258 results found
Lapidot T, Matar OK, Heng JYY, 2019, Calcium sulphate crystallisation in the presence of mesoporous silica particles: experiments and population balance modelling, Chemical Engineering Science, Vol: 202, Pages: 238-249, ISSN: 0009-2509
A population balance model is used to investigate the effect of mesoporous silica particles on calcium sulphate crystallisation in a stirred batch crystalliser. The model accounts for nucleation, growth, agglomeration, breakage, and particle-assisted nucleation, and the model equations are solved numerically using the method of classes over a logarithmic, non-uniform mesh. The crystallisation process is characterized experimentally using electrical conductivity to track the ion concentration and laser diffraction to measure the steady-state crystal size distribution obtained at the end of the experiments. The experiments are carried out over a range of temperatures, initial supersaturations, particle pore diameters, and particle loadings. The model is first fitted to experimental data obtained in the absence of particles to determine kinetic parameters of the nucleation, growth, agglomeration, and breakage for pure calcium sulphate crystallisation. Varying pore diameter did not influence the catalytic effect of the particles, however, particle loading was found to significantly decrease the nucleation induction time. The model was extended to account for the presence of particles by fitting two additional mechanisms. The first proposed a particle-assisted nucleation where nuclei are produced via heterogeneous crystallisation, then detach by particle-particle collision that is second-order with respect to particle loading. The second proposed that the crystal breakage frequency increases linearly with particle loading. Good agreement with the experimental data is demonstrated over the range of conditions examined.
Moran H, Magnini M, Markides C, et al., 2019, Inertial and buoyancy effects on horizontal flow of elongated bubbles in circular channels, ICMF 2019, Publisher: ICMF
The effects of gravity and inertia on the liquid film thickness surrounding elongated bubble flow in a horizontal tube of circularcross-section are studied through numerical simulations. At low Reynolds and Bond numbers, the inertial and buoyancy effectsare negligible and the liquid film thickness at the tube wall is a function of the Capillary number only; if tube diameter isincreased to the millimetre scale, however, buoyancy forces become significant. Simulations are performed with OpenFOAM(version 1606) and the built in Volume-of-Fluid method for a range of Reynolds, Bond and Capillary numbers, namelyRe= 1−1000,Bo= 0.05−0.42andCa= 0.02−0.09respectively. Two-dimensional simulations capture asymmetry ofthe liquid film thickness due to gravitational effects, but do not capture bubble inclination relative to the channel centreline,as has been demonstrated experimentally in the literature. Three-dimensional simulations capture the transverse flow of thefilm as it drains from the top to the bottom of the tube, and are thus able to demonstrate bubble inclination. Further simulationsthat introduce phase change to the elongated bubble model are underway, aiming to investigate boiling flows, with experimentsbeing performed for comparison and validation.
Conroy DT, Espin L, Matar OK, et al., 2019, Thermocapillary and electrohydrodynamic effects on the stability of dynamic contact lines, Physical Review Fluids, Vol: 4, ISSN: 2469-990X
Motivated by the need to understand how external fields influence the stability of dynamic contact lines, the linear stability of gravity-driven spreading of a thin liquid film in the presence of electric and temperature fields is studied. The film is confined from below by a flat substrate and from above by an air gap and another flat substrate. An electrostatic potential difference or temperature difference can be applied between the two substrates and the liquid is taken to be a perfect dielectric whose surface tension decreases linearly with temperature. Traveling-wave solutions are found for the film profile, and both electric and temperature fields influence the height of the capillary ridge of liquid that forms near the advancing contact line. The linear stability analysis shows that electric fields destabilize the film front to transverse perturbations and that temperature fields can either stabilize or destabilize the front, depending on the direction of the temperature gradient. An energy analysis reveals that the electric field in the capillary ridge is most responsible for the enhancement of the perturbation growth. For the case of temperature fields, the perturbed temperature gradients are the dominant mechanism through which the perturbation in film height is affected.
Magnini M, Khodaparast S, Matar OK, et al., 2019, Dynamics of long gas bubbles rising in a vertical tube in a cocurrent liquid flow, Physical Review Fluids, Vol: 4, ISSN: 2469-990X
When a confined long gas bubble rises in a vertical tube in a cocurrent liquid flow, its translational velocity is the result of both buoyancy and mean motion of the liquid. A thin film of liquid is formed on the tube wall and its thickness is determined by the interplay of viscous, inertial, capillary and buoyancy effects, as defined by the values of the Bond number (Bo≡ρgR2/σ with ρ being the liquid density, gthe gravitational acceleration, R the tube radius, and σ the surface tension), capillary number (Cab≡μUb/σ with Ub being the bubble velocity and μ the liquid dynamic viscosity), and Reynolds number (Reb≡2ρUbR/μ). We perform experiments and numerical simulations to investigate systematically the effect of buoyancy (Bo=0–5) on the shape and velocity of the bubble and on the thickness of the liquid film for Cab=10−3–10−1 and Reb=10−2–103. A theoretical model, based on an extension of Bretherton's lubrication theory, is developed and utilized for parametric analyses; its predictions compare well with the experimental and numerical data. This study shows that buoyancy effects on bubbles rising in a cocurrent liquid flow make the liquid film thicker and the bubble rise faster, when compared to the negligible gravity case. In particular, gravitational forces impact considerably the bubble dynamics already when Bo<0.842, with Bocr=0.842 being the critical value below which a bubble does not rise in a stagnant liquid in a circular tube. The liquid film thickness and bubble velocity in a liquid coflow may vary by orders of magnitude as a result of small changes of Bo around this critical value. The reduction of the liquid film thickness for increasing values of the Reynolds numbers, usually observed for Reb≲102 when Bo≪1, becomes more evident at larger Bond numbers. Buoyancy effects also have a significant influence on the features of the undulation appearing near the rear m
Shahruddin S, Jimenez-Serratos G, Britovsek G, et al., 2019, Fluid-solid phase transition of n-alkane mixtures: Coarse-grained molecular dynamics simulations and diffusion-ordered spectroscopy nuclear magnetic resonance, Scientific Reports, Vol: 9, ISSN: 2045-2322
Wax appearance temperature (WAT), defined as the temperature at which the first solid paraffin crystal appears in a crude oil, is one of the key flow assurance indicators in the oil industry. Although there are several commonly-used experimental techniques to determine WAT, none provides unambiguous molecular-level information to characterize the phase transition between the homogeneous fluid and the underlying solid phase. Molecular Dynamics (MD) simulations employing the statistical associating fluid theory (SAFT) force field are used to interrogate the incipient solidification states of models for long-chain alkanes cooled from a melt to an arrested state. We monitor the phase change of pure long chain n-alkanes: tetracosane (C24H50) and triacontane (C30H62), and an 8-component surrogate n-alkane mixture (C12-C33) built upon the compositional information of a waxy crude. Comparison to Diffusion Ordered Spectroscopy Nuclear Magnetic Resonance (DOSY NMR) results allows the assessment of the limitations of the coarse-grained models proposed. We show that upon approach to freezing, the heavier components restrict their motion first while the lighter ones retain their mobility and help fluidize the mixture. We further demonstrate that upon sub-cooling of long n-alkane fluids and mixtures, a discontinuity arises in the slope of the self-diffusion coefficient with decreasing temperature, which can be employed as a marker for the appearance of an arrested state commensurate with conventional WAT measurements.
Xiao D, Heaney CE, Mottet L, et al., 2019, A reduced order model for turbulent flows in the urban environment using machine learning, Building and Environment, Vol: 148, Pages: 323-337, ISSN: 0360-1323
To help create a comfortable and healthy indoor and outdoor environment in which to live, there is a need to understand turbulent air flows within the urban environment. To this end, building on a previously reported method , we develop a fast-running Non-Intrusive Reduced Order Model (NIROM) for predicting the turbulent air flows found within an urban environment. To resolve larger scale turbulent fluctuations, we employ a Large Eddy Simulation (LES) model and solve the resulting computational model on unstructured meshes. The objective is to construct a rapid-running NIROM from these results that will have ‘similar’ dynamics to the original LES model. Based on Proper Orthogonal Decomposition (POD) and machine learning techniques, this Reduced Order Model (ROM) is six orders of magnitude faster than the high-fidelity LES model and we demonstrate how ‘similar’ it can be to the high-fidelity model by comparing statistical quantities such as the mean flows, Reynolds stresses and probability densities of the velocities. We also include validation of the high-fidelity model against data from wind tunnel experiments.This paper represents a key step towards the use of reduced order modelling for operational purposes with the tantalising possibility of it being used in place of Gaussian plume models, and the potential for greatly improved model fidelity and confidence.
Balla M, Tripathi MK, Sahu KC, et al., 2019, Non-isothermal bubble rise dynamics in a self-rewetting fluid: three-dimensional effects, Journal of Fluid Mechanics, Vol: 858, Pages: 689-713, ISSN: 0022-1120
The dynamics of a gas bubble in a square channel with a linearly increasing temperature at the walls in the vertical direction is investigated via three-dimensional numerical simulations. The channel contains a so-called ‘self-rewetting’ fluid whose surface tension exhibits a parabolic dependence on temperature with a well-defined minimum. The main objectives of the present study are to investigate the effect of Marangoni stresses on bubble rise in a self-rewetting fluid using a consistent model fully accounting for the tangential surface tension forces, and to highlight the effects of three-dimensionality on the bubble rise dynamics. In the case of isothermal and non-isothermal systems filled with a ‘linear’ fluid, the bubble moves in the upward direction in an almost vertical path. In contrast, strikingly different behaviours are observed when the channel is filled with a self-rewetting fluid. In this case, as the bubble crosses the location of minimum surface tension, the buoyancy-induced upward motion of the bubble is retarded by a thermocapillary-driven flow acting in the opposite direction, which in some situations, when thermocapillarity outweighs buoyancy, results in the migration of the bubble in the downward direction. In the later stages of this downward motion, as the bubble reaches the position of arrest, its vertical motion decelerates and the bubble encounters regions of horizontal temperature gradients, which ultimately lead to the bubble migration towards one of the channel walls. These phenomena are observed at sufficiently small Bond numbers (high surface tension). For stronger self-rewetting behaviour, the bubble undergoes spiralling motion. The mechanisms underlying these three-dimensional effects are elucidated by considering how the surface tension dependence on temperature affects the thermocapillary stresses in the flow. The effects of other dimensionless numbers, such as Reynolds and Froude numbers, are also investig
Voulgaropoulos V, Aguiar GM, Matar OK, et al., Temperature and velocity field measurements of pool boiling using two-colour laser-induced fluorescence, infrared thermometry and particle image velocimetry, 10th International Conference on Multiphase Flow
We study nucleate pool boiling in water at saturation temperature and ambient pressure under low heat fluxes. A combinationof high-speed and spatially-resolved diagnostic tools are developed and applied to provide detailed insight into the flow andheat transfer mechanisms during bubble life cycle. Two fluorescent dyes with non-overlapping spectra are seeded into thewater and are excited by a Nd:YLF laser sheet at 527 nm. A two-colour laser-induced fluorescence method is employedto individually track the fluorescence of each dye by connecting two cameras, equipped with separate optical filters, to abeamsplitter and a lens. Tracer particles are also introduced in the water to perform simultaneous particle image velocimetrymeasurements. Finally, synchronised high-speed infrared thermometry is conducted to acquire the surface temperature fieldover the heater. The links between the interfacial/bubble dynamics, flow and heat transfer are investigated. Superheated liquidfrom the thermal boundary layer adjacent to the heater is displaced upwards, due to the growth and departure of the bubbles.Two counteracting vortices form on each side of the bubbles during their departure and rise, which contribute to the scavengingand mixing of the bulk water, resulting in a trail of superheated liquid below them.
Lei Q, Xie Z, Pavlidis D, et al., 2018, The shape and motion of gas bubbles in a liquid flowing through a thin annulus, Journal of Fluid Mechanics, Vol: 285, Pages: 1017-1039, ISSN: 0022-1120
We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique ‘tadpole-like’ shape with a semi-circular cap and a highly stretched tail. As the annulus is inclined, the bubble tail tends to vanish, resulting in a significant decrease of bubble length. To model the bubble evolution, the thin annulus is conceptualised as a ‘Hele-Shaw’ cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model, which achieved a close match to the experimental data. The numerical model is further used to investigate the effects of gap thickness and pipe diameter on the bubble behaviour. The mechanism for the semi-circular cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder, based on which a theoretical solution to the bubble velocity is derived. The bubble motion and cap geometry is mainly controlled by the gravitational component perpendicular to the flow direction. The bubble elongation in the horizontal annulus is caused by the buoyancy that moves the bubble to the top of the annulus. However, as the annulus is inclined, the gravitational component parallel to the flow direction becomes important, causing bubble separation at the tail and reduction in bubble length.
Kahouadji L, Nowak E, Kovalchuk N, et al., 2018, Simulation of immiscible liquid-liquid flows in complex microchannel geometries using a front-tracking scheme, MICROFLUIDICS AND NANOFLUIDICS, Vol: 22, ISSN: 1613-4982
The three-dimensional two-phase flow dynamics inside a microfluidic device of complex geometry is simulated using a parallel, hybrid front-tracking/level-set solver. The numerical framework employed circumvents numerous meshing issues normally associated with constructing complex geometries within typical computational fluid dynamics packages. The device considered in the present work is constructed via a module that defines solid objects by means of a static distance function. The construction combines primitive objects, such as a cylinder, a plane, and a torus, for instance, using simple geometrical operations. The numerical solutions predicted encompass dripping and jetting, and transitions in flow patterns are observed featuring the formation of drops, ‘pancakes’, plugs, and jets, over a wide range of flow rate ratios. We demonstrate the fact that vortex formation accompanies the development of certain flow patterns, and elucidate its role in their underlying mechanisms. Experimental visualisation with a high-speed imaging are also carried out. The numerical predictions are in excellent agreement with the experimental data.
Adebayo IT, Matar OK, 2018, Film control to study contributions of waves to droplet impact dynamics on thin flowing liquid films, Journal of Visualized Experiments, Vol: 138, ISSN: 1940-087X
Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.
Wright SF, Charogiannis A, Voulgaropoulos V, et al., 2018, Laser-based measurements of stratified liquid-liquid pipe flows interacting with jets in cross-flow, 19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics
At low velocities, horizontal liquid-liquid flows uder go gravitation ally-induced stratification, which in many practical applications complicatessignificantly the direct measurement of the average properties of theflow. The extent of flow stratification, however, can be limited through in line mixing leading to the formation of liquid-liquid dispersions withmore homogenous properties. In this work, we focus on the use of‘active’ mixing methods using jets in cross flows (JICFs). In this paper,a dedicated experimental flow facility for the investigation of such flowsis presented, along with the accompanying laser-based optical measurement techniques and associated algorithms that have beendeveloped for this investigation. The facility allows simultaneous,space-and time-resolved phase and velocity information to be generatedvia plan ar laser-induced fluorescence (PLIF) and particle velocimetry(PIV/ PTV), with stereo-PIV used to provide information on the third (out-of-plane) velocity component. Preliminary experimental results arepresented which demonstrate the capabilities of this arrangementfor optically examining stratified liquid-liquid flows interacting withJICFs, leading to new insights into these complex flows. The key resultsinclude phenomena of jets interacting with the liquid-liquid inter face,recirculation zones that lead to further mixing, the presence of complexcompound droplets, droplet size distributions, and water concentrationprofiles.
Matar OK, 2018, (One possible) future of multiphase flow modelling and simulation, 11th North American Conference on Multiphase Technology 2018, Pages: 341-351
© 11th North American Conference on Multiphase Technology. All rights reserved. Understanding the dynamics of multiphase flows to enable their accurate and efficient prediction is of central importance in the oil-and-gas industry. Despite the significant advances that have been made in modelling these flows over a number of decades, however, numerous challenges, and open problems, remain. We present examples of progress made recently in the Multi-scale Examination of MultiPHase physIcs in flowS (MEMPHIS) programme aimed at providing reliable predictive tools. We also highlight the limitations of purely CFD-based approaches, and provide our perspective on potential predictive strategies based on a 'multi-fidelity' approach: a true fusion between modelbased and data-driven modelling. We are grateful for fruitful discussions with Prof. Panagiota Angeli (University College London), Dr Benoit Chachuat, Prof. Mark A. Girolami, Prof. Chris C. Pain (all from Imperial College London), and Prof. Mark J. Simmons (University of Birmingham, UK). The contribution and inspiration provided by Prof. Geoff F. Hewitt (Imperial College London), and the late Prof. Barry J. Azzopardi (University of Nottingham, UK) are also gratefully acknowledged; along with PA, CCP, MJS and OKM, GFH and BJA were the original co-investigators of the MEMPHIS programme. We also thank Drs Damir Juric and Jalel Chergui (both from LIMSI, CNRS, France), and Dr Seungwon Shin (Hongik University, South Korea) for their contribution to MEMPHIS. Finally, we would like to acknowledge financial support from the Engineering and Physical Sciences Research Council, UK, through the Multi-scale Examination of MultiPHase physIcs in flowS (MEMPHIS) Programme Grant (grant number EP/K003976/1), and from BP, and Procter & Gamble.
Smith ER, Theodorakis PE, Craster RV, et al., 2018, Moving Contact Lines: Linking Molecular Dynamics and Continuum-Scale Modeling., Langmuir, ISSN: 0743-7463
Despite decades of research, the modeling of moving contact lines has remained a formidable challenge in fluid dynamics whose resolution will impact numerous industrial, biological, and daily life applications. On the one hand, molecular dynamics (MD) simulation has the ability to provide unique insight into the microscopic details that determine the dynamic behavior of the contact line, which is not possible with either continuum-scale simulations or experiments. On the other hand, continuum-based models provide a link to the macroscopic description of the system. In this Feature Article, we explore the complex range of physical factors, including the presence of surfactants, which governs the contact line motion through MD simulations. We also discuss links between continuum- and molecular-scale modeling and highlight the opportunities for future developments in this area.
Jaeger F, Matar OK, Muller EA, 2018, Bulk viscosity of molecular fluids, Journal of Chemical Physics, Vol: 148, ISSN: 0021-9606
The bulk viscosity of molecular models of gases and liquids is determined by molecular simulations as acombination of a dilute gas contribution, arising due to the relaxation of internal degrees of freedom, and aconfigurational contribution, due to the presence of intermolecular interactions. The dilute gas contributionis evaluated using experimental data for the relaxation times of vibrational and rotational degrees of freedom.The configurational part is calculated using Green-Kubo relations for the fluctuations of the pressure tensorobtained from equilibrium microcanonical molecular dynamics simulations. As a benchmark, the Lennard-Jones fluid is studied. Both atomistic and coarse-grained force fields for water, CO2and n-decane areconsidered and tested for their accuracy. Comparison to experimental data, where present, demonstratesthat the tested models show various degrees of success in predicting bulk viscosity values, although atomisticforce fields in general seem to perform more consistently than the corresponding coarse-grained counterparts.The dilute gas contribution to the bulk viscosity is seen to be significant only in the cases when intramolecularrelaxation times are in theμs range, and for low vibrational wave numbers (<1000 cm−1); This explainsthe abnormally high values of bulk viscosity reported for CO2. In all other cases studied, the dilute gascontribution is negligible, and the configurational contribution dominates the overall behaviour. In particular,the configurational term is responsible for the enhancement of the bulk viscosity near the critical point.
Stafford JP, Patapas A, Uzo N, et al., Towards scale‐up of graphene production via nonoxidizing liquid exfoliation methods, AIChE Journal, ISSN: 0001-1541
Graphene, the two‐dimensional form of carbon, has received a great deal of attention across academia and industry due to its extraordinary electrical, mechanical, thermal, chemical, and optical properties. In view of the potential impact of graphene on numerous and diverse applications in electronics, novel materials, energy, transport, and healthcare, large‐scale graphene production is a challenge that must be addressed. In the past decade, top–down production has demonstrated high potential for scale‐up. This review features the recent progress made in top–down production methods that have been proposed for the manufacturing of graphene‐based products. Fabrication methods such as liquid‐phase mechanical, chemical and electrochemical exfoliation of graphite are outlined, with a particular focus on nonoxidizing routes for graphene production. Analysis of exfoliation mechanisms, solvent considerations, key advantages and issues, and important production characteristics including production rate and yield, where applicable, are outlined. Future challenges and opportunities in graphene production are also highlighted.
Ibarra R, Zadrazil I, Matar O, et al., 2018, Dynamics of liquid-liquid flows in horizontal pipes using simultaneous two-line planar laser-induced fluorescence and particle velocimetry, International Journal of Multiphase Flow, Vol: 101, Pages: 47-63, ISSN: 0301-9322
Experimental investigations are reported of oil-water stratified and stratified-wavy flows in horizontal pipes using a simultaneous two-line (two-colour) technique based on combining planar laser-induced fluorescence with particle image/tracking velocimetry. This approach allows the study of fluid combinations with properties similar to those encountered in industrial field-applications in terms of density, viscosity, and interfacial tension, even though their refractive indices are not matched. The flow conditions studied span mixture velocities in the range 0.3 – 0.6 m/s and low water-cuts up to 20%, corresponding to in situ (local) Reynolds numbers of 1750 – 3350 in the oil phase and 2860 – 11650 in the water phase, and covering the laminar/transitional and transitional/turbulent flow regimes for the oil and water phases, respectively. Detailed, spatiotemporally-resolved in situ phase and velocity data in a vertical plane aligned with the pipe centreline and extending across the entire height of the channel through both phases are analysed to provide statistical information on the interface heights, mean axial and radial (vertical) velocity components, (rms) velocity fluctuations, Reynolds stresses, and mixing lengths. The mean liquid-liquid interface height is mainly determined by the flow water cut and is relatively insensitive (up to 20% the highest water cut) to changes in the mixture velocity, although as the mixture velocity increases the interfacial profile transitions gradually from being relatively flat to containing higher amplitude waves. The mean velocity profiles show characteristics of both laminar and turbulent flow, and interesting interactions between the two co-flowing phases. In general, mean axial velocity profiles in the water phase collapse to some extent for a given water cut when normalised by the mixture velocity; conversely, profiles in the oil phase do not. Strong vertical velocity components can modify the shape of th
Vitale A, Hennessy M, Matar O, et al., 2018, Controlling the evolution of frontal photopolymerization waves for 3D polymeric patterning, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Shahruddin S, Jimenez-Serratos G, Britovsek G, et al., 2018, Exploring wax precipitation in crude oils through diffusion ordered spectroscopy nuclear magnetic resonance combined with molecular dynamics simulations, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Britovsek G, Tomov A, Muller E, et al., 2018, Ethylene oligomerisation beyond Schulz-Flory distributions, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Matar O, Lee RY, Shaffee A, et al., 2018, Sand agglomeration in oil & gas reservoirs: Chemistry, experiments and simulations, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Che Z, Matar OK, 2018, Impact of droplets on immiscible liquid films, Soft Matter, Vol: 14, Pages: 1540-1551, ISSN: 1744-683X
The impact of droplets on liquid films is a ubiquitous phenomenon not only in nature but also in many industrial applications. Compared to the widely-studied impact of droplets on films of identical fluids, the impact of droplets on immiscible films has received far less attention. In the present work, we show using high-speed imaging that immiscibility has a profound effect on the impact dynamics. The impact of a water droplet on an oil film leads to the formation of a compound crown followed by a central jet, whereas that of an oil droplet on a water film results in rapid spreading on the film surface driven by a large, positive spreading factor. In the former scenario, the central jet occurs due to the severe stretching of the droplet during the formation of the crown and then the retraction of the droplet by capillarity, which leads to the collision of fluid at the impact point. A model for the elongation dynamics of the central jet is proposed based on energy conservation. The effects of key parameters controlling the impact process are analysed, including the droplet Ohnesorge and Weber numbers, the viscosity ratio, and the dimensionless film thickness. Different impact outcomes are discussed, such as bouncing, deposition, and oscillation of the impact droplet, the formation and collapse of the compound crown, and the formation and tip-pinching of the central jet. This study not only provides physical insights into the impact dynamics, but could also facilitate the control and optimisation of the droplet impact process in a number of applications as highlighted herein.
Seungwon S, Chergui J, Juric D, et al., 2018, A hybrid interface tracking – level set technique for multiphase flow with soluble surfactant, Journal of Computational Physics, Vol: 359, ISSN: 0021-9991
A formulation for soluble surfactant transport in multiphase flows recently presented by Muradoglu & Tryggvason (JCP 274 (2014) 737–757) is adapted to the context of the Level Contour Reconstruction Method, LCRM, (Shin et al. IJNMF 60 (2009) 753–778) which is a hybrid method that combines the advantages of the Front-tracking and Level Set methods. Particularly close attention is paid to the formulation and numerical implementation of the surface gradients of surfactant concentration and surface tension. Various benchmark tests are performed to demonstrate the accuracy of different elements of the algorithm. To verify surfactant mass conservation, values for surfactant diffusion along the interface are compared with the exact solution for the problem of uniform expansion of a sphere. The numerical implementation of the discontinuous boundary condition for the source term in the bulk concentration is compared with the approximate solution. Surface tension forces are tested for Marangoni drop translation. Our numerical results for drop deformation in simple shear are compared with experiments and results from previous simulations. All benchmarking tests compare well with existing data thus providing confidence that the adapted LCRM formulation for surfactant advection and diffusion is accurate and effective in three-dimensional multiphase flows with a structured mesh. We also demonstrate that this approach applies easily to massively parallel simulations.
Hennessy M, Vitale A, Matar O, et al., 2017, Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation, Soft Matter, Vol: 13, Pages: 9199-9210, ISSN: 1744-683X
Frontal photopolymerisation (FPP) is a directional solidification process that converts monomer-rich liquid into crosslinked polymer solid by light exposure and finds applications ranging from lithography to 3D printing. Inherent to this process is the creation of an evolving polymer network that is exposed to a monomer bath. A combined theoretical and experimental investigation is performed to determine the conditions under which monomer from this bath can diffuse into the propagating polymer network and cause it to swell. First, the growth and swelling processes are decoupled by immersing pre-made polymer networks into monomer baths held at various temperatures. The experimental measurements of the network thickness are found to be in good agreement with theoretical predictions obtained from a nonlinear poroelastic model. FPP propagation experiments are then carried out under conditions that lead to swelling. Unexpectedly, for a fixed exposure time, swelling is found to increase with incident light intensity. The experimental data is well described by a novel FPP model accounting for mass transport and the mechanical response of the polymer network, providing key insights into how monomer diffusion affects the conversion profile of the polymer solid and the stresses that are generated during its growth. The predictive capability of the model will enable the fabrication of gradient materials with tuned mechanical properties and controlled stress development.
Matar OK, Wilson SK, 2017, Preface to the special issue celebrating 50 years of the Journal of Engineering Mathematics, Journal of Engineering Mathematics, Vol: 107, Pages: 1-4, ISSN: 0022-0833
Poesio P, Damone A, Matar OK, 2017, A multiscale approach to interpret and predict the apparent slip velocity at liquid-liquid interfaces, 35th Italian-Union-of-Thermp-Fluid Dynamics (UIT) Heat Transfer Conference (UIT), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Adebayo IT, Matar OK, 2017, Droplet impact on flowing liquid films with inlet forcing: the splashing regime, Soft Matter, Vol: 13, Pages: 7473-7485, ISSN: 1744-683X
The impact process of droplets falling obliquely on thin flowing films is studied using a high-speed imaging system with a focus on splashing. Frequency-forcing of the flow rate at the inlet is applied in order to form solitary waves prior to droplet impact. The outcomes associated with impact on targeted regions of the waves are examined; these include the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effect of varying the film flow rate, droplet size, and speed on the splashing regime for each of these regions is elucidated. The results are further compared with those associated with uncontrolled flowing films, and with quiescent films. The present work has demonstrated, for the first time, the contribution made by the spatial structure of waves to the outcome of droplet impact on flowing films.
Conroy D, Matar OK, 2017, Dynamics and stability of three-dimensional ferrofluid films in a magnetic field, Journal of Engineering Mathematics, Vol: 107, Pages: 253-268, ISSN: 0022-0833
We consider the interfacial dynamics of a thin, ferrofluid film flowing down an inclined substrate, under the action of a magnetic field, bounded above by an inviscid gas. The fluid is assumed to be weakly conducting, and its dynamics are governed by a coupled system of the steady Maxwell, Navier–Stokes, and continuity equations. The magnetization of the film is a function of the magnetic field, and is prescribed by a Langevin function. We make use of a long-wave reduction in order to solve for the dynamics of the pressure, velocity, and magnetic fields inside the film. The potential in the gas phase is solved by means of Fourier Transforms. Imposition of appropriate interfacial conditions allows for the construction of an evolution equation for the interfacial shape, via use of the kinematic condition, and the magnetic field. We study the three-dimensional evolution of the film to spanwise perturbations by solving the nonlinear equations numerically. The constant-volume configuration is considered, which corresponds to a slender drop flowing down an incline. A parametric study is then performed to understand the effect of the magnetic field on the stability and structure of the interface.
Xie Z, Lu L, Stoesser T, et al., 2017, Numerical simulation of three-dimensional breaking waves and its interaction with a vertical circular cylinder, JOURNAL OF HYDRODYNAMICS, Vol: 29, Pages: 800-804, ISSN: 1001-6058
Wave breaking plays an important role in wave-structure interaction. A novel control volume finite element method with adaptive unstructured meshes is employed here to study 3-D breaking waves. The numerical framework consists of a “volume of fluid” type method for the interface capturing and adaptive unstructured meshes to improve computational efficiency. The numerical model is validated against experimental measurements of breaking wave over a sloping beach and is then used to study the breaking wave impact on a vertical circular cylinder on a slope. Detailed complex interfacial structures during wave impact, such as plunging jet formation and splash-up are captured in the simulation, demonstrating the capability of the present method.
Che Z, Matar OK, 2017, Impact of droplets on liquid films in the presence of surfactant, Langmuir, Vol: 33, Pages: 12140-12148, ISSN: 0743-7463
The impact of droplets on liquid films is ubiquitous in natural and industrial processes, and surfactants can significantly alter the impact process by changing the local surface tension. Here we study the impact of droplets on liquid films in the presence of surfactant using high-speed photography, and reveal the flow pattern by dye-tracing. The effects of the droplet size and speed, and the initial film thickness on the impact process are elucidated. The results show that the flow is significantly affected by adding surfactant to the droplet, the liquid film, or to both phases. In particular, the film dye patterns form concentric circles and flower-shaped structures at low and high droplet Weber numbers, respectively. We also show how surfactant-induced Marangoni stresses modify these flow patterns, and alter the characteristics of the phenomena associated with the impact process, such as the propagation of capillary waves, the evolution of the crown, and the formation of secondary droplets. During the impact of surfactant droplets on thin water films, the Marangoni stresses can be sufficiently strong so as to drive film dewetting.
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