Liquid Filament Research Articles

Research on the Numerical Effect of VTD Model in Cross Jet

To deeply study the specific characteristics of the cross-scale phenomenon of fragmentation and atomization in the cross-jet process, this paper is based on the adaptive grid technology, using the RANS model, RNG k-ε model, LES model turbulence models, and VOF models, DPM model, VTD model multiphase flow models have carried out numerical calculations on the lateral jet process, and compared and analyzed the results to better solve the cross-scale problem and obtain more suitable research methods for the phenomenon of lateral jet breaking and atomization. It is found that: compared to the RANS model and RNG k-ε model, the LES model is more suitable for jet breaking; compared with the VOF model and DPM model, the VTD model is closer to the actual physical phenomenon in the cross jet breaking shape, and the initial surface of the liquid column is broken, the columnar broken and the band-shaped liquid filament is broken by a series of broken and atomized phenomena. And a large number of banded or filamentous liquid lumps can be captured in the continuous liquid film stage and the primary crushing stage of the jet, while discrete misty droplets can also be captured in the secondary crushing area. Based on this, this paper finds that the cross-scale VTD model is combined the phase gradient adaptive grid technology can couple the advantages of the VOF model and the DPM model to better solve the transition problem between the continuous phase and the discrete phase, thereby obtaining more accurate jet atomization characteristics.

Breakup of a low-viscosity liquid thread

We examine the breakup of low-viscosity ($\ensuremath{\mu}$) liquid filaments. When $\ensuremath{\mu}$ is small, the filament initially thins as if it were inviscid and its minimum radius ${h}_{m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n}$ obeys a universal scaling law. Here, we use simulations to show that for fluids of sufficiently small $\ensuremath{\mu}$, a coefficient value in the scaling law predicted from computations agrees with theory to three decimal places and inviscid power-law behavior can be observed over 2-3 decades in ${h}_{m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n}$ as ${t}_{b}\ensuremath{-}t\ensuremath{\rightarrow}0$ where ${t}_{b}$ is the filament break up time. Transition from the inviscid regime to a viscous one is also demonstrated from simulations.

Numerical study of satellite droplet formation in dripping faucet

• Dynamic relation between the liquid/gas interface, pressure and velocity profile has been investigated. • Physics of capillary wave generation in liquid filament has been analyzed. • Satellite drop generation depends on the acceleration and distribution of the axial velocity at the cone-filament junction. • Satellite drop is less likely to be formed at high inlet velocity and low surface tension coefficient. Dynamics of primary and satellite drop formation has wide engineering applications. Drop formation from a vertical capillary tube in air goes through various stages that include necking, bifurcation, recoil, wave generation and secondary necking and bifurcation. In this study, the problem has been studied numerically where the free surface of the pendant drop is tracked by a coupled level set and volume-of-fluid method with the surface tension force determined by the continuum surface force model. The evolution of the complete droplet separation process has been simulated. The numerical results have been compared with published experimental data. The mechanism for the recoiling of the liquid filament after droplet detachment and the subsequent formation of a satellite droplet has been studied and dynamic correlation among liquid/gas interface topology, velocity and pressure field has been investigated. Bell shaped distribution of axial velocity profile and fast acceleration at the cone-filament junction has been found to lead to satellite drop generation. Parametric studies on the effects of inlet velocity, surface tension coefficient and viscosity have been performed. Satellite droplet is less likely to be formed at high inlet velocity and low surface tension coefficient. Viscosity, on the other hand, has been found to play an important role on the shape of liquid filament and satellite droplet. These findings can help better understanding of drop-on-demand liquid dispensing, inkjet printing, spray combustion and many more engineering applications.

Numerical simulation of jet atomization process of swirl nozzle

As the initial conditions of evaporative combustion, the jet atomization formed by the swirl nozzle has the characteristics of large distribution space and small atomization particle size, which is very important for the study of the jet atomization of the swirl nozzle. In this paper, the cross-scale VTD model is used to comprehensively consider the actual situation of multi-scale and multi-phase flow coupling in the atomization process. The evolution process and the breaking shape of the rotating cone liquid film during the atomization and crushing process are studied. The results show that the complete broken shape formed by the rotating cone liquid film can be divided into three areas along the axial direction: continuous liquid film, primary crushing, and secondary crushing. Among them, the appearance of irregularly distributed small holes is a significant feature of primary crushing. The sign of secondary crushing is that the liquid filaments are broken into small-scale droplet particles; after the primary and secondary crushing, the fully developed cone-shaped liquid film will appear to move up the position of the primary crushing cavity and reduce the scope. At the same time, the initial position of the secondary crushing will move up.

Application of the Composite Fibers Based on Chitosan and Chitin Nanofibrils in Cosmetology.

Chitosan and composite fibers containing chitin nanofibrils have been developed for use in cosmetology. The tensile strength of the chitosan multifilaments is 160.6 ± 19.0 MPa, and of the composite multifilaments containing chitin, nanofibrils are 198.0 ± 18.4 MPa. Chitin nanofibrils introduced into the chitosan solution contribute to the creation of a new spatial arrangement of chitosan chains and their denser packing. The studies carried out by optical, scanning electron, and atomic force microscopy has shown that the serum, consisting of a mixture of lactic acid and sodium lactate, contains extended oriented structures—“liquid filaments”. It has been also shown that a mixture of serum and composite fibers based on chitosan and chitin nanofibrils has mucoadhesive, film-forming properties. The introduction of composite fibers containing chitin nanofibrils into the serum promotes the reinforcing effect of liquid filaments, the lifting effect of the film. The obtained composition can be used in cosmetology as a skin care product.

Fragmentation of asymmetric liquid filaments and formation of satellite droplets in a microchannel

This study analyzes the breakup dynamic of fluid filament in a T-junction microchannel, including droplet stretching process, liquid filament retraction process and breakup process. The liquid filaments spontaneously evolve into droplets or break into multiple satellite droplets, which can be considered as the result of the competition between the retraction and elongation mechanisms of the liquid filaments. There is an obvious local interfacial tension difference during the shrinkage of the droplet filament, which promotes the discharge of the internal fluid, and then the filament is pinched, resulting in the formation of satellite droplets. On the basis of symmetrical filament pinch-off, and the fully considering of the influence of viscosity and the crossing flow, the elongation and retraction mechanism of droplet filaments in the microchannel are proposed. The study reveals the breakup of asymmetric liquid filaments in a microchannel and the formation of satellite droplets, which provides references for the manipulation of satellite droplets in a microchannel.

Visualization research on ash deposition characteristics of Zhundong coal in a vertical liquid slagging cyclone furnace

A 300 kW vertical liquid slagging cyclone furnace is built for the combustion experiment of Hami coal(HM), a kind of Zhundong coal. The sampling probe and CCD camera are used to monitor the growth process of deposition in the furnace online. In addition, the slags at various positions of the furnace are collected and analyzed to study the metal migration law and slagging characteristics of HM during combustion in the furnace. The results show that compared with the traditional furnace, deposition thickness on the probe of the liquid slagging cyclone furnace is much smaller. The stable thickness of the deposition on probes 1 and 2 is 0.85 mm and 3.30 mm respectively. Fe2O3 shows enrichment in liquid slag. The alkali metals in the gas phase condensed on the low-temperature surface of the sampling probe and existed in the initial layer of the deposition. The SO3 reacts with alkaline oxides at low temperature to form viscous compounds such as Na2SO4 and CaSO4. Due to the fast cooling rate, almost no diffraction peaks can be observed in the XRD curves of the liquid slag and glass filaments, which indicates that the material is a completely amorphous solid.

EFECTOS GRAVITATORIOS EN LA RETRACCIÓN DE FILAMENTOS LÍQUIDOS APOYADOS SOBRE UN PLANO INCLINADO

We study the retraction of liquid filaments resting on inclined solid substrates under partially wetting condition. This one causes each extreme of the filament to retract and form a region of mass accumulation (head) that subsequently develops a neck at its rear part. This neck then breaks up into a separate drop, while the rest of the filament restarts the sequence. In the horizontal case, this process is symmetric and leads to a regular arrangement of evenly spaced drops. When the substrate is inclined, gravity acts differently at each end, thus modifying the distance retracted by each end, the retraction speed and the final distance between drops, which also ceases to be uniform. We find that in the case of the inclined plane, the maximum distance retracted by the end that recedes against (in favor of) gravity decreases(increases) with respect to the horizontal case. This effect turns out to be a useful mechanism for the self–positioning of the resulting drops at the end of the process. Our experimental results, such as end positions, thickness profiles in the longitudinal section, etc., are contrasted with numerical simulations of the Navier–Stokes equations, obtaining a good agreement. This is achieved by using in the contact line the boundary condition known as Cox–Voinov–Blake (CVB)relationship, which includes both the effects of macroscopic hydrodynamics as well as molecular dynamics.

An approximated volume of fluid method with the modified height function method in the simulation of surface tension driven flows

Surface tension in two-phase flow problems plays a dominant role in many micro-flow phenomena and has an important influence on the development of flow instability phenomena that contain free surfaces. In this study, the multi-moment finite volume method is extended for direct numerical simulation of two-phase flow problems. A constraint interpolation profile–CSL (semi-Lagrangian) scheme is used for discretization of the advection part in the momentum equation. A compact volume of fluid method–approximated piecewise linear calculation method without flux limiter is proposed for capturing the moving interface. For modeling the surface tension accurately, the logic in curvature estimation is redesigned based on the height function (HF) method. The isolated volumetric fractions that may reduce accuracy in HF integration are excluded, and the numerical solution shows that the accuracy in the curvature estimation is improved for a coarse mesh. The present method is implemented with a parallel block-structured adaptive mesh refinement (BAMR) strategy; thus, the computational cost can be reduced significantly. Numerical tests show that the present BAMR solver is capable of reproducing the theoretical predictions of capillary wave instability problems with high accuracy. The simulation of droplet collisions further demonstrates the accuracy of the surface tension model. Finally, we extend it to the liquid jet atomization. The wavy disturbance, film breakup, liquid filament pinch-off, and droplet generation are well reproduced. The droplet size distribution satisfies the experimental measurement and theoretical predictions power-law. BAMR shows a huge advantage in computational efficiency than the traditional Cartesian grid. The findings of this study can help for a better understanding of the micro-mechanism of surface tension driven flows.

Spongy all-in-liquid materials by in-situ formation of emulsions at oil-water interfaces

Printing a structured network of functionalized droplets in a liquid medium enables engineering collectives of living cells for functional purposes and promises enormous applications in processes ranging from energy storage to tissue engineering. Current approaches are limited to drop-by-drop printing or face limitations in reproducing the sophisticated internal features of a structured material and its interactions with the surrounding media. Here, we report a simple approach for creating stable liquid filaments of silica nanoparticle dispersions and use them as inks to print all-in-liquid materials that consist of a network of droplets. Silica nanoparticles stabilize liquid filaments at Weber numbers two orders of magnitude smaller than previously reported in liquid-liquid systems by rapidly producing a concentrated emulsion zone at the oil-water interface. We experimentally demonstrate the printed aqueous phase is emulsified in-situ; consequently, a 3D structure is achieved with flexible walls consisting of layered emulsions. The tube-like printed features have a spongy texture resembling miniaturized versions of “tube sponges” found in the oceans. A scaling analysis based on the interplay between hydrodynamics and emulsification kinetics reveals that filaments are formed when emulsions are generated and remain at the interface during the printing period. Stabilized filaments are utilized for printing liquid-based fluidic channels.

Manifold death: A Volume of Fluid implementation of controlled topological changes in thin sheets by the signature method

A well-known drawback of the Volume-Of-Fluid (VOF) method is that the breakup of thin liquid films or filaments is mainly caused by numerical aspects rather than by physical ones. The rupture of thin films occurs when their thickness reaches the order of the grid size and by refining the grid the breakup events are delayed. When thin filaments rupture, many droplets are generated due to the mass conserving properties of VOF. Thus, the numerical character of the breakup does not allow obtaining the desired convergence of the droplet size distribution upon grid refinement. In this work, we present a novel algorithm to detect and perforate thin structures. First, thin films or ligaments are identified by taking quadratic moments of an indicator obtained from the volume fraction. A multiscale approach allows us to choose the critical film thickness independently of the mesh resolution. Then, the breakup is induced by making holes in the films before their thickness reaches the grid size. We show that the method improves the convergence upon grid refinement of the droplets size distribution and of enstrophy.

Deformation and breakup of water droplets containing polymer under a DC electric field

AbstractThe deformation and breakup behaviors of droplets containing polymer are investigated by high‐speed photography to reveal the mechanism of electric field collapse when treating the emulsion containing polymer. The results show the electric field collapse process caused by droplet breakup consists of three steps. First, the droplet containing polymer ejects liquid filament under strong electric field. Second, the liquid filament is continuously stretched until it touches positive and negative electrodes. Finally, the severe Joule heating effect appears because the strong electric current flows through the liquid filament. The release of a large amount of heat causes the formation, expansion, and collapse of bubble, resulting in the fracture of liquid filament and the collapse of electric field. The increase of polymer concentration strengthens the Joule heating effect and enlarges the influence scope of bubble expansion and collapse. These findings provide significant guidance for the stable operation of electric dehydrator.

Numerical Investigation on Spray Characteristics With Upstream Flow Pulsation of a Pintle Injector

The volume of fluid (VOF) model and the adaptive mesh refinement (AMR) method are used to study the spray characteristics of a gas–liquid pintle injector and the effects of mass flow pulsation of liquid on it. A pintle injector is a thrust adjusting device that changes the injection area using movable parts. Pressure pulsation in the supply pipeline is simulated by periodically changing the mass flow rate of the inlet. Spray characteristics with constant and pulsating upstream flows are compared with each other. The effect of frequency and amplitude of upstream liquid flow pulsation on the spray performance was studied. The results reveal that holding the mass flow rate of the upstream liquid flow constant, under the impact of gas flow, the liquid block, the liquid filament, and a large number of small droplets are peeled off from the liquid film. The film breakup position stays relatively fixed, and the spray has a conical shape. However, when the upstream liquid flow is pulsating, the film breakup position changes periodically, and the spray has a “Christmas tree”-shape. The pulsation frequency has little effect on the spray angle. But it strongly determines the droplet size and the spatial distribution of the spray. In addition, the pulsation amplitude can enhance the phenomenon of “Christmas tree.” With the increase in pulsation amplitude, the liquid film at the outlet of the pintle injector appears with a periodic phenomenon of “contraction–expansion.”

The Effect of Surface Wettability on Viscoelastic Droplet Dynamics under Electric Fields.

The effects of surface wettability and viscoelasticity on the dynamics of liquid droplets under an electric field are studied experimentally. A needle-plate electrode system is used as the power source to polarize a dielectric plate by the corona discharge emitted at the needle electrode, creating a new type of steerable electric field realized. The dynamics of droplets between the dielectric plate and a conductive substrate include three different phenomena: equilibrium to a stationary shape on substrates with higher wettability, deformation to form a bridge between the top acrylic plate and take-off on the substrates with lower wettability. Viscoelastic droplets differ from water in the liquid bridge and takeoff phenomena in that thin liquid filaments appear in viscoelastic droplets, not observed for Newtonian droplets. The equilibrated droplet exhibits more pronounced heights for Newtonian droplets compared to viscoelastic droplets, with a decrease in height with the increase in the concentration of the elastic constituent in the aqueous solution. In the take-off phenomenon, the time required for the droplet to contact the upper plate decreases with the concentration of the elastic constituent increases. It is also found that the critical voltage required for the take-off phenomenon to occur decreases as the elasticity increases.

Cumulative geometric frustration and superextensive energy scaling in a nonlinear classical XY -spin model

Geometric frustration results from a discrepancy between the locally favored arrangement of the constituents of a system and the geometry of the embedding space. Geometric frustration can be either noncumulative, which implies an extensive energy growth, or cumulative, which implies superextensive energy scaling and highly cooperative ground-state configurations which may depend on the dimensions of the system. Cumulative geometric frustration was identified in a variety of continuous systems including liquid crystals, filament bundles, and molecular crystals. However, a spin-lattice model which clearly demonstrates cumulative geometric frustration was lacking. In this paper we describe a nonlinear variation of the XY-spin model on a triangular lattice that displays cumulative geometric frustration. The model is studied numerically and analyzed in three distinct parameter regimes, which are associated with different energy minimizing configurations. We show that, despite the difference in the ground-state structure in the different regimes, in all cases the superextensive power-law growth of the frustration energy for small domains grows with the same universal exponent that is predicted from the structure of the underlying compatibility condition.

Dendritic Pattern Formation and Contact Line Forces during Dewetting of Dilute Polymer Solutions on a Hydrophobic Surface

The micro-scale morphology of the receding contact line of dilute polyethylene oxide solution drops (c ∼ 100 ppm) after impact and inertial spreading on a fluorinated hydrophobic surface is investigated. One can observe the formation of transient liquid filaments and dendritic structures that evolve into a bead-on-a-string structure similar to the well-known capillary breakup mechanism of dilute polymer solutions, which confirm the interaction between stetched polymer coils and the receding three-phase contact line. The estimation of the average polymer force per unit contact line lenght provides a quantitative explanation for the reduction of the contact line retraction velocity reduction observed experimentally.

Direct imaging of polymer filaments pulled from rebounding drops.

Polymer filaments form the foundation of biology from cell scaffolding to DNA. Their study and fabrication play an important role in a wide range of processes from tissue engineering to molecular machines. We present a simple method to deposit stretched polymer fibers between micro-pillars. This occurs when a polymeric drop impacts on and rebounds from an inclined superhydrophobic substrate. It wets the top of the pillars and pulls out liquid filaments which are stretched and can attach to adjacent pillars leaving minuscule threads, with the solvent evaporating to leave the exposed polymers. We use high-speed video at the microscale to characterize the most robust filament-forming configurations, by varying the impact velocity, substrate structure and inclination angle, as well as the PEO-polymer concentration. Impacts onto plant leaves or a randomized nano-structured surface leads to the formation of a branched structure, through filament mergers at the free surface of the drop. SEM shows the deposition of filament bundles which are thinner than those formed by evaporation or rolling drops. Raman spectroscopy identifies the native mode B stretched DNA filaments from aqueous-solution droplets.

The retraction of jetted slender viscoelastic liquid filaments

Long and slender liquid filaments are produced during inkjet printing, which can subsequently either retract to form a single droplet, or break up to form a primary droplet and one or more satellite droplets. These satellite droplets are undesirable since they degrade the quality and reproducibility of the print, and lead to contamination within the enclosure of the print device. Existing strategies for the suppression of satellite droplet formation include, among others, adding viscoelasticity to the ink. In the present work, we aim to improve the understanding of the role of viscoelasticity in suppressing satellite droplets in inkjet printing. We demonstrate that very dilute viscoelastic aqueous solutions ($\text {concentrations} \sim 0.003\,\%$ wt. polyethylene oxide, corresponding to nozzle Deborah number$De_{n}\sim 3$) can suppress satellite droplet formation. Furthermore, we show that, for a given driving condition, upper and lower bounds of polymer concentration exist, within which satellite droplets are suppressed. Satellite droplets are formed at concentrations below the lower bound, while jetting ceases for concentrations above the upper bound (for fixed driving conditions). Moreover, we observe that, with concentrations in between the two bounds, the filaments retract at velocities larger than the corresponding Taylor–Culick velocity for the Newtonian case. We show that this enhanced retraction velocity can be attributed to the elastic tension due to polymer stretching, which builds up during the initial jetting phase. These results shed some light on the complex interplay between inertia, capillarity and viscoelasticity for retracting liquid filaments, which is important for the stability and quality of inkjet printing of polymer solutions.

Effect of an axial electric field on the breakup of a leaky-dielectric liquid filament

We study experimentally and numerically the thinning of Newtonian leaky-dielectric filaments subjected to an axial electric field. We consider moderately viscous liquids with high electrical permittivity. We analyze the influence of the electric field on the formation of satellite droplets from the breakup of the filaments in the experiments. The electric force delays the free surface pinching. Two electrified filaments with the same minimum radius are thin at the same speed regardless of when the voltage is applied. The numerical simulations show that the polarization stress is responsible for the pinching delay observed in the experiments. Asymptotically close to the pinching point, the filament pinching is dominated by the diverging hydrodynamic forces. The polarization stress becomes subdominant even if this stress also diverges at this finite-time singularity.

Experiment on bubble formation through dynamical wetting transition in a square capillary

We experimentally study the displacement of viscous liquid by gas in a square capillary tube. The liquid is partially wetting in a way that no spontaneous imbibition along the interior corners would occur even in the absence of forced displacement. The gas–liquid interface exhibits a variety of morphologies with an increasing displacement rate. At a low displacement rate, a constantly moving meniscus can be observed, without any liquid deposition on the tube wall. An increase in the displacement rate gives rise to the deposition of two ultra-thin liquid filaments at each corner, which immediately break into tiny droplets. An additional thicker filament is entrained at each corner as the displacement rate further increases, connecting the thinner ones and the meniscus. When the displacement rate is high, liquid films are entrained on the tube wall and eventually collapse, entrapping an amount of gas in the form of Taylor bubbles. Quantitative measurements show that both the thicker filaments and the liquid films retract at constant speeds. Empirical relations predicting the film thickness and the bubble length are proposed and agree with the experimental results.