275 results found
Smith ER, Müller EA, Craster RV, et 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.
Hennessy MG, Ferretti GL, Cabral JT, et 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.
Morgan RG, Ibarra R, Zadrazil I, et 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
Xie Z, Pavlidis D, Salinas P, et 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.
Ibarra R, Morgan R, Zadrazil I, et 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.
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.
Tinguely M, Hennessy MG, Pommella A, et 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.
Muller EA, Matar OK, Jaeger F, et 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.
Vitale A, Hennessy M, Matar O, et al., 2016, Unified approach for polymeric patterning via controlling the propagation of frontal photopolymerization waves, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Charogiannis A, Heiles B, Mathie R, et al., 2016, 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
Ferretti GL, Nania M, Matar OK, et al., 2016, Wrinkling measurement of the mechanical properties of drying salt thin films, Langmuir, Vol: 32, Pages: 2199-2207, ISSN: 1520-5827
Kovalchuk NM, Matar OK, Craster RV, et 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.
Roumpea E, Chinaud M, Kahouadji L, et al., 2016, Plug flow of shear-thinning liquids in microchannels, Pages: 395-397
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
Theodorakis PE, Müller EA, Craster RV, et 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.
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.
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.
Vitale A, Hennessy M, Matar O, et 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.
Pavlidis D, Gomes JLMA, Xie Z, et 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.
Hennessy M, Vitale A, Cabral JT, et 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.
Ibarra R, Zadrazil I, Markides CN, et al., 2015, Towards a Universal Dimensionless Map of Flow Regime Transitions in Horizontal Liquid-Liquid Flows, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Hennessy MG, Vitale A, Matar OK, et 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.
Ibarra R, Matar OK, Markides CN, et 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.
Theodorakis P, Kovalchuk NM, Starov VM, et al., 2015, Superspreading: Mechanisms and Molecular Design, Mainz Material Simulation Days 2015, Pages: 29-29
Wray AW, Matar OK, Craster, et 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.
Saenz PJ, Sefiane K, Kim J, et 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.
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%.
Yang J, Tajudin ZB, Coletti F, et al., 2015, Numerical simulation of fouling in crude-oil heat exchangers: The interaction between different fouling routes, Pages: 833-842
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