Satellite Droplets Research Articles

Size-Controllable Monodispersed Microsphere Generation by Liquid Chopper Utilizing PZT Actuator

This paper describes a novel microfluidic method to prepare monodispersed water-in-oil (W/O) microspheres utilizing a piezoelectric actuator. In the main channel, a dispersed phase stream becomes focused and stabilized. In the side channels, oscillating streams are generated by the piezoelectric actuator and flow to the main channel to pinch off the focused stream to form a liquid chopper. By changing the frequencies of the liquid chopper, the size of the W/O microspheres can be successfully controlled in the range of 78.55 - 185.63 μm with very good uniformity (lower than 5% of the coefficient of variation). The present method has the advantages over conventional methods such as hydrodynamic flow focusing and T-junction. The proposed chopper method presents good sphericity, no satellite droplets, and size control of the microspheres.

Both Experimental and Numerical Investigation on Breakup Length of Cylindrical Falling Jet

Abstract The variation in breakup length of a liquid jet subjected to different physical parameters is investigated experimentally. The different parameters are jet velocity, Reynolds number and Weber number. Effect of gravity on breakup length is also studied. Jet breakup process is simulated numerically by solving the transient axisymmetric equations of continuity and motion using an algorithm based on the Volume of Fluid (VOF) method that was previously proven successful in simulations of steady-state liquid jets. The experimental results are compared with the numerical simulation and discrepancies are discussed. The experiment shows that the breakup length first decreases with the increase of jet velocity and then increases with much higher velocity. The simulation result shows the expected pattern of the jet breakup process with satellite drop formation and droplet oscillation as can be found in experiment. It also shows the transient variation in the jet breakup process.

Asymmetric Detachment from Angled Nozzles Plates in Drop-on Demand Inkjet Printing

The effects of nozzle plate defects on droplet jetting behavior and directionality are studied numerically and experimentally. In particular, we examine the mechanism responsible for the phenomenon of tail-hooking, where break-up occurs at a point on the side of the nozzle, rather than on the central axis. We investigate how nozzle defects lead to this breaking of jet axisymmetry. Fully three-dimensional simulations through nozzle geometry were performed using Flow3D software are compared with experiments performed on a large scale model of a drop on demand inkjet print-head. For nozzles plates in which the outer surface is not perfectly aligned with the nozzle axis, we observe asymmetric break-off. The directionality of main droplets is not greatly affected by the nozzle angle but the behavior and directionality of the satellite droplets is dramatically altered.

Morphology of water electrosprays in the simple-jet mode

Experiments were conducted in order to study and characterize electrohydrodynamic atomization in the simple-jet mode for inviscid liquids. The operational window of this mode regarding the electric potential and liquid flow rate is presented. From the data it could be concluded that this mode can be divided by the characteristics of its breakup mechanism and that these characteristics are a function of the liquid Weber number and the electric Bond number for a given setup. Additionally we were also able to calculate the average charge per droplet and define the average size of primary and satellite droplets. The dispersion of the spray was also studied regarding its relation to the liquid Weber number and to the electric Bond number. We conclude that simple-jet mode electrosprays are a good option for applications which require monodisperse micrometer droplets with high throughput.

Effects of pulsing frequency on characteristics of electrohydrodynamic inkjet using micro-Al and nano-Ag particles

Characteristics of electrohydrodynamic inkjet were examined upon varying the applied voltage and frequency as well as the base-fluid. Low-cost inks are sought and micro-aluminum particle suspensions are candidates. Inclusion of large particles engendered satellite droplets and hence degraded print quality. However, the formation of satellite droplets can be prevented by increasing fluid’s viscosity. Droplet sizes were measured as functions of the frequency; the higher the frequency, the smaller the droplet size. The low-cost inks with aluminum particles yielded reasonable performance on comparison with the expensive commercial ink comprised of silver nano-particles in case of printing dots that are a few hundred microns in size.

Research on the Influence of Ink Surface Tension to the Formation of Ink Droplet

Ink jet printing, as the key point of digital printing development, has already got some progress as technical be concerned, and also it has been used in industrial producing. But until now, ink jet printing still has some problems to be solved, include satellite droplet, droplet dimension, and spreading of droplet on substrate and so on. These problems involving parameters include dimension and geometry of the print-head, quality and characteristics of the control signal, surface tension of the ink, viscosity of the ink, characteristics of the substrate surface and so on. In this paper we just analysis the influence of ink surface tension to the process from jet to formation of the droplet by establishing mathematical model, but keep other parameters unchanged. At last, we get the relationship between ink surface tension and droplet parameters, so that it can be used as theoretical basement for print-head design and researching of ink.

Self-Similar Micron-Size and Nanosize Drops of Liquid Generated by Surface Acoustic Waves

A planar surface acoustic wave on a solid substrate and its radiated sound into a static liquid drop produce time-averaged, exponentially decaying acoustic and electric Maxwell pressures near the contact line. These localized contact-line pressures are shown to generate two sequences of hemispherical satellite droplets at the tens of microns and submicron scales, both obeying self-similar exponential scaling but with distinct exponents that correspond to viscous dissipation and field leakage length scales, respectively. The acoustic pressure becomes dominant when the film thickness exceeds (1/4π) of the surface acoustic wave wavelength and it affects the shape and stability of the mother drop. The Maxwell pressure of the nanodrops, which exceeds ten atmospheres, is sensitive to the contact angle.

Surface acoustic wave nebulization on nanocrystalline ZnO film

Surface acoustic wave (SAW) nebulization/atomization has been realised on thin ZnO film based SAW devices. The surface acoustic wave nebulization (SAWN) process has been observed to produce significant mist generation and ejected satellite droplets. By modifying the geometry of the interdigitated transducers to reduce the wavelengths from 400 μm to 120 μm, higher frequency SAWN has been achieved by increasing radio frequency driving frequencies from 11.8 MHz to 37.2 MHz, respectively. Compared with the commonly used LiNbO3 SAWN devices, ZnO film devices exhibit better thermal dissipation, and to date, they have shown no susceptibility to substrate failure during fabrication or operation. They also have the added advantage of the technology being suitable for direct integration with microsystems and integrated circuit microelectronics.

Dynamic Impact Behavior of Water Droplet on a Superhydrophobic Surface in the Presence of Stagnation Flow

The following study investigates splashing of impinging water droplets on superhydrophobic surfaces with and without the presence of a stagnation flow. Droplets were accelerated by either gravity or gravity and co-flow. By changing the height and the air flow velocity different combinations of stagnation flow and droplet velocity were created. The spreading diameter, spreading velocity and contact time were studied for different air and droplet speeds. It was clearly observed that for a fixed impact velocity (i.e. constant Weber number), the presence of the stagnation flow promotes splashing and formation of satellite droplets. Consequently, for the co-flow droplet impact experiments, the mass of the recoiled droplet is significantly smaller than that of the impinging droplet in still air.

How PEDOT:PSS solutions produce satellite-free inkjets

Satellite droplets are unwanted in inkjet printing and various approaches have been suggested for their reduction. Low jetting speeds limit applications of the process. Added surfactants for wetting and conductivity enhancement may help but dynamic surface tension effects may counteract improvements. A higher fluid viscosity delays ligament break-up, but also leads to slower jets, while viscoelasticity reduces satellite formation only in certain cases. We show here that aqueous solutions of PEDOT:PSS (1:2.5 by weight) are strongly shear-thinning. They exhibit low viscosity within the printing nozzle over a wide range of jet speeds, yet rapidly (<100μs) recover a higher viscosity at the low shear rates applicable once the jet has formed, which give the benefit of delayed satellite formation. The delay over a 0.8mm stand-off distance can be sufficient to completely suppress satellites, which is significant for many printing applications.

Direct numerical simulation of rising bubble interaction with free surface using level contour reconstruction method

Bubble rising phenomenon is widely found in many engineering applications including stream generators in power plants. Many experimental and numerical researches have been performed to predict the dynamic behavior of the bubble rising process. Most simulations so far have focused on evolution behavior of the rising bubble itself. Rising bubbles could penetrate through the top free surface which makes the problem much more complicated in addition to the curvature effect on the interface. With top surface, satellite droplets may be generated near breakup region and entrained to rising velocity field. In this paper, the effect of top free surface on rising bubble has been numerically investigated. High-order tetra-marching level contour reconstruction method (LCRM), which is a hybridization of fronttracking and level-set methods has been used to track the gas-liquid interface explicitly. Effects of free surface on the interfacial shape and behavior after breakup are studied with different size and depth of the initial bubble. It has been found that the initial diameter of the bubble is the dominant factor controlling the satellite droplet formation.

뉴튼유체와 전단희석유체의 액적분사 거동에 대한 수치해석적 연구

The droplet ejection behavior from drop-on-demand printhead are investigated numerically for Newtonian and shear-thinning fluid. The numerical simulation is performed using a volume-of-fluid model. In this study, we compare the printable range in terms of Z number and pinch-off time for Newtonian and shear-thinning fluids. The printability range are found to be 1.08 <TEX>$$\leq_-$$</TEX> Z <TEX>$$\leq_-$$</TEX> 12.9 for Newtonian fluid and 0.8 <TEX>$$\leq_-$$</TEX> Z <TEX>$$\leq_-$$</TEX> 12.9 for shear-thinning fluid. However, air entrainment is observed during merging of primary and satellite droplet within the printability range. The pinch-off time of the shear-thinning fluid is apparently shorter compared to the corresponding Newtonian fluid due to shear-thinning effects and the differences in the pinch-off time is enlarged significantly when the capillary number is larger than 0.5.

Cutting a drop of water pinned by wire loops using a superhydrophobic surface and knife.

A water drop on a superhydrophobic surface that is pinned by wire loops can be reproducibly cut without formation of satellite droplets. Drops placed on low-density polyethylene surfaces and Teflon-coated glass slides were cut with superhydrophobic knives of low-density polyethylene and treated copper or zinc sheets, respectively. Distortion of drop shape by the superhydrophobic knife enables a clean break. The driving force for droplet formation arises from the lower surface free energy for two separate drops, and it is modeled as a 2-D system. An estimate of the free energy change serves to guide when droplets will form based on the variation of drop volume, loop spacing and knife depth. Combining the cutting process with an electrofocusing driving force could enable a reproducible biomolecular separation without troubling satellite drop formation.

Numerical study on the effects of non-dimensional parameters on drop-on-demand droplet formation dynamics and printability range in the up-scaled model

The droplet formation dynamics from a drop-on-demand printhead is numerically investigated with regard to the printability range. The numerical simulation is carried out using a volume-of-fluid model, and the qualitative effects of non-dimensional parameters on the droplet formation dynamics are evaluated. To determine the printability range, within which the droplet is ejected in a stable manner without satellite droplets, extensive numerical simulations are carried out by varying the viscosity and surface tension. Generally, the printability range is determined by a Z number, which is the inverse of the Ohnesorge number (Oh). However, it is found that the Z number alone is insufficient for describing the droplet formation dynamics. Other important non-dimensional parameters such as the Reynolds number (Re), Weber number (We), and capillary number (Ca) should also be taken into consideration. For studying the printability, the droplet formation dynamics are divided into five different regimes, and a regime map based on the Z, We, and Ca is proposed.

액적의 분사 거동을 지배하는 무차원수에 대한 수치해석적 연구

The droplet ejection behavior from drop-on-demand printhead are investigated numerically in terms of the non-dimensional parameters. The numerical simulation is performed using a volume-of-fluid model. It is important to eject droplet within the printability range, where the droplet is ejected in stable manner without satellite droplets. Generally, the printability range has been determined by Z number, which is the inverse of Oh number. However, it is found that the ejection of droplets with same Z number can exhibit different behavior depending on the value of Ca and We number. Therefore, it is insufficient to determine the printability range only with Z number. Instead, other non-dimensional parameters, such as Ca and We number, should be considered comprehensively.

Effect of single aerosol droplets on plasma impedance in the inductively coupled plasma

The impedance of an inductively coupled plasma was indirectly monitored by two different means—through a RF-probe coil placed inside the torch housing and from tapping the phase-detector signal of the impedance-matching network. During single-droplet introduction, temporal spikes in both the RF-probe coil and the phase-detector signals were readily observed, indicating a momentary change in plasma impedance. The changes in plasma impedance were found to be due solely to plasma perturbation by droplet introduction, and not to an artifact caused by imperfect automatic impedance matching. The temporal changes in plasma impedance were found to be directly proportional to the temporally integrated atomic emission of hydrogen, which is assumed in turn to be directly proportional to the volume of the introduced droplet. A small satellite droplet, with an estimated diameter of 27μm (i.e., ~10pL in volume), caused a readily measurable change in plasma impedance. By assuming that the change in RF-probe voltage is directly proportional to the variation in RF power delivered by the load coil, the instantaneous power change coupled to the plasma during single-droplet introduction was estimated. Typical increases in peak RF power and total energy coupled to the plasma, for a single 50-μm droplet introduction, were thereby estimated to be around 8 to 11W and 0.03 to 0.04J, respectively.This impedance change was also exploited as a trigger to signal the droplet-introduction event into the plasma. This trigger signal was obtained through a combination of the RF-probe and the phase-detector signals and offered typical jitter from 1 to 2ms. With the proper choice of a trigger threshold, no trigger misfire resulted and the achievable efficiencies of the trigger signal were 99.95, 97.18 and 74.33% for plasma forward power levels of 900, 1200, and 1500W, respectively. The baseline noise on the RF-probe coil and the phase-detector signals, which increase with plasma forward power, contribute to both the jitter and the reduced trigger efficiency. The noise components that increase sharply with plasma forward power are at audio-frequencies between 150 and 300Hz, and are attributed to gas dynamics in the plasma.

Application of Axiomatic Design Theory to a Microfluidic Device for the Production of Uniform Water-in-Oil Microspheres Adapting an Integration Method

This work describes a novel microfluidic method to generate uniform water-in-oil (W/O) microspheres using the phase separation technique. Axiomatic design theory (ADT) was employed for the conceptual design of microchannel systems, and ADT verified that the proposed microfluidic system is a decoupled design. The integration of hydrodynamic flow focusing method and crossflow method is realized in a microfluidic device with oil phase and aqueous phase. The immiscible fluids are fed by continuous air pressure. By the hydrodynamic flow focusing method, the width of the dispersed focused aqueous phase is controlled. The focused flow enters T-junction geometry downstream, and the crossflow interferes with the focused flow. By varying the applied pressure to the crossflow, the W/O microspheres are formed at the T-junction. Based on this approach, the size of the W/O microspheres can be successfully controlled from 16 μm to 35 μm in diameter within about 5% of variation. The present method has advantages such as good sphericity, few satellite droplets, active control of the microsphere diameter, and high throughput with the simple and low cost process. To achieve the promising results, this integrating method reveals high potential for production of polymer based microspheres.

Temperature dependence of droplet breakup in 8CB and 5CB liquid crystals

Droplet breakup of many Newtonian fluids is well described by current experiments, theory, and simulations. Breakup in complex fluids where interactions between mesoscopic structural features can affect the flows remains poorly understood and a burgeoning area of research. Here, we report on our investigations of droplet breakup in thermotropic liquid crystals. We investigate breakup in the smectic, nematic, and isotropic phases of 4-cyano 4-octylbiphenyl (8CB) and the nematic and isotropic phases of 4-cyano 4-pentylbiphenyl (5CB). The experiment consists of varying the ambient temperature to control liquid crystalline phase and imaging breakup using a fast video camera at up to 110000 frames/s. We expand on previous work [John R. Savage et al., Soft Matter 6, 892 (2010)] that shows breakup in the smectic phase is symmetric, producing no satellite droplets, and is well described by a similarity solution for a shear-thinning power-law fluid. We show that in the nematic phase the breakup occurs in two stages. In the first stage, the breakup is symmetric and the power-law exponent for the minimum radius dependence on the time left to breakup is 1.2<n<1.9. In the second stage the drop develops two minima and the minimum radii shrink with a power-law exponent 0.6<n<1. We find that the exponents vary with temperature across the nematic phase. These results are surprising because rheological measurements of 8CB and 5CB in the nematic phases indicate Newtonian behavior that cannot account for the observed breakup dynamics. Finally, in the isotropic phase, the exponents are consistent with theoretical predictions and experiments for Newtonian fluid breakup in the inertial viscous regime.

Droplet characterisation of molten caprolactam for additive manufacturing applications

Droplet formation characteristics of molten caprolactam, the monomer of nylon 6, are reported in this article as part of research to develop a novel inkjet additive manufacturing process. Initial investigations confirmed the jettability of molten caprolactam at 80 °C with a printhead and recommended a range of parameters for a stable jet array. High-speed imaging was used to study the interactions between the melt and the printhead. Droplets were generated at voltages higher than 12.5 V. Nozzle wetting and crosstalk in adjacent nozzles were observed especially at higher voltages. Jetting frequency did not affect the droplet characteristics. By varying the jetting voltage though significant changes to the droplet shape, evolution and kinetics were observed. Satellite droplets were formed by disintegration of the tail at lengths of about 1 mm when a jetting voltage of 25.0 V was used. An increase of voltage from 15.0 V to 25.0 V increased the droplet velocity from 2 m/s to 8 m/s. Reynolds and Weber numbers predicted there would not be splashing upon droplet impact, which was confirmed by experiments when impinging molten caprolactam droplets on cold and heated substrates. The droplet formation study suggested a set of jetting parameters to be used for the next stage of the research on the inkjet process development.

Study on Droplets Formation Process of Polymer-Encapsulated Super-Fine Disperse Dye Ink

The polymer-encapsulated nanoscale disperse dye ink was prepared by phase separation technique using copolymer of styrene and maleic anhydride. The effects of glycerin and Tween80 on colloidal properties and droplets formation process of polymer-encapsulated nanoscale disperse dye ink were investigated. FTIR and TEM suggested that the disperse dye was encapsulated by copolymer successfully. The polymer-encapsulated nanoscale disperse dye ink had excellent stability, the viscosity increased with an increasing glycerin, and the surface tension decreased with an increase of amount of Tween80. Either viscosity increasing or surface tension reducing made the droplets break easily. In addition, the maximum volume of droplet would reduce with decreasing surface tension. Viscosity might be the main factor to control the &#x201C;satellite droplet&#x201D; forming, and the &#x201C;satellite droplet&#x201D; was increased when the viscosity was lower.