Drop Formation Research Articles

Experimental and numerical characterisation of two-phase flow in NETmix reactors

NETmix has been shown to be an efficient mixing technology for miscible fluids, promoting large heat and mass transfer rates. This work intends to assess NETmix for multiphase mixing, which is particularly relevant to evaluate the NETmix’s ability to couple mixing and heat transfer. The first step is to build fundamental knowledge regarding two-phase flow in these devices. A 2D CFD two-phase model was implemented based on the Euler‑Euler approach with the Volume of Fluid (VOF) method using the NETmix Unit Block (NUB). The two phases are liquids with equal density and viscosity, enabling the study of the impact of the interfacial tension and contact angle on the flow dynamics and drop formation. Mass transfer was assessed from the flow shear at the interface of the two phases. The CFD model was experimentally validated using a water-cyclohexane system. The NETmix technology promotes high flow dynamics and large interfacial area generation between the two phases. The successive split and recombining of flow contribute to an intense fluid renewal around the two phases’ interface, enhancing the mass transfer rate. NETmix shows potential for multiphase applications, particularly between two immiscible liquid phases, with a four-fold increase of the interfacial area of immiscible fluids.

Numerical simulation of multi-functional droplet production in microfluidics

In this work, we have investigated numerically the formation of stable Janus droplets in microfluidics flow focusing device. We have examined the effect of continuous phase capillary number (Ca) as it influences the drop size and the stability of a Janus drop for a fixed dispersed phase flow rate. The effect of flow rate ratio of disperse phases has also been examined to understand the relative distribution of the material in a single Janus droplet. Numerical simulations have been performed using the commercial software Ansys Fluent 19.2 with the Volume of Fluid (VOF) method to solve discrete versions of governing equations. We have extended our method to study the formation and stability of a ternary droplet. For this, a third inlet is incorporated at the middle of the initial channel geometry and fluid combination. In the case of ternary drop, we find that different phases re-arrange themselves after drop formation before reaching the final stable configuration.

Dynamics of Drop Formation in the Presence of Interfacial Mass Transfer.

The dynamics of drop formation have been investigated in the presence of interfacial mass transfer through controlled flow visualization experiments. The mixtures of n-hexane (solvent) and acetone (solute) were used as a dispersed phase, having different initial compositions varying over a broad range. Drops were formed at the submerged position in the continuous phase (water) at the same operating flow conditions. The unsteady force balance model is developed to analyze the implications of the simultaneously occurring interfacial transfer of the solute on the formation dynamics in real time, and predictions are validated with experimental results. Based on initial compositions, the analysis of the transient drop shape shows a sharp transition in the drop formation regime. At lower initial solute concentrations, i.e., ϕ0 < 0.2, axisymmetric drop formation occurs and the interfacial solute transfer has negligible effects on the formation dynamics. Over an intermediate range of solute concentrations, i.e., 0.2 < ϕ0 < 0.5, Marangoni instability is triggered along the evolving interface, and therefore, the interface deformations and contractions occur during the drop formation. At ϕ0 = 0.5, the drop takes highly nonaxisymmetric shapes and remains away from equilibrium until its detachment from an orifice. For ϕ0 > 0.5, the spontaneous ejection of plumes of the solute results in the rapid generation of multiple droplets of smaller size. This work shows that higher solute concentration gradients not only lead to faster solute transport but also induce strong interfacial instability simultaneously. Thus, the coupled effects of transient change in composition and fluid properties govern the drop size and its formation time in such systems under non-equilibrium.

Characterizations and Inkjet Printing of Carbon Black Electrodes for Dielectric Elastomer Actuators.

Dielectric elastomer actuators (DEAs) have been proposed as a promising technology for developing soft robotics and stretchable electronics due to their large actuation. Among available fabrication techniques, inkjet printing is a digital, mask-free, material-saving, and fast technology, making it versatile and appealing for fabricating DEA electrodes. However, there is still a lack of suitable materials for inkjet-printed electrodes. In this study, multiple carbon black (CB) inks were developed and tested as DEA electrodes inkjet-printed on acrylic membranes (VHB). Triethylene glycol monomethyl ether (TGME) and chlorobenzene (CLB) were selected to disperse CB. The inks' stability, particle size, surface tension, viscosity, electrical resistance, and printability were characterized. The DEA with Ink-TGME/CLB (mixture solvent) electrodes obtained 80.63% area strain, a new benchmark for the DEA actuation with CB powder electrodes on VHB. The novelty of this work involves the disclosure of a new ink recipe (TGME/CLB/CB) for inkjet printing that can obtain stable drop formations with a small nozzle (17 × 17 μm), high resolution (∼25 μm, approaching the limit of drop-on-demand inkjet printing), and the largest area strain of DEAs under similar conditions, distinguishing this contribution from the previous works, which is important for the fabrication and miniaturization of DEA-based soft and stretchable electronics.

Effect of substrate roughness on dynamics of wetting and decay of an ultrathin liquid film

In this study, we analyze the dynamics of wetting and decay of an ultrathin liquid film on a rough substrate. The dependence of disjoining pressure on liquid film thickness is derived by the direct numerical calculation for the Lennard-Jones pair interaction potential and 3D rough substrate with a random relief. It is shown that disjoining pressure for a rough substrate can be approximated by a similar function for a smooth one with highly fluctuating parameters over the substrate surface. The equation describing wetting and decay of 3D liquid film is modified, accounting for the calculated coordinate dependence of disjoining pressure the general form of capillary pressure depending on substrate relief. The approach is used for the simulation of drop formation from a nanosized liquid film for substrates with various roughness parameters. It is demonstrated that substrate roughness is responsible for accelerated film rupture and formation of “detectable” drops, increase in their number density, and decrease in average size. At the later stage, liquid film transformation is described by the power-law dependences of number density, average height and size with the exponents depending on substrate roughness. The blocking point of the size distribution function shifts to a larger value for a substrate with a more fine-grained structure. These changes in the growth rate and size distribution function of drops are associated with the change in conditions of liquid flow, which becomes inhomogeneous due to substrate roughness.

Fog formation, smog situations and air quality in high school physics education

Abstract. It has become a worldwide expectation that the physics curriculum includes everyday knowledge as well. One important field that can make the curriculum more colourful and exciting is the field of meteorology. In our paper, a three-lesson-long curriculum for high schools will be presented on how to teach the connection between fog formation and air quality. The international educational experience of this particular topic will be surveyed, mainly in the countries of the Carpathian Basin, moreover, the measurement processes and education methods used in the GLOBE Program will also be presented. The experimental curriculum consists of three parts. In the first part air humidity and the concepts of absolute and relative humidity are discussed. Through a few specific exercises, the students participating in the program learn to specify relative humidity and become acquainted with fog formation. It is shown via an experiment that air cooling at a saturated state is not enough to form fog because condensational nuclei are needed for the formation of tiny water drops. In the 2nd lesson, the concept of temperature inversion and its connection to fog and air pollution are discussed. Using Internet websites the students collect information about the formation of smog, its types, occurrence, and the conditions for declaring smog alerts. In the 3rd lesson, the methods of air pollution analysis and different air pollutants are discussed. Websites, where the students can follow the air pollution data of their area, are used. Based on these, problems related to the interpretation of the data will be solved. The information which is available on the website of the European Environment Agency is also touched on.

Dynamics of drop formation and characterization of additive manufactured CS/AgNP/PVA composite membranes

AbstractThis study investigated the dynamics of drop formation and the printability of chitosan/silver nanoparticles composite fluid (CS/AgNP) modified with polyvinyl alcohol (PVA) using an inkjet 3D printer. It specifically investigated the relationship between rheological properties and fluid mechanical properties in determining the minimum drop velocity for 3D printing of modified CS ink. Proper matching of modified CS fluid rheological properties to the established physical parameters generated adequate droplets on glass substrates and formed membrane composites. The dynamic viscosity of the CS ink decreased with an increase in the concentration of PVA, which resulted in the disruption of existing molecular forces within the CS/AgNP polymer chains, suggesting that the solvent (PVA) was responsible for reducing the shear stress, surface tension, and viscosity of the composite membranes and subsequently improving the flow rates of the CS matrix at the nozzle orifice. Optimized drop velocity of 1.77 m/s yielded adequate formation of the CS/AgNP/PVA membrane composite with enhanced surface morphology, while an increase in drop velocity to 2.5 m/s brought about the formation of satellite droplets with irregular surface structures in printed CS composites. Therefore, adherence to minimum fluid drop velocity is fundamental to effective inkjet printing of composite membranes.

Pressure drop model of DPF considering ash factor at different capture stages

In this paper, a pressure drop model of diesel particulate filter (DPF) is established on diesel bench loading tests and simulation studies. A proposed DPF soot loading prediction model takes into account the different factors and calculation methods of pressure drop formation in different capture stages, including deep bed capture stage, transitional capture stage and soot cake capture stage. Further, a pressure drop model of DPF is improved, and the model for different filter wall permeability in different capture stages is discussed. The calculated results of the model are verified with the experimental results. Finally, the influence of ash factor on the modification of DPF pressure drop model was investigated.The results show that, when the ash layer is 0.05 mm–0.15 mm in the deep bed capture stage, the modified values of filtration wall permeability are 8.2%–43.8% of the original values. In the transitional capture stage, the ash layer with thickness of 0.01 mm–0.05 mm needs to modify 0.75 g/L∼1.5 g/L soot loading. The DPF pressure drop and the particle soot cake layer thickness increase linearly with time in the soot cake capture stage. When the ash layer changes in the range of 0.05 mm–0.15 mm, the soot loading is modified to 13.4%–36.8% of the original value. When the axial scale of the ash plug is 1/5,1/4 and 1/3 of the DPF length, the soot loading is corrected to 33.3%, 37.3% and 45.1%.

A priori evaluation of the printability of water-based anode dispersions in inkjet printing

Inkjet printing represents a disruptive additive manufacturing technology that has emerged as an innovative approach to generate customized lithium-ion batteries by tailored dispersions. However, electrode dispersions cause a complex non-Newtonian behavior which hampers the processability. This paper demonstrates a novel procedure for an a priori evaluation of the printability of aqueous graphite dispersions. Therefore, dispersions with a varying active material content were prepared and the printability was examined through a characterization of the drop formation and the drop deposition behavior. While the drop formation was observed by in-situ monitoring, the drop deposition was analyzed in ex-situ test setups. The rheological properties were systematically determined to calculate nondimensional numbers that describe the dispensing behavior. Consequently, their capability to predict the stability of the drop formation was evaluated. The results revealed that a graphite dispersion with a content of 2 m% allowed for a stable drop formation. No splashing occurred on the substrate during the drop deposition and sufficient wetting can be assumed due to a contact angle of below 90∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}. Conclusions were drawn to further enhance the active material content. Due to the universality of the proposed approach, it is expected to be applicable to different dispersion systems.

De-wetting of evaporating drops on regular patterns of triangular posts.

Directional wicking and spreading of liquids can be achieved by regular micro-patterns of specifically designed topographic features that break the reflection symmetry of the underlying pattern. The present study aims to understand the formation and stability of wetting films during the evaporation of volatile liquid drops on surfaces with a micro-pattern of triangular posts arranged in a rectangular lattice. Depending on the density and aspect ratio of the posts, we observe either spherical-cap shaped drops with a mobile three-phase contact line or the formation of circular or angular drops with a pinned three-phase contact line. Drops of the latter class eventually evolve into a liquid film extending to the initial footprint of the drop and a shrinking cap-shaped drop sitting on the film. The drop evolution is controlled by the density and aspect ratio of the posts, while no influence of the orientation of the triangular posts on the contact line mobility becomes evident. Our experiments corroborate previous results of systematic numerical energy minimization, predicting that conditions for a spontaneous retraction of a wicking liquid film depend weakly on the orientation of the film edge relative to the micro-pattern.

Chromium hydroxide microsphere synthesis using droplet microfluidics

In this work, a capillary microfluidic platform based on microbore tubes is demonstrated to continuously synthesize chromium hydroxide microspheres with a narrow particle size distribution using sol-gel process. Chromium hydroxide microspheres are later calcined to obtain chromium oxide microspheres. An in-situ mixing technique is utilized to mix the chromium precursor with the gelling agent. The effects of different design (microfluidic junction configuration, size of the microbore tube) and operating variables (continuous to dispersed phase flow rate ratio, flow velocity, residence time) on the average diameter and polydispersity index of microspheres are studied systematically. Microspheres of an average size of 574 μm with polydispersity index of 6.6% could be prepared using a double T-junction at an organic to aqueous (O/A) flow rate ratio of 60 in an 800 µm diameter microbore tube. The detailed analysis of the flow conditions reveals that the regime of drop (of aqueous phase) formation varies between transition and dripping. Finally, a correlation is proposed to predict average diameter of the microspheres based on values of capillary number (Ca) and organic to aqueous (O/A) flow rate ratio.

Mechanisms of Translation of Deep-Seated Pulses into External Shells of the Modern Earth: Evidence from Late Cenozoic Global Tectonomagmatic Activation of Our Planet

It is known that the Earth’s history is characterized by periodic activation of tectonomagmatic processes, when they are intensified without visible reasons. This is obviously related to the evolution of deep-seated petrological processes, the peculiar reflect of which are events in the external shells of the modern Earth (tectonosphere), but the nature of these processes and mechanisms of their translation in tectonosphere remain weakly studied. This problem is considered by the Late Cenozoic (Neogene–Quaternary) global activation. The modern Earth represents a cooling body with solidifying liquid iron core. This process should be accompanied by several thermodynamic, physical, and physical-chemical effects, which could lead to the internal activation of our planet. We attempted to decipher these problems using available geological, petrological, geochemical, and geophysical data on the present-day activation. It is shown that main active element in the modern Earth is uninterruptedly upward moving thin crystallization zone located between completely solidified part of the core (solid inner core) and its completely liquid part (external liquid core). Diverse phase transitions in a cooling melt passing through bifurcation points are related to this zone. The phase transitions are represented by both a change of crystallizing solid phases which built up inner core and retrograde boiling with formation of drops of “core” fluids. These drops are floated in high-Fe host melt and are accumulated at the mantle base, where they are involved in the formation of mantle plumes, which are the main carriers of deep-seated pulsed into external geosphere, and finally leave the core with them. It is suggested that in one of such points the fluid solubility in cooling high-Fe liquid of external core sharply decreases. This should lead to the simultaneous intensification of retrograde boiling of this melt over the entire zone surface of zone of the core crystallization zone, i.e., on a global scale. This could provide the influx of excess “core” fluids required for large-scale generation of mantle plumes and serve as trigger for Late Cenozoic global tectonomagmatic activation of the Earth.

Mass transfer and size control of partially miscible fluid drops in a flow focusing microfluidic device

We conducted laboratory experiments and numerical simulations to investigate the formation and evolution of drops formed by partially miscible two-phase fluid, n-butanol (the continuous phase) and water (the dispersed phase), in a flow focusing microfluidic system. We carefully calibrated the numerical model to obtain good agreement with experimental data in drop velocity and mass transfer, demonstrating the model's capability to capture realistic drop dynamics. Our detailed investigation of the numerical results allowed us to determine the mechanism of drop formation and obtain a relevant criterion in terms of the disperse-to-continuous flow ratio beyond which the tubing patterns would occur. Additionally, we found that the mass transfer between the two phases, specifically at the drop interface, strongly depends on the local distribution of dissolved concentration of the dispersed phase. To enhance mass transfer, we conducted numerical simulations on alternating curved channels, which allows for the lateral advection of the dispersed phase concentration in the continuous phase at the curved section. We found that this lateral movement enhances mass transfer at the drop interface. Through detailed investigation of numerical results, we addressed mechanisms of mass transfer enhancement in the curved channel. Overall, our findings provide insight into the mechanisms of drop formation and mass transfer in partially miscible two-phase fluids in microfluidic systems, which could be useful in designing and optimizing such systems for various applications.

Mechanism of transition between separated and non-separated oil-water flows in inclined pipe

The analysis of drop formation and the transition between separated and non-separated oil–water flows enables us to better regulate flow parameters and pipe inclination angle, which is of great significance for improving the efficiency of oil transportation. Previous studies mainly carried out qualitative analysis through experiments and studied the transition between separated flows, very few studies are conducted on the transition between separated flow and dual continuous flow or intermittent flow. In addition, the influence of pipe inclination angle was not considered in the study of drop formation, which results in inadequate force analysis of drop. This article presents the use of dimensionless numbers to predict the transition between separated flow and dual continuous flow or intermittent flow. The relation expression between dimensionless numbers is obtained by regression of experimental data. The critical values of flow parameters and pipe inclination angle can be calculated by the relation expression between dimensionless numbers and flow parameters. In addition, this article proposes a three-dimensional interfacial wave shear deformation mechanism by adding volume forces to the force analysis for the possible inclined pipe. The prediction equation is obtained and the critical value of flow parameters and pipe inclination angle for drop formation can be calculated, which are in good agreement with the experimental data.

Printability of inkjet according to supply pressure

To obtain uniform and high-resolution jetting, the drop-on-demand inkjet can be operated with various physical properties of inks. To determine how supply pressure control affects the stability of the jetting, an experimental study was conducted on eight model inks in a range of Z number (1 &amp;lt; Z &amp;lt; 17). The velocity and volume of drop were measured by a visualization method to analyze the performance of piezoelectric inkjet head. Increasing negative supply pressure reduced both velocity and volume. The decline of volume was uniform regardless of driving voltage, whereas the decline of velocity increased with decreasing driving voltage. The printability diagram of Z–We was derived to analyze the jetting behavior according to the ink properties, such as viscosity and surface tension, and operating conditions, such as driving voltage and supply pressure. For dimensionless numbers, Z and We, the surface tension term can be compensated by the supplementary Laplace pressure force generated by the supply pressure. In the printability diagram of the modified Z* and We*, the suppression of the satellite drop formation by negative supply pressure can be identified as a shift from the outer to the inner stable region. The critical aspect ratio at the pinch-off was estimated from the Taylor–Culick analysis of the liquid filament breakup. The damping time of residual vibration was measured according to the supply pressure within the printable range. We conclude that control of the supply pressure with slight droplet velocity and volume reduction can improve the printing stability and frequency.

Effect of surfactants during drop formation in a microfluidic channel: a combined experimental and computational fluid dynamics approach

The effect of surfactants on the flow characteristics during rapid drop formation in a microchannel is investigated using high-speed imaging, micro-particle image velocimetry and numerical simulations; the latter are performed using a three- dimensional multiphase solver that accounts for the transport of soluble surfactants in the bulk and at the interface. Drops are generated in a flow-focusing microchannel, using silicone oil ( $4.6$ mPa s) as the continuous phase and a 52 % w/w glycerol solution as the dispersed phase. A non-ionic surfactant (Triton X-100) is dissolved in the dispersed phase at concentrations below and above the critical micelle concentration. Good agreement is found between experimental and numerical data for the drop size, drop formation time and circulation patterns. The results reveal strong circulation patterns in the forming drop in the absence of surfactants, whose intensity decreases with increasing surfactant concentration. The surfactant concentration profiles in the bulk and at the interface are shown for all stages of drop formation. The surfactant interfacial concentration is large at the front and the back of the forming drop, while the neck region is almost surfactant free. Marangoni stresses develop away from the neck, contributing to changes in the velocity profile inside the drop.

Nanoscopic jets and filaments of superfluid 4He at zero temperature: A DFT study.

The instability of a cryogenic 4He jet exiting through a small nozzle into vacuum leads to the formation of 4He drops, which are considered ideal matrices for spectroscopic studies of embedded atoms and molecules. Here, we present a He-density functional theory (DFT) description of droplet formation resulting from jet breaking and contraction of superfluid 4He filaments. Whereas the fragmentation of long jets closely follows the predictions of linear theory for inviscid fluids, leading to droplet trains interspersed with smaller satellite droplets, the contraction of filaments with an aspect ratio larger than a threshold value leads to the nucleation of vortex rings, which hinder their breakup into droplets.

Does the contact angle hysteresis control the droplet shapes on cylindrical fibers?

HypothesisEquilibrium droplets on cylindrical fibers are divided into two classes: barreled and clamshell droplets. In the barreled droplet, the liquid body fully envelops the fiber, and the drop forms two boundary contact lines. In the clamshell droplets, some fiber surface under the liquid body remains dry and only one boundary contact line exists. So far, the transition from one shape to the other was predicted by ignoring the contact angle hysteresis. A new series of experiments using drop-on-demand technology were conducted to analyze the shape of droplets. The existing theory cannot explain the obtained data. We hypothesized that the morphological clamshell-barrel transition of droplets significantly depends on the method of drop formation and is largely controlled by the contact angle hysteresis. ExperimentTo test this hypothesis, we investigated two scenarios of drop formation. In the first scenario, the hexadecane drop was growing on the fiber by printing smaller drops on it. In the second scenario, the drop was formed spontaneously from a coating glycerol film due to the Plateau-Rayleigh instability. To obtain a range of contact angles, a set of different silanes were adsorbed on the fiber surfaces. FindingsThe results showed that for small contact angles (<40°) and small contact angle hysteresis, both methods of drop formation led to the same theoretically explainable conditions for clamshell-barrel transition. As the contact angle increases and hysteresis becomes appreciable, the conditions for clamshell-barrel transition become significantly dependent on the method of drop formation. We discovered that no barreled droplets exist for contact angles greater than 60°. The experimental results were documented in a set of diagrams describing the clamshell-barrel transitions. These diagrams can be used in different engineering applications.

Electric-field-mediated morpho-dynamic evolution in drop–drop coalescence phenomena in the inertio-capillary regime

When two drops collide, they may either exhibit complete coalescence or selectively generate secondary drops, depending on their relative sizes and physical properties, as dictated by a decisive interplay of the viscous, capillary, inertia and gravity effects. Electric field, however, is known to induce distinctive alterations in the topological evolution of the interfaces post-collision, by influencing a two-way nonlinear coupling between electro-mechanics and fluid flow as mediated by a topologically intriguing interfacial deformation. While prior studies primarily focused on the viscous-dominated regime of the resulting electro-coalescence dynamics, several non-intuitive features of the underlying morpho-dynamic evolution over the intertio-capillary regime have thus far remained unaddressed. In this study, we computationally investigate electrically modulated coalescence dynamics along with secondary drop formation mechanisms in the inertio-capillary regime, probing the interactions of two unequal-sized drops subjected to a uniform electric field. Our results bring out an explicit mapping between the observed topological evolution as a function of the respective initial sizes of the parent drops as well as their pertinent electro-physical property ratios. These findings establish electric-field-mediated exclusive controllability of the observed topological features, as well as the critical conditions leading to the transition from partial to complete coalescence phenomena. In a coalescence cascade, an electric field is further shown to orchestrate the numbers of successive stages of coalescence before complete collapse. However, an increase of the numbers of cascade stages with the electric field strength and parent droplet size ratio is non-perpetual, and the same is demonstrated to continue until only a threshold number of cascade stages is reached. These illustrations offer significant insights into leveraging the interplay of electrical, inertial and capillary-driven interactions for controllable drop manipulation via multi-drop interactions for a variety of applications ranging from chemical processing to emulsion technology.

Surface tension gradient driven autonomous fatty acid-tetrahydrofuran liquid moving drops: Spreading to pinning

In this work, we have studied the drop dynamics of binary tetrahydrofuran (THF) solutions containing fatty acids (stearic acid and myristic acid) or surfactants (N-Cetyl-N, N, N trimethyl ammonium bromide, and sodium dodecyl sulfate). Highly volatile non-aqueous pure THF drop shows dynamic wetting behaviour i.e., droplet expansion and contraction behaviour·THF spreads continuously on fabricated total wetting surface without pinning, however addition of fatty acids causes formation of drops (instead of thin films). Such drops show autonomous motility which can be tuned by maintaining the concentration of fatty acids. The addition of fatty acids provides evaporation driven surface tension gradient (Marangoni stress) which initialize the autonomous drop motion and reduces the innate contact line pinning. It is found that addition of fatty acids showed drop dynamics which was concentration dependent. At low concentrations drop moves, while contact line pinning and subsequent evaporation was noticed at higher concentrations. The study of drop area with respect to time conveyed about the slowdown of evaporation timing of THF. This work proclaims the novel combination of highly evaporating THF with solutes (fatty acids/surfactants) showing its potential applications in self-propelled motion of non-aqueous drops.