Filament Breakup Research Articles

Satellite droplet formation and its inertial separation in integrated microchannels

The breakup of filament is often accompanied by the formation of satellite droplet, which affects the uniformity of droplets, and inertial microfluidic technology is usually used to separate droplets of different sizes. In this study, different filament breakup modes are observed, and the variation of satellite droplet size with operating conditions are analyzed. Based on the experimental results, predictive model for the satellite droplet size is proposed. To better control the separation of particles with different sizes, the focusing and separation process of main and satellite droplets in microdevices are investigated by numerical simulation. It is found that increasing the fluid flow rate, the width of microchannel and rear cavity, and reducing the satellite droplet size are all beneficial for the separation of droplets of different sizes. At last, models for predicting the focusing positions of main and satellite droplets in rear cavity are established, and an integrated microfluidic device is proposed.

Effects of dispersing media on the rheological and tribological properties of basil seed mucilage-based thickened liquids.

The development of thickening powders for the management of dysphagia is imperative due to the rapid growth of aging population and prevalence of the dysphagia. One promising thickening agent that can be used to formulate dysphagia diets is basil seed mucilage (BSM). This work investigates the effects of dispersing media, including water, milk, skim milk, and apple juice, on the rheological and tribological properties of the BSM-thickened liquids. Shear rheology results revealed that the thickening ability of BSM in these media in ascending order is milk < skim milk ≈ apple juice < water. On the other hand, extensional rheology demonstrated that the longest filament breakup time was observed when BSM was dissolved in milk, followed by skim milk, water, and apple juice. Furthermore, tribological measurements showed varying lubrication behavior, depending on the BSM concentration and dispersing media. Dissolution of BSM in apple juice resulted in the most superior lubrication property compared with that in other dispersing media. Overall, this study provides insights on BSM's application as a novel gum-based thickening powder in a range of beverages and emphasizes how important it is for consumers to have clear guidance for the use of BSM in dysphagia management.

Drop size characteristics of sprays emanating from circular and non-circular orifices in the atomization regime

Traditionally, circular orifices have been used for generating aerosols, however in recent times non-circular orifices are being considered due to their superior atomization and mixing features. In this work, we experimentally investigate spray morphology and drop size characteristics of kerosene (Jet A-1) jets issuing from three non-circular orifices geometries (elliptic, triangular and, square) and one circular orifice with the same exit cross sectional area. Our results show an unexpected and yet unreported coarsening of atomization for non-circular orifice jets quantified by an increase in the Sauter Mean Diameter (SMD) at all tested exit velocities represented by the liquid Weber number, Wel. We attribute this to two distinct spray morphologies: filament and core breakup which generate large size liquid structures identified as filaments and ligaments, noticeable in non-circular orifices jets compared to circular orifice jets. On exploring this further by undertaking a detailed examination of the drop size volume probability distribution at lower Wel corresponding to the location marking the end of primary breakup we see a bimodality due to a dominant distribution of fragments of small and larger sizes owing to the spray morphology. At higher Wel and larger distances from the injector exit we observe the bimodality converts to a unimodal distribution for all orifice jets with a single peak situated at lower drop diameters. Filament breakup is reasoned to be the cause of higher number of smaller drop sizes in triangular sprays among all non-circular orifice jets while thinner filaments in circular orifice sprays lead to smaller drop sizes compared to their triangular counterparts showing the same breakup morphology. We expect our results to help formulation of atomization strategies based on only changing the orifice geometry which heretofore have been achieved by complicated means thereby beneficially impacting applications as diverse as engine fuel combustion, pharmaceutical sprays and CO2 capture by NaOH sprays.

Breakup of quantum liquid filaments into droplets

We investigate how the Rayleigh-Plateau instability of a filament made of a $^{41}\mathrm{K}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ self-bound mixture may lead to an array of identical quantum droplets, with typical breaking times which are shorter than the lifetime of the mixture. If the filament is laterally confined, as it happens in a toroidal trap, and atoms of one species are in excess with respect to the optimal equilibrium ratio, the droplets are immersed into a superfluid background made by the excess species which provides global phase coherence to the system, suggesting that the droplet array in the unbalanced system may display supersolid character. This possibility is investigated by computing the nonclassical translational inertia coefficient. The filament may be a reasonable representation of a self-bound mixture subject to toroidal confinement when the bigger circle radius of the torus is much larger than the filament radius.

Effects of barium sulfate on rheological properties and IDDSI flow consistency of liquid stimuli prepared using commercial thickening powders.

During videofluoroscopic swallowing study (VFSS), barium sulfate (BaSO4 ) is commonly added into food samples as a radiopaque contrast media for bolus visualization and examination. Accordingly, the consistency and flow behavior of barium stimuli can differ significantly from their non-barium counterparts. Such differences may have a subsequent impact on the validity of VFSS. Therefore, in this study, effects of barium sulfate on the shear and extensional rheological properties and IDDSI (International Dysphagia Diet Standardization Initiative) flow consistency of liquids prepared using various commercial thickening powders were investigated. Results showed that all barium stimuli exhibited shear thinning behavior but with significantly higher shear viscosity compared to the non-barium counterparts. A shift factor of viscosity at shear rate 50 s-1 with values in range of 1.21-1.73 could be used to describe the increase in the viscosity for samples thickened with gum-based thickeners. However, the change in the viscosity was not invariant for the stimuli prepared starch-based thickener. The addition of BaSO4 had a negative impact on extensional properties of samples by demonstrating a faster filament rupture. The extent of impact on the decrease in filament breakup time was more pronounced in xanthan > guar gum ≈ tara gum-based thickeners. Based on the IDDSI flow test, no significant effect of BaSO4 was found on the gum-based thickeners, whereas there was a marked effect in the starch-based sample. These results can be used beneficially to assist clinicians in the dysphagia diagnosis for matching rheological properties of the barium stimuli to enhance effectiveness dysphagia interventions.

Investigation and prediction of the extensional viscosity of okra mucilage using the Giesekus model.

Okra is a vegetable that is widely grown around the world. Okra mucilage contains a high mucus concentration that can be useful for supporting the swallowing process. Although the extensional rheology of okra mucilage is essential to its flow, its extensional viscosity has not received much attention. Using a filament stretching rheometer, the extensional viscosity of the mucilage in okra was examined. The Giesekus model was used to predict this parameter. The okra mucilage with different concentrations was extracted from fresh okra. The extensional viscosity was measured using a filament breakup apparatus. The diameter of the liquid bridge was measured by a laser micrometer and it was also observed by a high-speed camera. A rotational rheometer was used to measure the shear viscosity. In addition, the master curves for the shear viscosity were plotted to eliminate the influence of solvent and shear rate and evaluate the influence of concentration on the elasticity of okra mucilage. The okra mucilage shear and extensional viscosity were predicted using the Giesekus model. Every sample of okra mucilage exhibited shear thinning behavior. In addition to having a high extensional viscosity that is hundreds of times higher than its shear viscosity, okra mucilage also exhibited stretching phenomena. The master curves demonstrated that the pseudoplasticity of the okra mucilage increased along with the concentration. The rheological behavior of the mucilage in okra can be explained by the Giesekus model. Okra mucilage's shear viscosity exhibited shear thinning behavior and a strong extensional viscosity that was significantly higher than its shear viscosity. The shear and extensional viscosity of okra mucilage can be described and predicted using the Giesekus model.

Structural and shear and extensional rheological properties of hairy basil seed mucilage for potential application as oropharyngeal dysphagia diets

In this study, mucilage was extracted from the swollen hairy basil (Ocimum × africanum Lour.) seeds using ultrasonic-assisted extraction. Clear mucilage (BSM) was subsequently obtained by centrifugation for studies on its structural and shear and extensional rheological properties. The results revealed that the extracted BSM was rich in galactose and xylose, but low in mannose and galacturonic acid contents which indicated the presence of arabinogalactan and xylan. Small amounts of proteins (∼5%) were also detected. The average molecular weight of BSM was estimated as ∼480 kDa. Shear rheology results showed that aqueous dispersions of BSM at 1.0–2.5%w/v exhibited solid-like viscoelastic behavior with small frequency dependency, resembling a cross-linked gel. All BSM samples also exhibited shear thinning behavior that could be described by the power law model. However, the Cox-Merz rule was not obeyed for all studied BSM gels. The gel's elasticity was enhanced with increasing BSM concentration. For extensional rheology, the apparent extensional viscosity showed an extensional strain-softening behavior and demonstrated higher filament breakup time and extensional viscosity with increasing BSM concentration. A comparison between BSM and commercial thickening agents widely used in dysphagia management showed comparable rheological characteristics, thus suggesting the promising potential of BSM as a thickening agent for this application.

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.

Extensional rheology of hydrophobically associating polyacrylamide solution used in chemical flooding: Effects of temperature, NaCl and surfactant

The study provides an insight into the oil washing mechanism of hydrophobically associating polyacrylamide (HAHPAM) in chemical flooding from the perspective of extensional rheology. The extensional rheology of HAHPAM solution was systematically investigated under different influencing factors (concentration, temperature, NaCl and surfactant) using capillary breakup extensional rheometry (CaBER). Compared with partially hydrolyzed polyacrylamide (HPAM), the HAHPAM solution shows better extensibility due to the additional hydrophobic interaction between molecular chains, particularly at high temperature and high salinity. By increasing the concentration and decreasing the temperature, the filament breakup time and extensional viscosity of the HAHPAM solution increase. NaCl delays the attenuation of the HAHPAM filament by increasing the solution extensional viscosity and surface tension. In the presence of sodium dodecyl sulfate (SDS), the reduced surface tension dominates the stability of the filament and weakens the extensibility of the HAHPAM/SDS composite solution.

Thermally-modulated shape transition at the interface of soft gel filament and hydrophobic substrate

A liquid filament may pinch off into different shapes on interacting with a soft surface, as modulated by the interplay of inertial, capillary, and viscous forces. While similar shape transitions may intuitively be realized for more complex materials such as soft gel filaments as well, their intricate controllability towards deriving precise and stable morphological features remains challenging, as attributed to the complexities stemming from the underlying interfacial interactions over the relevant length and time scales during the sol–gel transition process. Circumventing these deficits in the reported literature, here we report a new means of precisely-controlled fabrication of gel microbeads via exploiting thermally-modulated instabilities of a soft filament atop a hydrophobic substrate. Our experiments reveal that abrupt morphological transitions of the gel material set in at a threshold temperature, resulting in spontaneous capillary thinning and filament breakup. We show that this phenomenon may be precisely modulated by an alteration in the hydration state of the gel material that may be preferentially dictated by its intrinsic glycerol content. Our results demonstrate that the consequent morphological transitions give rise to topologically-selective microbeads as an exclusive signature of the interfacial interactions of the gel material with the deformable hydrophobic interface underneath. Thus, intricate control may be imposed on the spatio-temporal evolution of the deforming gel, facilitating the inception of highly ordered structures of specific shapes and dimensionalities on demand. This is likely to advance the strategies of long shelf-life analytical biomaterial encapsulations via realizing one-step physical immobilization of bio-analytes on the bead surfaces as a new route to controlled materials processing, without demanding any resourced microfabrication facility or delicate consumable materials.

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.

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.

Bubble production by air filament and cavity breakup in plunging breaking wave crests

Air filaments and cavities in plunging breaking waves, generically cylinders, produce bubbles through an interface instability. The effects of gravity, surface tension and surface curvature on cylinder breakup are explored. A generalized dispersion relation is obtained that spans the Rayleigh–Taylor and Plateau–Rayleigh instabilities as cylinder radius varies. The analysis provides insight into the role of surface tension in the formation of bubbles from filaments and cavities. Small filaments break up into bubbles through a Plateau–Rayleigh instability driven through the action of surface tension. Large air cavities produce bubbles through a Rayleigh–Taylor instability driven by gravity and moderated by surface tension, which has a stabilizing effect. Surface tension, interface curvature and gravity are all important for cases between these two extremes. Predicted unstable mode wavenumber and bubble size show good agreement with direct numerical simulations of plunging breaking waves and air cylinders.

Extensional flow of stiff‐chain polymer solutions in the semidilute regime

AbstractStiff‐chain polymers are preponderant in biology as structural components of tissue and living cells. Hence, understanding their solution behavior under different flow histories is fundamental. Here, the extensional flow under capillary breakup of isotropic semidilute solutions of stiff‐chain poly‐n‐butyl isocyanate (PBIC) and hydroxypropyl cellulose (HPC) has been studied. PBIC‐chloroform and HPC‐water were studied as the polymer‐solvent interaction tunes the molecular conformation (Romo‐Uribe, J. Appl. Polym. Sci. 2021, 138, e49712). PBIC in chloroform exhibits a rod‐like conformation, and capillary breakup at constant concentration and temperature initially showed an exponential decay of filament diameter D(t) followed by a sudden decrease of D, and finally slow filament thinning. Increasing the molecular weight by threefold increased the breakup time τb by twofold. HPC in water adopts a semiflexible conformation and the filament thinning also exhibited three stages, however the breakup time was shorter than PBIC. Hence, the extensional viscosity as a function of strain of PBIC and HPC solutions exhibited three regimes, where initial constant extensional viscosity ηE was followed by strain hardening and finally a sudden decrease of extensional viscosity as strain increased. It was found that the filament breakup time τb is a function of the radius of gyration Rg.

Atomistic Insights on the Full Operation Cycle of a HfO2-Based Resistive Random Access Memory Cell from Molecular Dynamics.

We characterize the atomic processes that underlie forming, reset, and set in HfO2-based resistive random access memory (RRAM) cells through molecular dynamics (MD) simulations, using an extended charge equilibration method to describe external electric fields. By tracking the migration of oxygen ions and the change in coordination of Hf atoms in the dielectric, we characterize the formation and dissolution of conductive filaments (CFs) during the operation of the device with atomic detail. Simulations of the forming process show that the CFs form through an oxygen exchange mechanism, induced by a cascade of oxygen displacements from the oxide to the active electrode, as opposed to aggregation of pre-existing oxygen vacancies. However, the filament breakup is dominated by lateral, rather than vertical (along the filament), motion of vacancies. In addition, depending on the temperature of the system, the reset can be achieved through a redox effect (bipolar switch), where oxygen diffusion is governed by the applied bias, or by a thermochemical process (unipolar switch), where the diffusion is driven by temperature. Unlike forming and similar to reset, the set process involves lateral oxygen atoms as well. This is driven by field localization associated with conductive paths.

Inkjet Printability Assessment of Weakly Viscoelastic Fluid: A Semidilute Polyvinylpyrrolidone Solution Ink Case Study.

Here, we present an integrated approach to the weakly viscoelastic fluid printability assessment by using global dimensionless criteria (DC). The problem was studied on a model semidiluted polyvinylpyrrolidone water-based ink. For the study purpose, the ink composition was kept as simple as possible. First, the solution density, viscosity, and surface tension were determined. Obtained data were used for testing limitations of DC printability diagrams already available for Newtonian fluids. A replotted version of the original Kim and Baek's map was developed emphasizing the importance of surface tension in the drop formation process. Another set of DC (e.g., Ec and De) was also used for a real evaluation of the viscoelasticity effect on both jetting conditions and drop formation. The polymer relaxation time as a crucial parameter for viscoelasticity was shown to be calculated using the Kuhn segment length rather than from Zimm and Rouse theories for diluted polymer systems. Then, a two-dimensional diagram using four DC (Oh and De with Ec and El as parameters) is proposed based on the famous McKinley's work. The diagram describes the interplay of possible forces responsible for filament thinning and breakup processes. Obtained results were supported by further experiments involving drop ejection and formation, determination of critical polymer concentration, and others. The proposed diagram promises a useful initial step in further investigations of viscoelasticity of polymer compounds by inkjet printing.

Highlighting the need for high-speed imaging in capillary breakup extensional rheometry

The capillary breakup extensional rheometer is commonly used to determine material properties of complex fluids. This is achieved by tracking the diameter evolution of a liquid bridge undergoing capillary thinning and breakup in a uniaxial extensional flow. Typically, the filament diameter evolution is tracked at the mid-plane between the two end-plates using a laser micrometer. We show using high-speed imaging that while this arrangement is satisfactory in flows where the filament is long (relative to its initial diameter), slender and approximately cylindrical, errors can be significant when the filaments are short (and with a non-negligible curvature) such as encountered when using the so-called slow-retraction-method and ‘Dripping-onto-Substrate’ rheometry. We will further highlight the need for high-speed imaging in CaBER experiments by considering errors induced when the laser micrometer is misaligned with the location of filament breakup. This latter source of error will be particularly relevant for capillary breakup experiments where the location of filament breakup is not typically known a priori, such as the case for many so-called ‘yield-stress’ fluids.

Simulation of a liquid jet in supersonic crossflow by a hybrid CLSVOF-LPT method

This study investigates the atomization process of a liquid jet in supersonic crossflow at a Mach number of 2.1 by using a multi-scale simulation method. This hybrid numerical method can realize accurate simulation of the primary and secondary breakup of the liquid jet with large scale instabilities and small droplet dynamics under compressible supersonic crossflow. The Eulerian framework is applied for capturing the gas-liquid interface by a coupled Level set and Volume-of-fluid (CLSVOF) method. Based on the Delayed Detached Eddy Simulation (DDES), the adaptive mesh refinement (AMR) method is introduced to predict the breakup of liquid column, blocks and filaments. The Lagrangian framework is applied for the dispersed droplets generated under the conversion criteria during the atomization process. The hybrid CLSVOF-LPT method can greatly reduce the loss of liquid phase mass and save computing resources. The distribution morphology of droplets in a cross-sectional plane from the simulation agreed well with the experimental data. Besides, the trends of the jet trajectory and primary breakup behavior of liquid column are well captured.

An improved Capillary Breakup Extensional Rheometer to characterize weakly rate-thickening fluids: Applications in synthetic automotive oils

We describe a customized Capillary Breakup Extensional Rheometer (CaBER) with improved dynamic performance and added features for temperature control over the range from room temperature up to 250 °C. The system is aimed at characterizing the extensional rheological behavior of weakly rate-thickening fluids that are widely utilized in the automotive industry. We examine the shear rheology and filament-thinning dynamics of two commercially available automotive fluids with the same viscosity index. Comparisons of the rheological properties of the two samples reveal that although they have identical shear viscosities, they exhibit significant and distinct rate-thickening behavior in the strong extensional flow that is generated close to filament breakup. For the more elastic sample, the exponential filament-thinning dynamics are well-described by the Oldroyd-B model; however, this viscoelastic model poorly describes the response of the more weakly rate-thickening fluid. To address this limitation, we propose a simple Inelastic Rate-Thickening (IRT) model that more robustly describes the measured material response. The two constitutive parameters of the model represent the zero-shear-rate viscosity of the fluid and the rate of extensional thickening in the fluid. Numerical calculations with the IRT model show that the radii of thinning fluid filaments deviate from a linear decay in time and approach a quadratic dependence very close to breakup. By carefully fitting the measured temporal evolution of the mid-plane radius we can therefore systematically differentiate the extensional rheological response of the two oils. More generally, we show that the weakly rate-thickening regime can be distinguished from the well-known elasto-capillary response predicted by the Oldroyd-B model, via a constraint on the relaxation time (or more specifically the elasto-capillary number) of the fluid. The weakly rate-thickening behavior documented in these oils is representative of the relatively unstudied extensional rheology of many industrial fluids at large extensional strain rates (100–1000 s-1) and influences many complex industrial processes such as jetting, coating and stamping.

A numerical study of liquid compound filament contraction

Droplets resulting from liquid filament contraction have been widely used in industrial processes. However, detailed investigations of liquid compound filament contraction processes are lacking in the literature. Therefore, this study provides a numerical investigation of the contraction of a two-layered compound filament. The simulations are based on an axisymmetric front-tracking method. It is found that because of the interfacial tension force, the initially long cylindrical filament contracts to a compound droplet without any breakup or breaks up into smaller droplets during contraction. Unlike simple filaments, the presence of the inner filament inside the compound filament results in a more complicated compound filament breakup process with various droplet types, e.g., simple droplets, single-core compound droplets, and multi-core compound droplets. We find that the inner filament breaks up into droplets, while the outer does not induce breakup. Such a breakup mode produces a multi-core compound droplet after contraction. In some cases, while the inner filament only contracts to a single droplet, its enclosing filament breaks up to produce simple droplets at each end. We also find a breakup mode that combines these two modes, where both the inner and outer filaments perform breakup. In addition, the breakup of the compound filament occurs via one of two mechanisms: end-pinching and necking. These breakup modes and mechanisms are affected by various parameters such as the inner and outer aspect ratios, the Ohnesorge number, the interfacial tension ratio, and the viscosity ratios. Based on these parameters, various regime diagrams of breakup and non-breakup are proposed.