Imperial College London

Professor Omar K. Matar

Faculty of EngineeringDepartment of Chemical Engineering

Vice-Dean (Education), Faculty of Engineering
 
 
 
//

Contact

 

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

 
 
//

Assistant

 

Miss Nazma Mojid +44 (0)20 7594 3918

 
//

Location

 

506ACE ExtensionSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

263 results found

Roumpea E, Chinaud M, Kahouadji L, Matar OK, Angeli Pet al., 2016, Plug flow of shear-thinning liquids in microchannels, Pages: 395-397

Conference paper

Tinguely M, Matar OK, Garbin V, 2015, Tracking the deformation of a tissue phantom induced by ultrasound-driven bubble oscillations, 9th International Symposium on Cavitation (CAV2015), Publisher: IOP Publishing Ltd, ISSN: 1742-6588

Conference paper

Theodorakis PE, Müller EA, Craster RV, Matar OKet al., 2015, Modelling the superspreading of surfactant-laden droplets with computer simulation., Soft Matter, Vol: 11, Pages: 9254-9261, ISSN: 1744-6848

The surfactant-driven superspreading of droplets on hydrophobic substrates is considered. A key element of the superspreading mechanism is the adsorption of surfactant molecules from the liquid-vapour interface onto the substrate through the contact line, which must be coordinated with the replenishment of interfaces with surfactant from the interior of the droplet. We use molecular dynamics simulations with coarse-grained force fields to provide a detailed structural description of the droplet shape and surfactant dynamics during the superspreading process. We also provide a simple method for accurate estimation of the contact angle subtended by the droplets at the contact line.

Journal article

Conroy DT, Matar OK, 2015, Thin viscous ferrofluid film in a magnetic field, Physics of Fluids, Vol: 27, ISSN: 1089-7666

We consider a thin, ferrofluidic film flowing down an inclined substrate, under the action of a magnetic field, bounded above by an inviscid gas. Its dynamics are governed by a coupled system of the steady Maxwell’s, the Navier-Stokes, and the continuity equations. The magnetization of the film is a function of the magnetic field and may be prescribed by a Langevin function. We make use of a long-wave reduction in order to solve for the dynamics of the pressure and velocity fields inside the film. In addition, we investigate the problem in the limit of a large magnetic permeability. Imposition of appropriate interfacial conditions allows for the construction of an evolution equation for the interfacial shape via use of the kinematic condition. The resultant one-dimensional equations are solved numerically using spectral methods. The magnetic effects give rise to a non-local contribution. We conduct a parametric study of both the linear and nonlinear stabilities of the system in order to evaluate the effects of the magnetic field. Through a linear stability analysis, we verify that the Maxwell’s pressure generated from a normally applied magnetic field is destabilizing and can be used to control the size and shape of lobes and collars on the free surface. We also find that in the case of a falling drop, the magnetic field causes an increase in the velocity and capillary ridge of the drop.

Journal article

Che Z, Deygas A, Matar OK, 2015, Impact of droplets on inclined flowing liquid films, Physical Review E, Vol: 92, ISSN: 1539-3755

The impact of droplets on an inclined falling liquid film is studied experimentally using high-speed imaging.The falling film is created on a flat substrate with controllable thicknesses and flow rates. Droplets with differentsizes and speeds are used to study the impact process under various Ohnesorge and Weber numbers, and filmReynolds numbers. A number of phenomena associated with droplet impact are identified and analyzed, suchas bouncing, partial coalescence, total coalescence, and splashing. The effects of droplet size, speed, as well thefilm flow rate are studied culminating in the generation of an impact regime map. The analysis of the lubricationforce acted on the droplet via the gas layer shows that a higher flow rate in the liquid film produces a largerlubrication force, slows down the drainage process, and increases the probability of droplet bouncing. Our resultsdemonstrate that the flowing film has a profound effect on the droplet impact process and associated phenomena,which are markedly more complex than those accompanying impact on initially quiescent films.

Journal article

Vitale A, Hennessy M, Matar O, Cabral JPet al., 2015, A unified approach for patterning via frontal photopolymerization, Advanced Materials, Vol: 27, Pages: 6118-6124, ISSN: 1521-4095

A unified patterning strategy via frontal photopolymerization (FPP) that is robust to a wide range of radical photopolymerizing systems, including thiol–ene and acrylic monomers is reported. The factors governing the spatiotemporal solidification process, including front position, profile shape, and thermal effects, are investigated and modeled theoretically, resulting in the predictive FPP patterning of polymer networks with prescribed dimensions.

Journal article

Pavlidis D, Gomes JLMA, Xie Z, Percival JR, Pain CC, Matar OKet al., 2015, Compressive advection and multi-component methods for interface-capturing, International Journal for Numerical Methods in Fluids, Vol: 80, Pages: 256-282, ISSN: 1097-0363

This paper develops methods for interface-capturing in multiphase flows. The main novelties of these methods are as follows: (a) multi-component modelling that embeds interface structures into the continuity equation; (b) a new family of triangle/tetrahedron finite elements, in particular, the P1DG-P2(linear discontinuous between elements velocity and quadratic continuous pressure); (c) an interface-capturing scheme based on compressive control volume advection methods and high-order finite element interpolation methods; (d) a time stepping method that allows use of relatively large time step sizes; and (e) application of anisotropic mesh adaptivity to focus the numerical resolution around the interfaces and other areas of important dynamics. This modelling approach is applied to a series of pure advection problems with interfaces as well as to the simulation of the standard computational fluid dynamics benchmark test cases of a collapsing water column under gravitational forces (in two and three dimensions) and sloshing water in a tank. Two more test cases are undertaken in order to demonstrate the many-material and compressibility modelling capabilities of the approach. Numerical simulations are performed on coarse unstructured meshes to demonstrate the potential of the methods described here to capture complex dynamics in multiphase flows.

Journal article

Hennessy M, Vitale A, Cabral JT, Matar OKet al., 2015, Role of heat generation and thermal diffusion during frontal photopolymerization, Physical Review E, Vol: 92, Pages: 022403-022403, ISSN: 1539-3755

Frontal photopolymerisation (FPP) is a rapid and versatile solidification process that can be used to fabricate complex three-dimensional structures by selectively exposing a photosensitive monomer-rich bath to light. A characteristic feature of FPP is the appearance of a sharp polymerisation front that propagates into the bath as a planar travelling wave. In this paper, we introduce a theoretical model to determine how heat generation during photopolymerisation influences the kinetics of wave propagation as well as the monomer-to-polymer conversion profile, both of which are relevant for FPP applications and experimentally measurable. When thermal diffusion is sufficiently fast relative to the rate of polymerisation, the system evolves as if it were isothermal. However, when thermal diffusion is slow, a thermal wavefront develops and propagates at the same rate as the polymerisation front. This leads to an accumulation of heat behind the polymerisation front which can result in a significant sharpening of the conversion profile and acceleration of the growth of the solid. Our results also suggest that a novel way to tailor the dynamics of FPP is by imposing a temperature gradient along the growth direction.

Journal article

Tripathi MK, Sahu KC, Karapetsas G, Matar OKet al., 2015, Bubble rise dynamics in a viscoplastic material, JOURNAL OF NON-NEWTONIAN FLUID MECHANICS, Vol: 222, Pages: 217-226, ISSN: 0377-0257

Journal article

Ibarra R, Zadrazil I, Markides CN, Matar OKet al., Towards a Universal Dimensionless Map of Flow Regime Transitions in Horizontal Liquid-Liquid Flows, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

Conference paper

Hennessy MG, Vitale A, Matar OK, Cabral JTet al., 2015, Controlling frontal photopolymerization with optical attenuation and mass diffusion, Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Vol: 91, ISSN: 1063-651X

Frontal photopolymerization (FPP) is a versatile directional solidification process that can be used to rapidly fabricate polymer network materials by selectively exposing a photosensitive monomer bath to light. A characteristic feature of FPP is that the monomer-to-polymer conversion profiles take on the form of traveling waves that propagate into the unpolymerized bulk from the illuminated surface. Practical implementations of FPP require detailed knowledge about the conversion profile and speed of these traveling waves. The purpose of this theoretical study is to (i) determine the conditions under which FPP occurs and (ii) explore how optical attenuation and mass transport can be used to finely tune the conversion profile and propagation kinetics. Our findings quantify the strong optical attenuation and slow mass transport relative to the rate of polymerization required for FPP. The shape of the traveling wave is primarily controlled by the magnitude of the optical attenuation coefficients of the neat and polymerized material. Unexpectedly, we find that mass diffusion can increase the net extent of polymerization and accelerate the growth of the solid network. The theoretical predictions are found to be in excellent agreement with experimental data acquired for representative systems.

Journal article

Ibarra R, Matar OK, Markides CN, Zadrazil Iet al., 2015, An experimental study of oil-water flows in horizontal pipes, Multiphase 2015, Publisher: BHR Group

This paper reports an effort to investigate the effect of flow velocities and inlet configurations on horizontal oil-water flows in a 32 mm ID acrylic pipe using water and an aliphatic oil (Exxsol D140) as test fluids. The flows of interest were analysed using pressure drop measurements and high-speed photography in an effort to obtain a flow pattern map, pressure gradient profiles and measures of the in situ phase fractions. The experiments reveal a particular effect of the inlet configuration on the observed flow pattern. A horizontal plate, installed at the inlet, generates a transition to stratified flow when the plate height closely matched the in situ water height at high input oil fractions.

Conference paper

Theodorakis P, Kovalchuk NM, Starov VM, Muller EA, Craster RV, Matar OKet al., 2015, Superspreading: Mechanisms and Molecular Design, Mainz Material Simulation Days 2015, Pages: 29-29

Conference paper

Wray AW, Matar OK, Craster, Sefiane K, Papageorgiou DTet al., 2015, Electrostatic Suppression of the "Coffee-stain Effect", Procedia IUTAM, Vol: 15, Pages: 172-177, ISSN: 2210-9838

The dynamics of a slender, nano-particle laden droplet are examined when it is subjected to an electric field. Under a long-waveassumption, the governing equations are reduced to a coupled pair of nonlinear evolution equations prescribing the dynamics of theinterface and the depth-averaged particle concentration. This incorporates the effects of viscous stress, capillarity, electrostaticallyinducedMaxwell stress, van der Waals forces, evaporation and concentration-dependent rheology. It has previously been shown27that electric fields can be used to suppress the ring effect typically exhibited when such a droplet undergoes evaporation. Wedemonstrate here that the use of electric fields affords many diverse ways of controlling the droplets.

Journal article

Saenz PJ, Sefiane K, Kim J, Matar OK, Valluri Pet al., 2015, Evaporation of sessile drops: a three-dimensional approach, Journal of Fluid Mechanics, Vol: 772, Pages: 705-739, ISSN: 1469-7645

The evaporation of non-axisymmetric sessile drops is studied by means of experiments and three-dimensional direct numerical simulations (DNS). The emergence of azimuthal currents and pairs of counter-rotating vortices in the liquid bulk flow is reported in drops with non-circular contact area. These phenomena, especially the latter, which is also observed experimentally, are found to play a critical role in the transient flow dynamics and associated heat transfer. Non-circular drops exhibit variable wettability along the pinned contact line sensitive to the choice of system parameters, and inversely dependent on the local contact-line curvature, providing a simple criterion for estimating the approximate contact-angle distribution. The evaporation rate is found to vary in the same order of magnitude as the liquid–gas interfacial area. Furthermore, the more complex case of drops evaporating with a moving contact line (MCL) in the constant contact-angle mode is addressed. Interestingly, the numerical results demonstrate that the average interface temperature remains essentially constant as the drop evaporates in the constant-angle (CA) mode, while this increases in the constant-radius (CR) mode as the drops become thinner. It is therefore concluded that, for increasing substrate heating, the evaporation rate increases more rapidly in the CR mode than in the CA mode. In other words, the higher the temperature the larger the difference between the lifetimes of an evaporating drop in the CA mode with respect to that evaporating in the CR mode.

Journal article

Nania M, Matar OK, Cabral JT, 2015, Frontal vitrification of PDMS using air plasma and consequences for surface wrinkling, Soft Matter, Vol: 11, Pages: 3067-3075, ISSN: 1744-6848

We study the surface oxidation of polydimethylsiloxane (PDMS) by air plasma exposure and its implications for the mechanically-induced surface wrinkling of the resulting glass–elastomer bilayers. The effect of plasma frequency (kHz and MHz), oxygen content (from O2 to air), pressure (0.5 ≤ P ≤ 1.5 mbar), as well as exposure time and power, is quantified in terms of the resulting glassy skin thickness h, inferred from wrinkling experiments. The glassy skin thickness is found to increase logarithmically with an exposure time t, for different induction powers p, and all data collapse in terms of a plasma dose, D ≡ p × t. The kinetics of film propagation are found to increase with the oxygen molar fraction yO2 and decrease with the gas pressure P, allowing both the wrinkling wavelength λ and amplitude A to be effectively controlled by gas pressure and composition. A generalised relationship for frontal vitrification is obtained by re-scaling all λ and h data by D/P. A coarse-grained wave propagation model effectively describes and quantifies the process stages (induction, skin formation and propagation) under all the conditions studied. Equipped with this knowledge, we further expand the capabilities of plasma oxidation for PDMS wrinkling, and a wavelength of λ ≈ 100 nm is readily attained with a modest strain εprestrain ≈ 20%.

Journal article

Theodorakis PE, Mueller EA, Craster RV, Matar OKet al., 2015, Superspreading: Mechanisms and Molecular Design, Langmuir, Vol: 31, Pages: 2304-2309, ISSN: 1520-5827

Journal article

Yang J, Tajudin ZB, Coletti F, Muller A, MacChietto S, Matar OKet al., 2015, Numerical simulation of fouling in crude-oil heat exchangers: The interaction between different fouling routes, Pages: 833-842

Conference paper

Yang J, Serratos MGJ, Fari-Arole DS, Muller EA, Matar OKet al., 2015, Crude Oil Fouling: Fluid Dynamics, Reactions and Phase Change, IUTAM SYMPOSIUM ON MULTIPHASE FLOWS WITH PHASE CHANGE: CHALLENGES AND OPPORTUNITIES, Vol: 15, Pages: 186-193, ISSN: 2210-9838

Journal article

Matar O, 2015, No more empirical correlations, TCE The Chemical Engineer, Pages: 42-45, ISSN: 0302-0797

Journal article

Pavlidis D, Gomes JLMA, Xie Z, Pain CC, Tehrani AAK, Moatamedi M, Smith PN, Jones AV, Matar OKet al., 2015, Numerical modelling of melt behaviour in the lower vessel head of a nuclear reactor, IUTAM SYMPOSIUM ON MULTIPHASE FLOWS WITH PHASE CHANGE: CHALLENGES AND OPPORTUNITIES, Vol: 15, Pages: 72-77, ISSN: 2210-9838

Journal article

Pavlidis D, Gomes JLMA, Salinas P, Pain CC, Tehrani AAK, Moatamedi M, Smith PN, Jones AV, Matar OKet al., 2015, Numerical modelling of debris bed water quenching, IUTAM SYMPOSIUM ON MULTIPHASE FLOWS WITH PHASE CHANGE: CHALLENGES AND OPPORTUNITIES, Vol: 15, Pages: 64-71, ISSN: 2210-9838

Journal article

Karapetsas G, Craster RV, Matar OK, 2015, Surfactant Enhanced Spreading of Liquid Drops on Solid Surfaces, International Conference of Computational Methods in Sciences and Engineering (ICCMSE), Publisher: AMER INST PHYSICS, Pages: 425-428, ISSN: 0094-243X

Conference paper

Tripathi MK, Sahu KC, Karapetsas G, Sefiane K, Matar OKet al., 2015, Non-isothermal bubble rise: non-monotonic dependence of surface tension on temperature, JOURNAL OF FLUID MECHANICS, Vol: 763, Pages: 82-108, ISSN: 0022-1120

Journal article

Saenz PJ, Valluri P, Sefiane K, Matar OKet al., 2015, Stability and two-phase dynamics of evaporating Marangoni-driven flows in laterally-heated liquid layers and sessile droplets, IUTAM Symposium on Multiphase Flows with Phase Change Challenges and Opportunities, Publisher: ELSEVIER SCIENCE BV, Pages: 116-123, ISSN: 2210-9838

Conference paper

Swain PAP, Karapetsas G, Matar OK, Sahu KCet al., 2015, Numerical simulation of pressure-driven displacement of a viscoplastic material by a Newtonian fluid using the lattice Boltzmann method, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS, Vol: 49, Pages: 197-207, ISSN: 0997-7546

Journal article

Angeli P, Azzopardi BJ, Hewakandamby B, Hewitt GF, Pain CC, Simmons MJH, Matar OKet al., 2015, Multi-scale exploration of multiphase physics in flows (MEMPHIS): A framework for the next-generation predictive tools for multiphase flows, Pages: 242-249

Ins this paper, we outline the framework that we are developing as part of the Multi-scale Exploration of Multiphase PHysIcs in flowS (MEMPHIS) programme to create the next generation modelling tools for complex multiphase flows. These flows are of central importance to micro-fluidics, oil-and-gas, nuclear, and biomedical applications, and every processing and manufacturing technology. This framework involves the establishment of a transparent linkage between input and prediction to allow systematic error-source identification, and, optimal, model-driven experimentation, to maximise prediction accuracy. The framework also involves massively-parallelisable numerical methods, capable of running efficiently on 105-106 core supercomputers, with optimally-adaptive, three-dimensional resolution, and sophisticated multi-scale physical models. The overall aim of this framework is to provide unprecedented resolution of multi-scale, multiphase phenomena, thereby minimising the reliance on correlations and empiricism.

Conference paper

Vitale A, Hennessy MG, Matar OK, Cabral JTet al., 2014, Interfacial profile and propagation of frontal photopolymerization waves, Macromolecules, Vol: 48, Pages: 198-205, ISSN: 0024-9297

We investigate the frontal photopolymerization of a thiol–ene system with a combination of experiments and modeling, focusing on the interfacial conversion profile and its planar wave propagation. We spatially resolve the solid-to-liquid front by FT-IR and AFM mechanical measurements, supplemented by differential scanning calorimetry. A simple coarse-grained model is found to describe remarkably well the frontal kinetics and the sigmoidal interface, capturing the effects of UV light exposure time (or dose) and temperature, as well as the front position and resulting patterned dimensions after development. Analytical solutions for the conversion profile enable the description of all conditions with a single master curve in the moving frame of the front position. Building on this understanding, we demonstrate the design and fabrication of gradient polymer materials, with tunable properties along the direction of illumination, which can be coupled with lateral patterning by modulated illumination or grayscale lithography.

Journal article

Percival JR, Pavlidis D, Xie Z, Gomes JLM, Sakai M, Shigeto Y, Takahashi H, Matar OK, Pain CCet al., 2014, Control volume finite element modelling of segregation of sand and granular flows in fluidized beds, INTERNATIONAL JOURNAL OF MULTIPHASE FLOW, Vol: 67, Pages: 191-199, ISSN: 0301-9322

Journal article

Xie Z, Pavlidis D, Percival JR, Gomes JLMA, Pain CC, Matar OKet al., 2014, Adaptive unstructured mesh modelling of multiphase flows, International Journal of Multiphase Flow, Vol: 67, Pages: 104-110, ISSN: 0301-9322

© 2013 Elsevier Ltd. Multiphase flows are often found in industrial and practical engineering applications, including bubbles, droplets, liquid film and waves. An adaptive unstructured mesh modelling framework is employed here to study interfacial flow problems, which can modify and adapt anisotropic unstructured meshes to better represent the underlying physics of multiphase problems and reduce computational effort without sacrificing accuracy. The numerical framework consists of a mixed control volume and finite element formulation, a 'volume of fluid'-type method for the interface capturing based on a compressive control volume advection method and second-order finite element methods. The framework also features a force-balanced algorithm for the surface tension implementation, minimising the spurious velocities often found in such flows. Numerical examples of the Rayleigh-Taylor instability and a rising bubble are presented to show the ability of this adaptive unstructured mesh modelling framework to capture complex interface geometries and also to increase the efficiency in multiphase flow simulations.

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: id=00169227&limit=30&person=true&page=3&respub-action=search.html