Stable Droplet Formation Research Articles

Inkjet printing of pixel arrays: droplet formation and pattern uniformity of a non-aqueous ink with tunable viscosity

Inkjet printing is a rapid and material-efficient process that is suitable for the fabrication of large-area microarrays from a range of optoelectronic materials. In order to ensure stable droplet formation and a uniform print image with very smooth surfaces, however, the ink properties such as viscosity and surface tension have to be precisely adjusted. In this study, a non-aqueous ink formulation is proposed whose viscosity can be conveniently adjusted by controlling the mixing ratio of propylene carbonate (PC) as the low-viscosity solvent and propylene glycol (PG) as the high-viscosity solvent. Using a combination of advanced imaging techniques, we show that raising the PG content from 20% to 80% increased the viscosity of the ink from 3.36 cP to 26.70 cP, resulting in stable droplet formation and a more evenly printed image. At a spacing of 5 dots/pixel, the roughness value decreased dramatically, from root mean square (RMS): 11.28 (20% PG) to RMS: 0.09 (80% PG). Alternatively, more homogeneous patterns (albeit with a rough surface) were also produced with the low-viscosity ink (20% PG) when a conditioned substrate with low surface energy and selective liquid repellency was used. With this we present a simple but effective strategy to improve droplet formation while obtaining highly uniform pixel arrays. The knowledge gained will be particularly useful for inkjet printing of pixel-patterned color conversion layers in devices such as organic light-emitting diodes (LEDs) and micro-LED displays.

Experimental study of the stable droplet formation process during micro-valve-based three-dimensional bioprinting

Three-dimensional (3D) bioprinting offers great potential for the fabrication of complex 3D cell-laden constructs for clinical and research applications. The droplet formation process is the important first step in droplet-based 3D bioprinting, affecting the positional accuracy and printing fidelity. In this paper, the drop ejection behavior, thresholds for stable droplet generation, and formation of satellite drops are studied, under various ink properties, printing conditions, and input cell concentrations using a micro-valve-based 3D bioprinter. Three droplet ejection behaviors are identified under different conditions: an isolated stable droplet, satellites coalescing into a single droplet, and the presence of one/multiple satellites. The droplet state is represented by a phase diagram bounded by a dimensionless Z number (the inverse of the Ohnesorge number) and a jet Weber number, Wej, to define the printability of the utilized bioprinter. The printability range is defined as 2 < Z < 15 and 10 < Wej < 25 by considering characteristics, such as stable single droplet formability and sufficient drop falling velocity. There is no fatal damage on cells within this printability range. The results show there is no strong influence of an actuation system on droplet-based bioprinting printability. As the input cell concentration increases, the bioink's density and viscosity increases, and surface tension decreases, which, in turn, causes the Z number to slightly decrease. The change in the cell concentration (from 0 to 1×107 cells/ml), within a Newtonian bioink, has negligible impact on the droplet volume, falling velocity, drop ejection behavior, breakup time, and ligament length in microvalve-based bioprinting.

Design insights for upscaling spontaneous microfluidic emulsification devices based on behavior of the Upscaled Partitioned EDGE device

Microfluidic emulsification has the potential to produce emulsions with very controlled droplet sizes in a subtle manner. To support in unleashing this potential, we provide guidelines regarding upscaling based on the performance of Upscale Partitioned EDGE (UPE) devices, using rapeseed oil as the to-be-dispersed phase and whey proteins as the emulsifier. The UPE5x1 device (11,000 droplet formation units (DFUs) of 5 × 1 µm) produced 3.5-µm droplets (CV 3.2 %) at 0.3 mL/h; UPE10x2 (8,000 DFUs of 10 × 2 µm) produced 7-µm droplets (CV 3.2 %) at 0.5 mL/h, and at higher pressures, 32-µm droplets (CV 3–4 %) at 4 mL/h. These productivities are relatively high compared to those of other devices reported in literature (e.g., Microchannel, Tsukuba and Millipede, Harvard).Based on these results, and on others from literature, we conclude that: (1) the continuous phase channel dimensions need to be chosen such that they allow for gradual filling of this channel with droplets without decreasing the pressure over the droplet formation units significantly; (2) the dispersed phase supply channel design should create a wide stable droplet formation pressure range to increase productivity; and (3) higher productivities can be obtained through the choice of the ingredients used; low viscosity dispersed phase and an emulsifier that increases the interfacial tension without negatively affecting device wettability is preferred (e.g., whey protein outperforms Tween 20). These results and design guidelines are expected to contribute to the first food emulsion products prepared with microfluidics.

Asymptotic dynamics of certain solutions for an extended nonlinear Gross–Pitaevskii equation with critical nonlinear damping in [formula omitted

Recent experiments have revealed the formation of stable droplets in a dipolar Bose–Einstein condensate (BEC). This surprising result has been explained experimentally by the stabilization provided by the three-body loss process that appears in the form of a critical damping term in an extended Gross–Pitaevskii equation (eGPE) to model the formation of these dipolar quantum droplets. This paper examines the dynamics of solutions to this equation (eGPE). We validate this prediction by proving that nonlinear damping prevents collapse and ensures the existence of global-in-time solutions. The asymptotic dynamics of these solutions are discussed as well, especially when the system is free (without potential). We show that all global solutions asymptotically behave as free waves in time.

Study on the dynamic characteristics of stable formation of single droplet in gas-liquid co-flow device

The dynamic characteristics of the stable formation of a single droplet in a gas-liquid co-flow device are studied experimentally. The scaling laws of the dimensionless necking radius R of a droplet with dimensionless remaining time τ for different parameters are analyzed, and the reasons for the transformation between these scaling laws are discussed. The results show that the change in the main pressure has little effect on the scaling law. In the earlier necking, R follows a 1/5 power law with respect to τ. In the later collapse, a 2/3 power law is matched. When the auxiliary pressure changes, the pneumatic shear force has a large impact on the earlier necking, causing the scaling law to change, but only a slight effect on the later collapse. Simultaneously, the droplet size and jet limit length decrease significantly with an increase in auxiliary pressure. The results are of great significance for revealing the mechanism of droplet formation and reducing droplet formation size in gas-liquid co-flow devices and provide a theoretical reference for further study of stable droplet formation.

Justification of the parameters and operating modes of the disinfecting system

Increasing yields is an important task for our country. This is possible due to removing the tip of the shoot, which stimulates the potential of the plant itself, and such products can be sold as organic since no other chemicals are used to increase yields. When the shoot tip is removed, a wound is formed through which pathogens can enter. To reduce the likelihood of plant contamination, the device provides a system for applying a disinfectant solution (hydrogen peroxide) to the knife blade. The settings of the injector are taken into account. When choosing a spray gun, it is necessary to take into account the average working pressure, equal to P = 0.28 MPa, which provides a solution flow rate щa Q = 0.8 l • min-1 with a nozzle diameter of 0.1 mm. These requirements can be met by spray guns of caliber 04 or 05. At a spray angle of 800, the recommended spray angle is 300; the mounting distance is 0.3 m below the plane of rotation of the cut-off wheel. The initial flow rate of the liquid of 17.33 m • s-1 will provide stable droplet formation, providing the recommended dispersion of the solution of 150-200 microns.

Improved inkjet printability of dye-based inks through enhancing the interaction of dye molecules and polymer nanospheres

Inkjet printability of dye-based inks including stability, jetting morphology, and color performance is difficult to maintain due to the strong dye molecular interaction. Herein, carboxyl-rich polymer nanospheres were introduced to the dye-based inks, forming a novel kind of multilayer dye/polymer nanospheres to enhance the inkjet printability. Disperse dyes were firstly loaded into poly (styrene-butyl acrylate-methacrylate) nanospheres at 95 °C, and then encapsulated by poly (styrene-methacrylate) polymer layer through seed polymerization to form multilayer structure. The results showed that hydrogen bonds were formed between disperse dyes and nanospheres, and the interaction was enhanced when more carboxyl groups were polymerized on nanosphere surface. Moreover, due to the high surface carboxyl group content, the stability was significantly improved in comparison with disperse dye suspension. After inkjet printing, the dye/nanosphere dispersion showed more stable droplet formation and higher color depth. Besides, because of the self-curing ability of polymer nanospheres, satisfactory fastness was obtained for practical application. The results from this study can promote the optimization of dye-based ink formulations, and show great potential in obtaining high quality inkjet printing products.

Effects of nozzle and fluid properties on the drop formation dynamics in a drop-on-demand inkjet printing

The droplet formation dynamics of a Newtonian liquid in a drop-on-demand (DOD) inkjet process is numerically investigated by using a volume-of-fluid (VOF) method. We focus on the nozzle geometry, wettability of the interior surface, and the fluid properties to achieve the stable droplet formation with higher velocity. It is found that a nozzle with contracting angle of 45° generates the most stable and fastest single droplet, which is beneficial for the enhanced printing quality and high-throughput printing rate. For this nozzle with the optimal geometry, we systematically change the wettability of the interior surface, i.e., different contact angles. As the contact angle increases, pinch-off time increases and the droplet speed reduces. Finally, fluids with different properties are investigated to identify the printability range.

Reactive inkjet printing of polyethylene glycol and isocyanate based inks to create porous polyurethane structures

ABSTRACTReactive inkjet printing offers a direct way to create polymeric structures in situ on a substrate. Therefore, two component polyurethane formulations can be utilized to be used in multicomponent inkjet printing. In this contribution, the use of polyethylene glycol (M = 200 g mol−1), glycerol ethoxylate (M = 1,000 g mol−1), and water (blowing agent) in combination with aliphatic 1,6‐hexamethylene diisocyanate or aromatic methylene diphenyl diisocyanate for reactive inkjet printing is evaluated. The inks are jettable on a Dimatix DMP‐3000 inkjet printer using a 10 pL piezo driven drop‐on‐demand printhead showing stable droplet formation. Solid films on glass are formed using a drop‐by‐drop printing strategy. Layer‐by‐Layer strategy gives best results on polycarbonate substrates forming porous polyurethane structures. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46977.

Hybrid Analytical–Monte Carlo Model of In/GaAs(001) Droplet Epitaxy: Theory and Experiment

We study the droplet epitaxy of indium on the As‐stabilized GaAs(001) substrate using the analytical theory of nucleation combined with kinetic Monte Carlo simulations. The developed model allows considering the atomistic processes of nucleation and growth of droplets without strict binding to the zinc blende structure. We assume that the formation of stable droplets results from the alternation of the processes of assembly and disassembly of subcritical islands. We use a concept of nonmonotonic dynamics of critical size and supersaturation to explain the physical processes on the surface. The thickness of the indium wetting layer exceeds a value of one monolayer and increases gradually with decreasing temperature, which is due to the long‐range interaction of indium with the GaAs substrate. The simulation results are in good agreement with the experiments in a wide range of growth temperatures.

Zein self-assembly using the built-in ultrasonic dialysis process: microphase behavior and the effect of dialysate properties

The microphase behavior at different time scales was investigated to understand the mechanism of zein self-assembly during the built-in ultrasonic dialysis process (BUDP) in this study. The results indicated that the mutual mass transfer between zein solution and dialysate changed the solvent polarity, which caused the conformational transition of zein molecules, formation of stable droplets under the ultrasonic field, and further solidification of the compact microspheres. The effects of dialysate properties on zein self-assembly were investigated. The results showed that both pH and ionic strength had a great effect on the size, zeta potential, and morphology of the obtained zein particles, where zein microspheres with a size around 1–2 μm and zeta potential > 30 mV were obtained at pH 4–pH 8 and NaCl concentration ≤ 10 mM. And non-ionic Tween 60 could be used as a potential surfactant to improve the stability of zein samples through a physical combination.

Rapid Acquisition of X-Ray Scattering Data from Droplet-Encapsulated Protein Systems.

Encapsulating reacting biological or chemical samples in microfluidic droplets has the great advantage over single-phase flows of providing separate reaction compartments. These compartments can be filled in a combinatoric way and prevent the sample from adsorbing to the channel walls. In recent years, small-angle X-ray scattering (SAXS) in combination with microfluidics has evolved as a nanoscale method of such systems. Here, we approach two major challenges associated with combining droplet microfluidics and SAXS. First, we present a simple, versatile, and reliable device, which is both suitable for stable droplet formation and compatible with in situ X-ray measurements. Second, we solve the problem of "diluting" the sample signal by the signal from the oil separating the emulsion droplets by multiple fast acquisitions per droplet and data thresholding. We show that using our method, even the weakly scattering protein vimentin provides high signal-to-noise ratio data.

Ground-state properties and elementary excitations of quantum droplets in dipolar Bose-Einstein condensates

Recent experiments have revealed the formation of stable droplets in dipolar Bose-Einstein condensates. This surprising result has been explained by the stabilization given by quantum fluctuations. We study in detail the properties of a BEC in the presence of quantum stabilization. The ground-state phase diagram presents three main regimes: mean-field regime, in which the quantum correction is perturbative, droplet regime, in which quantum stabilization is crucial, and a multi-stable regime. In the absence of a multi-stable region, the condensate undergoes a crossover from the mean-field to the droplet solution marked by a characteristic growth of the peak density that may be employed to clearly distinguish quantum stabilization from other stabilization mechanisms. Interestingly quantum stabilization allows for three-dimensionally self-bound condensates. We characterized these self-bound solutions, and discuss their realization in experiments. We conclude with a discussion of the lowest-lying excitations both for trapped condensates, and for self-bound solutions.

Systematic design of jettable nanoparticle-based inkjet inks: rheology, acoustics, and jettability.

Drop-on-demand inkjet printing of functional inks has received a great deal of attention for realizing printed electronics, rapidly prototyped structures, and large-area systems. Although this method of printing promises high processing speeds and minimal substrate contamination, the performance of this process is often limited by the rheological parameters of the ink itself. Effective ink design must address a myriad of issues, including suppression of the coffee-ring effect, proper drop pinning on the substrate, long-term ink reliability, and, most importantly, stable droplet formation, or jettability. In this work, by simultaneously considering optimal jetting conditions and ink rheology, we develop and experimentally validate a jettability window within the capillary number-Weber number space. Furthermore, we demonstrate the exploitation of this window to adjust nanoparticle-based ink rheology predictively to realize a jettable ink. Finally, we investigate the influence of mass loading on jettability to establish additional practical limitations on nanoparticle ink design.

Influence of electrolyte concentration on microchannel oil-in-water emulsification using differently charged surfactants

The purpose of this study was to investigate the formation characteristics of oil-in-water (O/W) emulsion droplets in the presence of electrolyte by microchannel (MC) emulsification using differently charged surfactants. Droplet formation was conducted by pressurizing a dispersed phase (refined soybean oil) via 23,348 asymmetric through-holes micro fabricated on a 24mm×24-mm silicon chip into a continuous phase in the presence of a surfactant (1.0wt%) and an electrolyte (NaCl) (0–1.0mol/L). Monodisperse O/W emulsions with an average droplet diameter (dav,drop) of 26μm and a coefficient of variation (CV) below 5% were produced from the asymmetric through-holes when the NaCl concentration was lower than a threshold level that is 0.3mol/L for sodium dodecyl sulfate (SDS) and 0.5mol/L for polyoxyethylene (20) sorbitan monolaurate (Tween 20). Further increase in the NaCl concentration resulted in increase of the dav,drop and CV of the resultant O/W emulsions, which is due to unstable and unsmooth motion of the oil–water interface during droplet formation. In the presence of SDS, it was found that interfacial tension must be higher than a threshold level (c.a. 0.9mN/m) in order to achieve stable droplet formation by MC emulsification. The ζ-potential on the droplet and chip surfaces was negative, and their absolute value decreased with increasing the NaCl concentration. The ζ-potential results also indicated that repulsive electrostatic interaction between the droplet and chip surfaces in the presence of NaCl is high for SDS and negligible for Tween 20. The difference in their electrostatic interaction is assumed to affect the smoothness of droplet formation and the behavior of the droplets formed.

Lipid Droplet De Novo Formation and Fission Are Linked to the Cell Cycle in Fission Yeast

Cells sequester neutral lipids in bodies called lipid droplets. Thus, the formation and breakdown of the droplets are important for cellular metabolism; unfortunately, these processes are difficult to quantify. Here, we used time-lapse confocal microscopy to track the formation, movement and size changes of lipid droplets throughout the cell cycle in fission yeast Schizosaccharomyces pombe. In theory, the number of lipid droplets in these cells must increase for daughter cells to have the same number of droplets as the parent at a reference point in the cell cycle. We observed stable droplet formation events in G2 phase that were divided evenly between de novo formation of nascent droplets and fission of preexisting droplets. The observations that lipid droplet number is linked to the cell cycle and that droplets can form via fission were both new discoveries. Thus, we scrutinized each fission event for multiple signatures to eliminate possible artifacts from our microscopy. We augmented our time-lapse confocal microscopy with electron microscopy, which showed lipid droplet 'intermediates': droplets shaped like dumbbells that are potentially in transition states between two spherical droplets. Using these complementary microscopy techniques and also dynamic simulations, we show that lipid droplets can form by fission.

Liquid–liquid two phase flow of millichannel with a dynamic mixer

A novel millimeter-sized reactor, named millireactor in the present study, is proposed for oil droplet formation aiming at heterogeneous enzymatic hydrolysis. The reactor consists of a circular ditch with a small stirring bar and a straight channel. To obtain optimal operation conditions and a reactor configuration for droplet formation, the stable droplet formation region was explored by mapping analysis, and the relationship between the specific interfacial area of droplets and other physical parameters were investigated by dimension analysis. It was found that the shear stress parameter and interfacial tension played important roles in the droplet formation. Considering parameters such as the length of the stirring bar, rotating speed, diameter of the stirring section, gap width, interfacial tension, viscosity and residence time in the stirring section, a correlation equation including three dimensionless numbers was obtained. The optimal design guide for obtaining high specific area of oil droplets involves increasing the volume of the stirring section and reducing the gap width as narrow as possible. The reactor performance was evaluated comparing the amount of fatty acid produced in the millireactor versus the batch reactor. The productivity of the millireactor was higher than the batch reactor for the first 10 min.

Effect of channel structure on preparation of a water‐in‐oil emulsion by polymer microchannels

AbstractMonodisperse water‐in‐oil emulsions were successfully prepared using a polymer microchannel (PMC) with hydrophobic properties and high anti‐alkalinity and fabricated by the molding injection method. We investigated the effect of the channel structure on the droplet preparation characteristics, including the average droplet diameter, the coefficient of variation (CV), the pressure range for stable droplet formation, and the critical flow velocity of the dispersed phase. Various PMC with different structures were fabricated and used for emulsification. The results indicated that the average droplet diameter and the CV did not change remarkably and that the critical flow velocity of the dispersed phase was increased slightly when we used PMC with different small rectangular‐shaped convexes in the channel. However, the pressure range for stable droplet formation widened, the CV decreased, and the critical flow velocity of the dispersed phase increased significantly with a decrease of the PMC equivalent diameter and an increase of the PMC length, which revealed greater stability and productivity of the emulsion droplets.

Production of monodispersed water‐in‐oil emulsions using polymer microchannels

AbstractMonodispersed water‐in‐oil (W/O) emulsions were prepared using new fabricated acrylic polymer microchannel (MC) plates. Acrylic plate MC‐A1 was 20 μm wide, 10 μm deep, and 120 μm long; and MC‐A2 was 40 μm wide, 20 μm deep, and 120 μm long with respect to MC sizes. Both MC plates had a terrace length of 30 μm. Monodispersed waterin‐triolein emulsions with average diameters of 42 to 60 μm (MC‐A1) and 62 to 98 μm (MC‐A2) were prepared successfully using tetraglycerin condensed ricinoleic acid ester (CR310) as a surfactant. The CV for droplets produced was less than 10%, demonstrating monodispersity. When we used decane (which is less viscous than triolein) as the continuous phase and CR310 and hexaglycerin pentaoleate (PO500) as surfactants, the average diameters and CV of the droplets became smaller, and the pressure ranges for stable droplet formation were much wider than for the water/triolein system. For comparison, a silicon MC plate (MC‐Si) with dimensions similar to the polymer MC‐A1 was tested and exhibited little significant difference in the average diameter and CV of droplets between MC‐A1 and MC‐Si.

Effect of interfacial tension on the dynamic behavior of droplet formation during microchannel emulsification

Microchannel (MC) emulsification is a novel technique for producing monodisperse emulsions. In this study, we investigated the effect of interfacial tension on the dynamic behavior of droplet formation with various surfactant concentrations. Interfacial tension did not affect the resultant droplet diameter in lower flow velocity ranges, but it did affect the time-scale parameters. These results were interpreted using the droplet formation mechanism reported in our previous study. At surfactant concentrations below 0.3%, the emulsification behavior was differed from that at higher surfactant concentrations. An analysis of diffusional transfer indicated that dynamic interfacial tension affects the emulsification behavior at lower surfactant concentrations. Dynamic interfacial tension that exceeded the equilibrium value led to a shorter detachment time. This resulted in stable droplet formation of monodispersed emulsions by spontaneous transformation, even at flow velocities above the predicted critical flow velocity. A previous study predicted that the droplet formation would become unstable and polydispersed larger droplets would form over critical flow velocity. Wetting of the MC with the dispersed phase at lower surfactant concentrations induced formation of larger polydispersed droplets at high flow velocities.