Imperial College London

Professor Omar K. Matar

Faculty of EngineeringDepartment of Chemical Engineering

Vice-Dean (Education), Faculty of Engineering
 
 
 
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Contact

 

+44 (0)20 7594 9618o.matar Website

 
 
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Assistant

 

Miss Nazma Mojid +44 (0)20 7594 3918

 
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Location

 

506ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

263 results found

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

Conference paper

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.

Journal article

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.

Journal article

Xie Z, Lu L, Stoesser T, Lin J-G, Pavlidis D, Salinas P, Pain CC, Matar OKet 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.

Journal article

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.

Journal article

Matar OK, Wilson SK, 2017, Preface to the inaugural "Perspectives" article entitled "The importance of being thin" by Stephen H. Davis, Journal of Engineering Mathematics, Vol: 105, Pages: 1-2, ISSN: 0022-0833

Journal article

Adebayo I, Xie Z, Che Z, Matar Oet al., 2017, Doubly-excited pulse waves on thin liquid films flowing down an inclined plane: An experimental and numerical study, Physical Review E, Vol: 96, ISSN: 1539-3755

The interaction patterns between doubly excited pulse waves on thin liquid films flowing down an inclined plane are studied both experimentally and numerically. The effect of varying the film flow rate, interpulse interval, and substrate inclination angle on the pulse interaction patterns is examined. Our results show that different interaction patterns exist for these binary pulses, which include solitary wave behavior, partial or complete pulse coalescence, and pulse noncoalescence. A regime map of these patterns is plotted for each inclination angle examined, parametrized by the film Reynolds number and interpulse interval. Finally, the individual effect of the system parameters mentioned above on the coalescence distance of binary pulses in the “complete pulse coalescence” mode is studied; the results are compared to numerical simulations of the two-dimensional Navier-Stokes equations yielding good agreement.

Journal article

Nowak E, Xie Z, Kovalchuk NM, Matar OK, Simmons MJHet al., 2017, Bulk advection and interfacial flows in the binary coalescence of surfactant-laden and surfactant-free drops, Soft Matter, Vol: 13, Pages: 4616-4628, ISSN: 1744-683X

This work focuses on the study of bulk flows accompanying the coalescence of two aqueous drops, one containing surfactant and the other surfactant-free, in silicone oils of various viscosities. It is observed that the surfactant-free drop intrudes into the surfactant-laden drop in the form of a penetrating jet whose speed increases and average radius decreases with increasing outer phase viscosity. Mixing patterns within the coalescing drops are due to the force imbalance caused by capillary pressure difference and surfactant-induced Marangoni stresses. The driving force for mixing associated with the difference in interfacial tension between the drops is considerably stronger than that related to the drop size. The long timescale mixing of the drops is driven by rapid interior convection, and the subsequent, slow, diffusive process. Three-dimensional numerical simulations show excellent qualitative and quantitative agreement with the experimental results. The implications of our results to formulation strategies of complex microstructures in practical applications are also discussed.

Journal article

Wray AW, Matar OK, Papageorgiou DT, 2017, Accurate low-order modeling of electrified falling films at moderate Reynolds number, Physical Review Fluids, Vol: 2, ISSN: 2469-990X

The two- and three-dimensional spatio-temporal dynamics of a falling, electrified leakydielectric film are studied. The method of weighted residuals is used to derive high-ordermodels that account for both inertia as well as second-order electrostatic effects. Themodels are validated against both linear theory and direct numerical simulations of theNavier-Stokes equations. It is shown that a simplified model offers a rapid computationaloption at the cost of a minimal decrease in accuracy. This model is then used to perform aparametric study in three dimensions.

Journal article

Ibarra R, Matar OK, Markides CN, 2017, Flow structures in low-inclination stratified oil-water pipe-flows using laser-based diagnostic techniques, International Conference on Multiphase Production, Publisher: BHR Group, Pages: 71-85

In this work, a novel two-line laser-based diagnostic measurement technique was developed and applied to obtain combined space- and time-resolved phase and velocity information in low-inclination upward (<+5°) stratified flows of oil (Exxsol D140) and water. The strength of this technique is in enabling direct measurements in the non-refractive-index-matched fluids of interest, as opposed to substitute (optically matched) fluids whose properties may be less representative of those in real field-applications. The experimental test-section consisted of a 32-mm internal diameter pipe with a total length of 8.5 m. Results reveal interesting interactions between the co-flowing liquid phases. The velocity gradients at the interface are enhanced at high pipe inclinations for upward flows as the oil and water velocities increase and decrease, respectively. This also has a direct effect on the velocity fluctuations (quantified through their rms) and on the interfacial instabilities, which in turn affect the local velocity distributions in both phases.

Conference paper

Theodorakis PE, Muller EA, Craster RV, Matar OKet al., 2017, Physical insights into the blood-brain barrier translocation mechanisms, Physical Biology, Vol: 14, ISSN: 1478-3975

The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood–brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.

Journal article

Wray AW, Papageorgiou DT, Matar OK, 2017, Reduced models for thick liquid layers with inertia on highly curved substrates, SIAM Journal on Applied Mathematics, Vol: 77, Pages: 881-904, ISSN: 0036-1399

A method is presented for deriving reduced models for fluid flows over highly curved substrates with wider applicability and accuracy than existing models in the literature. This is done by reducing the Navier--Stokes equations to a novel system of boundary layer like equations in a general geometric setting. This is accomplished using a new, relaxed set of scalings that assert only that streamwise variations are “slow”. These equations are then solved using the method of weighted residuals, which is demonstrated to be applicable regardless of the geometry selected. A large number of results in the literature can be derived as special cases of our general formulation. A few of the more interesting cases are demonstrated. Finally, the formulation is applied to two thick annular flow systems as well as a conical system in both linear and nonlinear regimes, which traditionally has been considered inaccessible to such reduced models. Comparisons are made with direct numerical simulations of the Stokes equations. The results indicate that reduced models can now be used to model systems involving thick liquid layers.

Journal article

Poesio P, Damone A, Matar OK, 2017, Slip at liquid-liquid interfaces, PHYSICAL REVIEW FLUIDS, Vol: 2, ISSN: 2469-990X

We address a problem of fundamental importance in the physics of interfaces, which is central to the description of multiphase fluid dynamics. This work is important to study interfaces in systems such as polymer melts and solutions, where velocity jumps have been observed and interpreted as a manifestation of slip. This is in violation of classical interfacial conditions that require continuity of velocity and has been remedied in the literature via use of ad hoc models, such as the so-called Navier slip condition. This paper suggests that it is possible to obviate completely the need for such an approach. Instead, we show that one simply requires knowledge of the density field and the molar fraction of the fluid components and the dependence of the viscosity on the density. This information can be obtained easily through molecular dynamics simulations.

Journal article

Xie Z, Hewitt G, Pavlidis D, Salinas P, Pain C, Matar Oet al., 2017, Numerical study of three-dimensional droplet impact ona flowing liquid film in annular two-phase flow, Chemical Engineering Science, Vol: 166, Pages: 303-312, ISSN: 1873-4405

Annular flow with liquid entrainment occurs in a wide variety of two-phase flow system. A novel control volume finite element method with adaptive unstructured meshes is employed here to study three-dimensional droplet deposition process in annular two-phase flow. The numerical framework consists of a ‘volume of fluid’ type method for the interface capturing and a force-balanced continuum surface force model for the surface tension on adaptive unstructured meshes. The numerical framework is validated against experimental measurements of a droplet impact problem and is then used to study the droplet deposition onto a flowing liquid film at atmospheric and high pressure conditions. Detailed complex interfacial structures during droplet impact are captured during the simulation, which agree with the experimental observations, demonstrating the capability of the present method. It is found that the effect of the ambient pressure on the fluid properties and interfacial tension plays an important role in the droplet deposition process and the associated interfacial phenomena.

Journal article

Saenz PJ, Wray AW, Che Z, Matar OK, Valluri P, Kim J, Sefiane Ket al., 2017, Dynamics and universal scaling law in geometrically-controlled sessile drop evaporation, NATURE COMMUNICATIONS, Vol: 8, ISSN: 2041-1723

The evaporation of a liquid drop on a solid substrate is a remarkably common phenomenon. Yet, the complexity of the underlying mechanisms has constrained previous studies to spherically symmetric configurations. Here we investigate well-defined, non-spherical evaporating drops of pure liquids and binary mixtures. We deduce a universal scaling law for the evaporation rate valid for any shape and demonstrate that more curved regions lead to preferential localized depositions in particle-laden drops. Furthermore, geometry induces well-defined flow structures within the drop that change according to the driving mechanism. In the case of binary mixtures, geometry dictates the spatial segregation of the more volatile component as it is depleted. Our results suggest that the drop geometry can be exploited to prescribe the particle deposition and evaporative dynamics of pure drops and the mixing characteristics of multicomponent drops, which may be of interest to a wide range of industrial and scientific applications.

Journal article

Nania M, Foglia F, Matar OK, Cabral JTet al., 2017, Sub-100 nm wrinkling of polydimethylsiloxane by double frontal oxidation, Nanoscale, Vol: 9, Pages: 2030-2037, ISSN: 2040-3364

We demonstrate nanoscale wrinkling on polydimethylsiloxane (PDMS) at sub-100 nm length scales via a(double) frontal surface oxidation coupled with a mechanical compression. The kinetics of the glassy skinpropagation is resolved by neutron and X-ray reflectivity, and atomic force microscopy, combined withmechanical wrinkling experiments to evaluate the resulting pattern formation. In conventional PDMSsurface oxidation, the smallest wrinkling patterns attainable have an intrinsic lower wavelength limit dueto the coupling of skin formation and front propagation at fixed strain εprestrain, whose maximum is, inturn, set by material failure. However, combining two different oxidative processes, ultra-violet ozonolysisfollowed by air plasma exposure, we break this limit by fabricating trilayer laminates with excellent interfacialproperties and a sequence of moduli and layer thicknesses able to trivially reduce the surface topographyto sub-100 nm dimensions. This method provides a powerful, yet simple, non-lithographicapproach to extend surface patterning from visible to the deep UV range.

Journal article

Uppal AS, Matar OK, Craster RV, 2017, Dynamics of spreading thixotropic droplets, Journal of Non-Newtonian Fluid Mechanics, Vol: 240, Pages: 1-14, ISSN: 1873-2631

The e ect of thixotropy on the two-dimensional spreading of a sessiledrop is modelled using lubrication theory. Thixotropy is incorporated by theinclusion of a structure parameter, , measuring structure build-up governedby an evolution equation linked to the droplet micromechanics. A number ofmodels are derived for coupled to the interface dynamics; these range frommodels that account for the cross-stream dependence of to simpler ones inwhich this dependence is prescribed through appropriate closures. Numericalsolution of the governing equations show that thixotropy has a profound e ecton the spreading characteristics; the long-time spreading dynamics, however,are shown to be independent of the initial structural state of the droplet. Wealso compare the predictions of the various models and determine the rangeof system parameters over which the simple models provide su ciently goodapproximations of the full, two-dimensional spreading dynamics.

Journal article

Nikolaou A, Booth P, Gordon F, Yang J, Matar O, Chachuat Bet al., 2017, Multi-physics modeling of light-limited microalgae growth in raceway ponds, IFAC Proceedings Volumes (IFAC-PapersOnline), Vol: 49, Pages: 324-329, ISSN: 1474-6670

This paper presents a multi-physics modeling methodology for the quantitative prediction of microalgae productivity in raceway ponds by combining a semi-mechanistic model of microalgae growth describing photoregulation, photoinhibition and photoacclimation, with models of imperfect mixing based on Lagrangian particle-tracking and heterogeneous light distribution. The photosynthetic processes of photoproduction, photoregulation and photoinhibition are represented by a model of chlorophyll fluorescence developed by Nikolaou et al. (2015), which is extended to encompass photoacclimation. The flow is simulated with the commercial CFD package ANSYS, whereas light attenuation is described by the Beer-Lambert law as a first approximation. Full-scale simulation results are presented on extended time horizons. Comparisons are made in terms of areal productivities under both imperfect and idealized (CSTR) mixing conditions, and for various extraction rates and water depths.

Journal article

Smith ER, Müller EA, Craster RV, Matar OKet al., 2016, A Langevin model for fluctuating contact angle behaviour parametrised using molecular dynamics, Soft Matter, Vol: 12, Pages: 9604-9615, ISSN: 1744-6848

Molecular dynamics simulations are employed to develop a theoretical model to predict the fluid-solid contact angle as a function of wall-sliding speed incorporating thermal fluctuations. A liquid bridge between counter-sliding walls is studied, with liquid-vapour interface-tracking, to explore the impact of wall-sliding speed on contact angle. The behaviour of the macroscopic contact angle varies linearly over a range of capillary numbers beyond which the liquid bridge pinches off, a behaviour supported by experimental results. Nonetheless, the liquid bridge provides an ideal test case to study molecular scale thermal fluctuations, which are shown to be well described by Gaussian distributions. A Langevin model for contact angle is parametrised to incorporate the mean, fluctuation and auto-correlations over a range of sliding speeds and temperatures. The resulting equations can be used as a proxy for the fully-detailed molecular dynamics simulation allowing them to be integrated within a continuum-scale solver.

Journal article

Hennessy MG, Ferretti GL, Cabral JT, Matar OKet al., 2016, A minimal model for solvent evaporation and absorption in thin films, Journal of Colloid and Interface Science, Vol: 488, Pages: 61-71, ISSN: 0021-9797

We present a minimal model of solvent evaporation and absorption in thin films consisting of a volatile solvent and non-volatile solutes. An asymptotic analysis yields expressions that facilitate the extraction of physically significant model parameters from experimental data, namely the mass transfer coefficient and composition-dependent diffusivity. The model can be used to predict the dynamics of drying and film formation, as well as sorption/desorption, over a wide range of experimental conditions. A state diagram is used to understand the experimental conditions that lead to the formation of a solute-rich layer, or “skin”, at the evaporating surface during drying. In the case of solvent absorption, the model captures the existence of a saturation front that propagates from the film surface towards the substrate. The theoretical results are found to be in excellent agreement with data produced from dynamic vapour sorption experiments of ternary mixtures comprising an aluminium salt, glycerol, and water. Moreover, the model should be generally applicable to a variety of practical contexts, from paints and coatings, to personal care, packaging, and electronics.

Journal article

Morgan RG, Ibarra R, Zadrazil I, Matar OK, Hewitt GF, Markides CNet al., 2016, On the role of buoyancy-driven instabilities in horizontal liquid–liquid flow, International Journal of Multiphase Flow, Vol: 89, Pages: 123-135, ISSN: 0301-9322

Horizontal flows of two initially stratified immiscible liquids with matched refractive indices, namely an aliphatic hydrocarbon oil (Exxsol D80) and an aqueous-glycerol solution, are investigated by combining two laser-based optical-diagnostic measurement techniques. Specifically, high-speed Planar Laser-Induced Fluorescence (PLIF) is used to provide spatiotemporally resolved phase information, while high-speed Particle Image and Tracking Velocimetry (PIV/PVT) are used to provide information on the velocity field in both phases. The two techniques are applied simultaneously in a vertical plane through the centreline of the investigated pipe flow, illuminated by a single laser-sheet in a time-resolved manner (at a frequency of 1–2 kHz depending on the flow condition). Optical distortions due to the curvature of the (transparent) circular tube test-section are corrected with the use of a graticule (target). The test section where the optical-diagnostic methods are applied is located 244 pipe-diameters downstream of the inlet section, in order to ensure a significant development length. The experimental campaign is explicitly designed to study the long-length development of immiscible liquid–liquid flows by introducing the heavier (aqueous) phase at the top of the channel and above the lighter (oil) phase that is introduced at the bottom, which corresponds to an unstably-stratified “inverted” inlet orientation in the opposite orientation to that in which the phases would naturally separate. The main focus is to evaluate the role of the subsequent interfacial instabilities on the resulting long-length flow patterns and characteristics, also by direct comparison to an existing liquid–liquid flow dataset generated in previous work, downstream of a “normal” inlet orientation in which the oil phase was introduced over the aqueous phase in a conventional stably-stratified inlet orientation. To the best knowledge of the authors this

Journal article

Xie Z, Pavlidis D, Salinas P, Percival J, Pain C, Matar Oet al., 2016, A balanced-force control volume finite element method for interfacial flows with surface tension using adaptive anisotropic unstructured meshes, Computers & Fluids, Vol: 138, Pages: 38-50, ISSN: 0045-7930

A balanced-force control volume finite element method is presented for three-dimensional interfacial flows with surface tension on adaptive anisotropic unstructured meshes. A new balanced-force algorithm for the continuum surface tension model on unstructured meshes is proposed within an interface capturing framework based on the volume of fluid method, which ensures that the surface tension force and the resulting pressure gradient are exactly balanced. Two approaches are developed for accurate curvature approximation based on the volume fraction on unstructured meshes. The numerical framework also features an anisotropic adaptive mesh algorithm, which can modify unstructured meshes to better represent the underlying physics of interfacial problems and reduce computational effort without sacrificing accuracy. The numerical framework is validated with several benchmark problems for interface advection, surface tension test for equilibrium droplet, and dynamic fluid flow problems (fluid films, bubbles and droplets) in two and three dimensions.

Journal article

Ibarra R, Morgan R, Zadrazil I, Matar O, Markides Cet al., 2016, Investigation of oil-water flow in horizontal pipes using simultaneous two-line planar laser-induced fluorescence and particle velocimetry, HEFAT, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

The flow of oil and water in pipes represents a challenging configuration in multiphase flows due to complex hydrodynamics which are still not fully understood. This can be observed in the large number of flow regimes encountered, which extend from smooth stratified flows to complex dispersions such as droplets of oil-in-water and water-in-oil. These flow configurations are the result of the inherent properties of the liquid phases, e.g., their densities and viscosities, interfacial tension and contact angle, as well as of flow conditions and related phenomena, such as turbulence, which have a direct effect on the interface instabilities giving rise to flow regime transitions. In this paper, experimental data are reported that were acquired at low water cuts and low mixture velocities using an aliphatic oil (Exxsol D140) and water as the test fluids in an 8.5 m long and 32 mm internal diameter horizontal pipe. A copper-vapour laser, emitting two narrow-band laser beams, and two high-speed cameras were used to obtain quantitative simultaneous information of the flow (specifically, spatiotemporally resolved fluid-phase and velocity information in both phases) based on simultaneous two-line Planar Laser-Induced Fluorescence (PLIF) and Particle Image and Tracking Velocimetry (PIV/PTV). To the best knowledge of the authors this is the first such instance of the application of this combined technique to these flows. It is found that the rms of the fluctuating velocity show peaks in high shear regions, i.e. at the pipe wall and interface.

Conference paper

Karapetsas G, Chandra Sahu K, Matar OK, 2016, Evaporation of Sessile Droplets Laden with Particles and Insoluble Surfactants, Langmuir, Vol: 32, Pages: 6871-6881, ISSN: 1520-5827

We consider the flow dynamics of a thin evaporating droplet in the presence of an insoluble surfactant and noninteracting particles in the bulk. On the basis of lubrication theory, we derive a set of evolution equations for the film height, the interfacial surfactant, and bulk particle concentrations, taking into account the dependence of liquid viscosity on the local particle concentration. An important ingredient of our model is that it takes into account the fact that the surfactant adsorbed at the interface hinders evaporation. We perform a parametric study to investigate how the presence of surfactants affects the evaporation process as well as the flow dynamics with and without the presence of particles in the bulk. Our numerical calculations show that the droplet lifetime is affected significantly by the balance between the ability of the surfactant to enhance spreading, suppressing the effect of thermal Marangoni stresses-induced motion, and to hinder the evaporation flux through the reduction of the effective interfacial area of evaporation, which tend to accelerate and decelerate the evaporation process, respectively. For particle-laden droplets and in the case of dilute solutions, the droplet lifetime is found to be weakly dependent on the initial particle concentration. We also show that the particle deposition patterns are influenced strongly by the direct effect of the surfactant on the evaporative flux; in certain cases, the "coffee-stain" effect is enhanced significantly. A discussion of the delicate interplay between the effects of capillary pressure and solutal and thermal Marangoni stresses, which drive the liquid flow inside of the evaporating droplet giving rise to the observed results, is provided herein.

Journal article

Tinguely M, Hennessy MG, Pommella A, Matar OK, Garbin Vet al., 2016, Surface waves on a soft viscoelastic layer produced by an oscillating microbubble, Soft Matter, Vol: 12, Pages: 4247-4256, ISSN: 1744-6848

Ultrasound-driven bubbles can cause significant deformation of soft viscoelastic layers, for instance in surface cleaning and biomedical applications. The effect of the viscoelastic properties of a boundary on the bubble–boundary interaction has been explored only qualitatively, and remains poorly understood. We investigate the dynamic deformation of a viscoelastic layer induced by the volumetric oscillations of an ultrasound-driven microbubble. High-speed video microscopy is used to observe the deformation produced by a bubble oscillating at 17–20 kHz in contact with the surface of a hydrogel. The localised oscillating pressure applied by the bubble generates surface elastic (Rayleigh) waves on the gel, characterised by elliptical particle trajectories. The tilt angle of the elliptical trajectories varies with increasing distance from the bubble. Unexpectedly, the direction of rotation of the surface elements on the elliptical trajectories shifts from prograde to retrograde at a distance from the bubble that depends on the viscoelastic properties of the gel. To explain these behaviours, we develop a simple three-dimensional model for the deformation of a viscoelastic solid by a localised oscillating force. By using as input for the model the values of the shear modulus obtained from the propagation velocity of the Rayleigh waves, we find good qualitative agreement with the experimental observations.

Journal article

Muller EA, Matar OK, Jaeger F, Muscatello Jet al., 2016, Optimising water transport through graphene-based membranes: Insights from non-equilibrium molecular dynamics, ACS Applied Materials & Interfaces, Vol: 8, Pages: 12330-12336, ISSN: 1944-8244

Recent experimental results suggest that stacked layers of graphene oxide exhibitstrong selective permeability to water. To construe this observation the transportmechanism of water permeating through a membrane consisting of layered graphenesheets is investigated via non-equilibrium and equilibrium molecular dynamics simulations.The effect of sheet geometry is studied by changing the offset between theentrance and exit slits of the membrane. The simulation results reveal that the permeabilityis not solely dominated by entrance effects; the path traversed by watermolecules has a considerable impact on the permeability. We show that contrary tospeculation in the literature, water molecules do not pass through the membrane as ahydrogen-bonded chain; instead, they form well-mixed fluid regions confined betweenthe graphene sheets. The results of the present work are used to provide guidelinesfor the development of graphene and graphene oxide membranes for desalination andsolvent separation.

Journal article

Vitale A, Hennessy M, Matar O, Cabral Jet al., 2016, Unified approach for polymeric patterning via controlling the propagation of frontal photopolymerization waves, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727

Conference paper

Charogiannis A, Heiles B, Mathie R, Matar O, Markides Cet al., Spatiotemporally resolved heat transfer measurements in falling-film flows over an inclined heated foil, International Symposium and School of Young Scientists INTERFACIAL PHENOMENA AND HEAT TRANSFER

Conference paper

Ferretti GL, Nania M, Matar OK, Cabral JTet al., 2016, Wrinkling measurement of the mechanical properties of drying salt thin films, Langmuir, Vol: 32, Pages: 2199-2207, ISSN: 1520-5827

Journal article

Kovalchuk NM, Matar OK, Craster RV, Miller R, Starov VMet al., 2016, The effect of adsorption kinetics on the rate of surfactant-enhanced spreading, Soft Matter, Vol: 12, Pages: 1009-1013, ISSN: 1744-6848

A comparison of the kinetics of spreading of aqueous solutions of two different surfactants on an identical substrate and their short time adsorption kinetics at the water/air interface has shown that the surfactant which adsorbs slower provides a higher spreading rate. This observation indicates that Marangoni flow should be an important part of the spreading mechanism enabling surfactant solutions to spread much faster than pure liquids with comparable viscosities and surface tensions.

Journal article

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