solution of NH4F and (NH4)2SO4, which is not toxic, and a small, specially-devised etching cradle to achieve precise control of the etched fiber length. A fiberized saturable absorber was implemented by depositing the graphene oxide particles on top of the residual cladding for our cladding-etched fiber by using the drop & dry method. The saturable absorber was incorporated into an erbium-doped fiber ring cavity to test its suitability as a mode-locker with a modulation depth larger than 4.3%. Femtosecond pulses with a temporal width of ∼615 fs were readily obtained at a repetition rate of ∼17.09 MHz. The saturable absorption and laser performance of our saturable absorber were compared to those of an absorber on a core-etched fiber platform prepared through the formation of microdroplets of HF and surface tension-driven flow, as in [57] .

A pneumatically driven inkjet printing system for highly viscous microdroplet formation

This paper introduces a pneumatically driven inkjet printing system that forms highly viscous microdroplets in the nanoliter volume range. The printing system has a unique printing mechanism that uses a flexible membrane and an effective backflow stopper. While typical inkjet systems can handle liquids with a limited range of viscosity due to energy loss by viscous dissipation at the nozzle and ineffective backflow management within their systems, our printing system can print liquids with viscosity as high as 384.5 cP. In the viscosity range 1–384.5 cP, we investigated printing characteristics such as printed droplet volume, standoff distance, and maximum possible frequency. The droplet formation showed outstanding reliability, with the droplet volume exhibiting a coefficient of variation less than 1.07 %. Our printing system can be directly used in inkjet applications with functional liquids over a broad viscosity range.

Observation and micro-electrochemical characterisation for micro-droplets in initial marine atmospheric corrosion

Micro-droplets formed around NaCl droplet on carbon steel surface were observed with a confocal laser scanning microscope. Micro-electrochemical characterisation of the micro-droplets zone was also performed by a scanning Kelvin probe (SKP) and the scanning vibrating electrode technique (SVET). Results show that the electric current density and potential distribution of carbon steel under the NaCl droplet are asymmetrical; the peripheral regions of the droplet are cathodic, whilst its centre is anodic. The potential difference between the anode and cathode is 0.36 V, and the cathodic current density reaches 2.02 μA cm−2. This kind of asymmetrical distribution of electrochemical characterisation results in cathodic polarisation at the fringe of the NaCl droplet, thus inducing the formation of OH–, which could promote water adsorption and subsequent formation of micro-droplets around the NaCl droplet. The electrochemical potential difference of the oxygen concentration cell formed between the central and peripheral regions of the NaCl droplet is the main driving force for micro-droplet formation on metal surfaces.

Microdroplet formation in rounded flow-focusing junctions

Herein we report microfluidic droplet formation in flow-focusing geometries possessing varying degrees of rounding. Rounding is incorporated in all four corners (symmetric) or only in the two exit corners (asymmetric). The ratios of the radius of curvature, R, to channel width, w, are varied where R/w = 0, 0.5 and 1. In all cases, monodisperse droplets are produced, with the largest droplets being produced at the junctions with the largest rounding. Junctions without rounding are shown to produce droplets at higher frequencies than those with rounding. Droplet pinch-off position is found to be dependent on both geometry and volumetric flow rates; the location shifts toward the interior of the rounded junctions with increasing oil-to-water flow rate ratios. Accordingly, we find that rounding within microfluidic flow-focusing junctions strongly influences droplet formation. Junction rounding may be deliberate due to the selected fabrication method or occur as an unintended result of microfabrication processes not held to strict tolerances. Indeed, understanding droplet characteristics for those formed in such structures is critical for microfluidic applications where droplet volume or reagent mass must be well controlled. Thus, rounding can be a valuable design parameter when tuning the size and production frequency for emulsion collection or ensuing downstream operations such as chemical reactions.

Shape-tunable wax microparticle synthesis via microfluidics and droplet impact.

Spherical and non-spherical wax microparticles are generated by employing a facile two-step droplet microfluidic process which consists of the formation of molten wax microdroplets in a flow-focusing microchannel and their subsequent off-chip crystallization and deformation via microdroplet impingement on an immiscible liquid interface. Key parameters on the formation of molten wax microdroplets in a microfluidic channel are the viscosity of the molten wax and the interfacial tension between the dispersed and continuous fluids. A cursory phase diagram of wax morphology transition is depicted depending on the Capillary number and the Stefan number during the impact process. A combination of numerical simulation and analytical modeling is carried out to understand the physics underlying the deformation and crystallization process of the molten wax. The deformation of wax microdroplets is dominated by the viscous and thermal effects rather than the gravitational and buoyancy effects. Non-isothermal crystallization kinetics of the wax illustrates the time dependent thermal effects on the droplet deformation and crystallization. The work presented here will benefit those interested in the design and production criteria of soft non-spherical particles (i.e., alginate gels, wax, and polymer particles) with the aid of time and temperature mediated solidification and off-chip crosslinking.

Alternating Force Based Drop-on-Demand Microdroplet Formation and Three-Dimensional Deposition

Drop-on-demand (DOD) microdroplet formation and deposition play an important role in additive manufacturing, particularly in printing of three-dimensional (3D) in vitro biological models for pharmacological and pathological studies, for tissue engineering and regenerative medicine applications, and for building of cell-integrated microfluidic devices. In development of a DOD based microdroplet deposition process for 3D cell printing, the droplet formation, controlled on-demand deposition and at the single-cell level, and most importantly, maintaining the viability and functionality of the cells during and after the printing are all remaining to be challenged. This report presents our recent study on developing a novel DOD based microdroplet deposition process for 3D printing by utilization of an alternating viscous and inertial force jetting (AVIFJ) mechanism. The results include an analysis of droplet formation mechanism, the system configuration, and experimental study of the effects of process parameters on microdroplet formation. Sodium alginate solutions are used for microdroplet formation and deposition. Key process parameters include actuation signal waveforms, nozzle dimensional features, and solution viscosity. Sizes of formed microdroplets are examined by measuring the droplet diameter and velocity. Results show that by utilizing a nozzle at a 45 μm diameter, the size of the formed microdroplets is in the range of 52–72 μm in diameter and 0.4–2.0 m/s in jetting speed, respectively. Reproducibility of the system is also examined and the results show that the deviation of the formed microdroplet diameter and the droplet deposition accuracy is within 6% and 6.2 μm range, respectively. Experimental results demonstrate a high controllability and precision for the developed DOD microdroplet deposition system with a potential for precise cell printing.

Numerical simulation of droplet‐based microfluidic chip interfacing with laser desorption/ionisation mass spectrometry target substrate

Droplet-based microfluidic devices have shown great potential in biomedical applications. When proteomics bioanalysis or biomarker identification is required, off-line microfluidic chip interfacing with mass spectrometry (MS) is necessary. This reported work has focused on the use of numerical simulation of microdroplet formation and dripping for a droplet-based microfluidic-chip interfacing with off-line laser desorption/ionisation MS target substrate. Unlike most studies on the microdroplet formation at the liquid–liquid phase, this work has investigated the microdroplet formation at the liquid–gas phase at various injection flow rates and inner microchannel surface wettability to meet microfluidic-chip–MS interfacing requirements. After the microdroplet formation, subsequent microdroplet dripping analysis was performed. The effects of inner microchannel and outer orifice surface wettability are discussed.