Showing papers on "Marangoni effect published in 2002"
TL;DR: The origin of the formation of this core microstructure can be explained by Marangoni motion on the basis of the temperature dependence of the interfacial energy, which shows that this type of powder can be formed even if the cooling rate is very high.
Abstract: The egg-type core microstructure where one alloy encases another has previously been obtained during experiments in space. Working with copper-iron base alloys prepared by conventional gas atomization, we were able to obtain this microstructure under gravity conditions. The minor liquid phase always formed the core of the egg, and it sometimes also formed a shell layer. The origin of the formation of this core microstructure can be explained by Marangoni motion on the basis of the temperature dependence of the interfacial energy, which shows that this type of powder can be formed even if the cooling rate is very high.
270 citations
TL;DR: In this article, the formation and stability of stationary laser weld keyholes were investigated using a numerical simulation, where the effect of multiple reflections in the keyhole was estimated using the ray tracing method, and the free surface profile, flow velocity and temperature distribution were calculated numerically.
Abstract: The formation and stability of stationary laser weld keyholes are investigated using a numerical simulation. The effect of multiple reflections in the keyhole is estimated using the ray tracing method, and the free surface profile, flow velocity and temperature distribution are calculated numerically. In the simulation, the keyhole is formed by the displacement of the melt induced by evaporation recoil pressure, while surface tension and hydrostatic pressure oppose cavity formation. A transition mode having the geometry of the conduction mode with keyhole formation occurs between the conduction and keyhole modes. At laser powers of 500 W and greater, the protrusion occurs on the keyhole wall, which results in keyhole collapse and void formation at the bottom. Initiation of the protrusion is caused mainly by collision of upward and downward flows due to the pressure components, and Marangoni flow has minor effects on the flow patterns and keyhole stability.
237 citations
TL;DR: It is suggested that the spreading of trisiloxane ethoxylates is controlled by a surface tension gradient, which forms when a drop of surfactant solution is placed on a solid surface, and that the aggregates and vesicles formed in trisiles do not initiate the spreading process and therefore these structures are not a requirement for the superspreading process.
195 citations
TL;DR: Data demonstrate that, by tuning the liquid-gas boundary condition, the patterns formed from an evaporating drop can be controlled, which was not anticipated in prior studies.
Abstract: Evaporating drops provide a means of organizing particles suspended within them. Here, the manner in which surfactants alter these patterns is studied as a function of the surface state of an insoluble monolayer at the drop interface. The surface state is visualized throughout the drop evolution using fluorescence microscopy. A regime of surfactant coverage is identified that creates conditions that enhance the Marangoni-Benard instability. This result was not anticipated in prior studies, in which surfactants are predicted to prevent this instability. These data demonstrate that, by tuning the liquid-gas boundary condition, the patterns formed from an evaporating drop can be controlled.
193 citations
TL;DR: In this article, partial differential equations governing the spatio-temporal evolution of the interface and surfactant concentrations are derived in the long wavelength approximation for the breakup of viscous liquid threads covered with insoluble surfactants.
Abstract: The breakup of viscous liquid threads covered with insoluble surfactant is investigated here; partial differential equations governing the spatio-temporal evolution of the interface and surfactant concentrations are derived in the long wavelength approximation. These one-dimensional equations are solved numerically for various values of initial surfactant concentration, surfactant activity and the Schmidt number (a measure of the importance of momentum, i.e., kinematic viscosity, to surfactant diffusion). The presence of surfactant at the air–liquid interface gives rise to surface tension gradients and, in turn, to Marangoni stresses, that drastically affect the transient dynamics leading to jet breakup and satellite formation. Specifically, the size of the satellite formed during breakup decreases with increasing initial surfactant concentration and surfactant activity. The usual self-similar breakup dynamics found in the vicinity of the pinchoff location for jets without surfactant [Eggers, Phys. Rev. L...
105 citations
TL;DR: In this article, the authors investigated the effect of active flux on the Marangoni convection in the welding pool, and showed that the depth/width ratio of the weld pool was closely related to the oxygen content in the pool.
Abstract: Active flux can modify the fusion zone geometry dramatically in GTA welding (A-TIG). In the present study, in order to investigate the effect of the active flux on the Marangoni convection in the welding pool, bead-on-plate specimens are made on SUS304 stainless steel pre-placed with single active flux, Cu 2 O, NiO, Cr 2 O 3 , SiO 2 and TiO 2 by the GTA process. Weld pool cross-sections and the bead surface morphology are analyzed by optical microscopy after welding. The oxygen content in the weld metal is measured using a HORIBA EMGA-520 Oxygen/Nitrogen Analyzer. The results showed that the depth/width ratio of the weld pool was closely related to the oxygen content in the pool. The oxygen content in the weld metal increases with the quantity of fluxes, Cu 2 O, NiO, Cr 2 O 3 , SiO 2 and TiO 2 . However, for the TiO 2 oxide flux, the highest oxygen content in the weld metal is below 200 ppm. As the oxygen content in the weld metal is in a certain range of 70-300 ppm, the depth/width ratio increases to 1.5 to 2.0 times. Too low or too high oxygen content in the pool does not increase the depth/width ratio. The oxygen from the decomposition of the flux in the welding pool alters the surface tension gradients on the weld pool surface, and hence, changes the Marangoni convection direction and the weld pool penetration depth.
103 citations
TL;DR: In this paper, three characteristic dynamical regimes of spreading drops (viscous capillary, viscous-inertia-capillary, and inertia capillary) are studied using a non-invasive optical technique.
100 citations
TL;DR: In this paper, a semi-empirical approach is proposed to study the dynamics of shrinking of thin alkane droplets evaporating on isothermal surfaces, where only evaporation is taken into account.
Abstract: The dynamics of shrinking of thin alkane droplets evaporating on isothermal surfaces has been studied in the case of complete wetting, where no pinning of the contact line is expected. As the resolution of the complete hydrodynamic equation is still an open problem, we propose a semiempirical approach, where only evaporation is taken into account. A careful analysis of the mathematical consequences of the form chosen for the evaporation term is given. The agreement with experiments is extremely good, suggesting that hydrodynamic contributions and Marangoni flows are probably negligible for the simple liquids considered.
95 citations
TL;DR: In this article, the linear stability of a two-fluid shear flow with an insoluble surfactant on the flat interface is investigated in the Stokes approximation, and asymptotic long-wave expressions for the growth rates are obtained.
Abstract: The linear stability of a two-fluid shear flow with an insoluble surfactant on the flat interface is investigated in the Stokes approximation. Gravity is neglected in order to isolate the Marangoni effect of the surfactant. In contrast to all earlier studies of related fluid systems, we encounter (i) the destabilization (here, of a shear flow) caused solely by the introduction of an interfacial surfactant and (ii) the destabilization (here, of a system with a surfactant) caused solely by the imposition of a Stokes flow. Asymptotic long-wave expressions for the growth rates are obtained.
94 citations
TL;DR: In this paper, a hanging evaporating drop is investigated and the velocity field inside the droplet is evaluated by monitoring the motion of tracers within the drop, in the meridian plane, using a charge-coupled device (CCD) camera.
Abstract: Experiments and numerical simulations are carried out to study Marangoni and buoyancy effects in a hanging evaporating drop. The liquids investigated are n-octane, which exhibits Marangoni effect, and water, which does not exhibit thermal Marangoni effect. The disk sustaining the drop (diameter of a few millimeters) is held at a constant temperature. A temperature difference arises in the droplet as a consequence of the energy exchange with the ambient and of the evaporative cooling. In the presence of surface tension gradients (Marangoni effect), convective flows are established, and small surface temperature differences are measured at the drop-ambient interface. When the thermal Marangoni effect is absent (as in the water droplet), the surface temperature is stratified, and much larger surface temperature differences are established over the drop surface. The velocity field inside the droplet is evaluated by monitoring the motion of tracers within the drop, in the meridian plane, using a charge-coupled device (CCD) camera. The surface temperature distribution is detected by an infrared camera
93 citations
TL;DR: In this article, a mathematical model for the drainage of a surfactant-stabilised foam lamella, including capillary, Marangoni and viscous effects, is presented.
Abstract: We present a mathematical model for the drainage of a surfactant-stabilised foam lamella, including capillary, Marangoni and viscous effects and allowing for diffusion, advection and adsorption of the surfactant molecules.
We use the slender geometry of a lamella to formulate the model in the thin-film limit and perform an asymptotic decomposition of the liquid domain into a capillary-static Plateau border, a time-dependent thin film and a transition region between the two. By solving a quasi-steady boundary-value problem in the transition region, we obtain the flux of liquid from the lamella into the Plateau border and thus are able to determine the rate at which the lamella drains.
Our method is illustrated initially in the surfactant-free case. Numerical results are presented for three particular parameter regimes of interest when surfactant is present. Both monotonic profiles and those exhibiting a dimple near the Plateau border are found, the latter having been previously observed in
experiments. The velocity field may be uniform across the lamella or of parabolic Poiseuille type, with fluid either driven out along the centre-line and back along the free surfaces or vice versa. We find that diffusion may be negligible for a typical real surfactant, although this does not lead to a reduction in order because of the inherently diffusive nature of the fluid-surfactant interaction.
Finally, we obtain the surprising result that the flux of liquid from the lamella into the Plateau border increases as the lamella thins, approaching infinity at a finite lamella thickness.
TL;DR: In this article, the effects of welding speed, Marangoni convection and natural convection on melt flow and heat transfer are all included in the modelling, and thus a three-dimensional (3D) approach is employed.
Abstract: Modelling results are presented concerning the laser full-penetration welding characteristics. The effects of welding speed, Marangoni convection and natural convection on melt flow and heat transfer are all included in the modelling, and thus a three-dimensional (3-D) approach is employed. Comparison of the present 3-D modelling results with corresponding two-dimensional ones shows that besides the welding speed, Marangoni convection also plays critical role in determining the temperature distribution in the workpiece and melt flow in the weld pool and cannot be ignored even for the full-penetration welding of a thin plate. A method is described concerning how to use the present 3-D modelling results to estimate the keyhole radius or predict the energy efficiency in the laser full-penetration welding.
TL;DR: In this paper, the two-dimensional dewetting dynamics of ultrathin liquid films are studied in the presence of insoluble surfactant; surfactants can drive a flow due to surface tension gradients and additionally the coefficients of the intermolecular potential, which are usually assumed to be constant, can depend on the surface tension gradient.
Abstract: Many industrially and technologically important situations involve thin films covered with either pre-existing or introduced surfactant, which can potentially affect dewetting and spreading processes. The two-dimensional dewetting dynamics of ultrathin liquid films are studied here in the presence of insoluble surfactant; surfactants can drive a flow due to surface tension gradients and additionally the coefficients of the intermolecular potential, which are usually assumed to be constant, can depend on the surfactant concentration. Coupled evolution equations for the film height and surfactant concentration are derived using lubrication theory. These equations are parameterized by a Marangoni parameter, M, and the equilibrium film thickness, lc, obtained by setting the intermolecular potential to zero. A linear stability analysis of these equations shows that the presence of surfactant can widen the band of unstable wave numbers and that, for relatively large lc, the selected wave number is minimized for...
10 Jan 2002
TL;DR: Fluid Science Relevance in Microgravity Research Mechanical Behaviour of Liquid Bridges in microgravity Interfacial Phenomena Thermal Marangoni Flows INTERfacial Patterns and Waves Fluid Mechanics of Bubbles and Drops Diffusion and Thermodiffusion in Micro-gravity Critical and Supercritical Fields and Related Phenomenas Microgravity Two Phase Flow and Heat Transfer Transient and Sloshing Motions in an Unsupported Container Pool Boiling and Bubble Dynamics in Micro -gravity Combustion PhenomenA at Microgravity Fluid Flow and Solute Segregation
Abstract: Fluid Science Relevance in Microgravity Research Mechanical Behaviour of Liquid Bridges in Microgravity Interfacial Phenomena Thermal Marangoni Flows Interfacial Patterns and Waves Fluid Mechanics of Bubbles and Drops Diffusion and Thermodiffusion in Microgravity Critical and Supercritical Fields and Related Phenomena Microgravity Two Phase Flow and Heat Transfer Transient and Sloshing Motions in an Unsupported Container Pool Boiling and Bubble Dynamics in Microgravity Combustion Phenomena at Microgravity Fluid Flow and Solute Segregation in Crystal Growth from the Melt Fluid Flows and Macromolecular Crystal Growth in Microgravity Fluid Dynamics Experiment Sensitivity to Accelerations Prevailing on Microgravity Platforms Facilities for Microgravity Fluid Science Research Onboard Appendix A: ISs Assembly Sequence Appendix B: Flight Control Positions and Their Call Signs in the International Space Station.
TL;DR: In this paper, the authors use linear stability analysis of the lubrication approximation to show that spatially nonuniform time-dependent radiative heating can indeed have a stabilizing effect.
Abstract: In the process of drying, thin volatile liquid films often undergo a long-wavelength instability leading to nonuniformities or formation of dry spots, with the strength of the instability increasing with the volatility and temperature of the liquid. Perhaps counterintuitively, this evaporative instability can be actively suppressed by an appropriate heating procedure. We use linear stability analysis of the lubrication approximation to show that spatially nonuniform time-dependent radiative heating can indeed have a stabilizing effect. Evaporation is shown to introduce several fundamentally new aspects into the control problem for heated liquid films, compared to the relatively well-studied case of thermal convection. The control problem becomes especially interesting and nontrivial for mixtures and solutions with negative Marangoni numbers due to a peculiar cancellation effect rendering the system insensitive to temperature control at a certain wavelength. It is shown that taking the time dependence of the mean thickness of the film into account is necessary to circumvent this insensitivity.
TL;DR: In this article, the optimal leveling of the ridge by means of a Marangoni stress such as might be produced by a localized heater creating temperature variations at the film surface is investigated.
Abstract: A thin viscous film flowing over a step down in topography exhibits a capillary ridge preceding the step. In applications, a planar liquid surface is often desired and hence there is a need to level the ridge. This paper investigates optimal leveling of the ridge by means of a Marangoni stress such as might be produced by a localized heater creating temperature variations at the film surface. The differential equation for the free surface based on lubrication theory and incorporating the effects of topography and temperature gradients is solved numerically for steps down in topography with different temperature profiles. Both rectangular “top-hat” and parabolic profiles, chosen to model physically realizable heaters, were found to be effective in reducing the height of the capillary ridge. Leveling the ridge is formulated as an optimization problem to minimize the maximum free-surface height by varying the heater strength, position, and width. With the optimized heaters, the variation in surface height is...
TL;DR: In this article, the authors show that the initial rate of spreading is not influenced by the forces at the contact line or at the solid-solution interface, leading to the conclusion that the major driving force for spreading of trisiloxane surfactant solution on hydrophobic surfaces is the surface tension gradient over the air surface.
TL;DR: In this paper, the authors investigated a mechanism that could create and drive van der Waals rupture via numerical simulations coupled with analytical techniques, where the spreading process itself is modelled with a pair of coupled evolution equations for the fluid film thickness and surfactant concentration that are derived in the lubrication approximation.
Abstract: An intriguing, dramatic and, at present, not fully understood instability often accompanies surfactant driven flows on thin films. This paper investigates a candidate mechanism that could create and drive this instability, van der Waals rupture, via numerical simulations coupled with analytical techniques. The spreading process itself is modelled with a pair of coupled evolution equations for the fluid film thickness and surfactant concentration that are derived in the lubrication approximation. These equations are then linearized about a base state that corresponds to the one-dimensional rupturing solution, and equations for the evolution of the transverse disturbances are derived. These linearized equations are investigated in several ways: numerical simulations where the perturbations are driven by the time evolving base state, or where the base state is frozen at a time tf close to the rupture event. The quasistatic initial value problem is also investigated as an eigenvalue problem, where the eigenva...
TL;DR: In this paper, a microscale heater array 2: 7m m� 2:7 mm in size along with a video camera was used to provide subcooled pool boiling heat transfer measurements at gravity levels ranging from 1.8g to 10 � 6 g.
TL;DR: In this article, the authors examine the evolution of a temperature gradient at the free surface of a coating solution during the spin coating process and compare these temperature gradient values with their relevant threshold values, determined from prior reports in the literature, in order to deduce the magnitude of the instabilities they represent.
Abstract: The evolution of a temperature gradient at the free surface of a coating solution during the spin coating process is examined. Solvent evaporation causes localized cooling at the top that can result in thermocapillary instability within the coating solution, and thereby driving convective flows that may result in non-uniform coatings. We examine the evolution of these temperature gradients by using a one dimensional finite difference model that simultaneously describes the thinning behavior (both by flow and by evaporation) and the temperature evolution within the solution. The entire system is initially isothermal but is subject to evaporation-driven cooling at the free surface of the gradually thinning fluid. The model is then used to determine the magnitude of the thermocapillary effects during the spin coating process. As test systems we simulate the spin coating of several pure alcohol solutions having different volatilities and therefore different evaporative-cooling powers. As the fluid thins, we calculate the instantaneous Marangoni (Mn) number, which signifies the magnitude of thermocapillary-driven convection. We compare these Mn values against their relevant threshold values, determined from prior reports in the literature, in order to deduce the magnitude of the instabilities they represent. If the Mn value is super-critical, then the instability that it represents will be sufficient for the onset of convection cells within a stagnant fluid layer of corresponding thickness. Because the radial outflow is fully laminar under normal conditions, super-critical Mn values imply that similar instabilities would arise within a spinning solution. Super-critical Mn values were observed under numerous conditions suggesting that thermocapillary instability may be responsible for striation features that develop in coatings made by spin coating. Trends related to spin-speed, solvent volatility, and initial solution thickness are discussed with the goal of improving the flatness of coatings that are made by this process.
TL;DR: In this paper, the effect of different basic temperature gradients on the onset of ferroconvection driven by combined surface tension and buoyancy forces is studied, where the lower boundary is assumed to be rigid and either conducting or insulating to temperature perturbations while the upper boundary at which the surface tension acts is free insulating and non-deformable.
TL;DR: The effect of physical compatibilization on the deformation and coalescence of droplets in immiscible polymer blends is discussed in this article, where evidence is provided for the existence of concentration gradients in block copolymers along the interface during deformation.
Abstract: The effect of physical compatibilization on the deformation and coalescence of droplets in immiscible polymer blends is discussed. Evidence is provided for the existence of concentration gradients in block copolymers along the interface during deformation. This causes complex changes in droplet shapes during deformation and relaxation. These concentration gradients also result in Marangoni stresses, which stabilize the droplets against deformation and breakup. Coalescence experiments have been performed, varying both the compatibilizer concentration and the shear rate. Existing coalescence models have been evaluated. An empirical extension of Chesters' partially mobile interface model is presented, that treats the effects of Marangoni stresses on the coalescence process as a higher effective viscosity ratio.
TL;DR: In this paper, the authors examined the hydrodynamics of dip-coated, finite-length films of evaporative fluids, from the film tip through the film body all the way to the connection with the main meniscus.
Abstract: We examine experimentally the hydrodynamics of dip-coated, finite-length films of evaporative fluids, from the film tip through the film body all the way to the connection with the main meniscus. The characteristic film thickness has a power-law dependence on the withdrawing speed similar to that for the thickness of “infinite” films formed by nonvolatile liquids. The film length and cross-sectional area have power-law dependence on the withdrawing speed as well, but the prefactors of the power laws are controlled by the evaporation rate of the fluid. These power laws are consistent with the global mass balance over the film between mass lost by evaporation and mass input by the solid motion. We have also found that the apparent contact angle and the curvature at the film tip both have power-law dependencies on the withdrawing speed that are consistent with those found for the length and the film thickness. Film shape measurements near the film tip reach thicknesses ∼100 A from the solid; but we did not detect any influence of the inner scale hydrodynamics and van der Waals forces on this shape. We have developed a systematic method for measuring the contributions of gravity, capillary force, viscous force, and vapor recoil on the pressure and flow fields in the film. This exercise reveals detailed information about the flow in evaporative films. The combined effects of evaporation and Marangoni flow on the hydrodynamics are deduced from experimental data, independent of evaporation models.
TL;DR: Three-dimensional surface-tension-driven Bénard convection at zero Prandtl number is computed in the smallest possible doubly periodic rectangular domain that is compatible with the hexagonal flow structure at the linear stability threshold of the quiescent state.
Abstract: Three-dimensional surface-tension-driven Benard convection at zero Prandtl number is computed in the smallest possible doubly periodic rectangular domain that is compatible with the hexagonal flow structure at the linear stability threshold of the quiescent state. Upon increasing the Marangoni number beyond this threshold, the initially stationary flow becomes quickly time dependent. We investigate the transition to chaos for the case of a free-slip bottom wall by means of an analysis of the kinetic energy time series. We observe a period-doubling scenario for the transition to chaos of the energy attractor, intermittent behavior of a component of the mean velocity field, three characteristic energy levels, and two frequencies that contain a considerable amount of the power spectral density connected with the kinetic energy time series.
TL;DR: The role of magnetic field in the inhibition of natural convection driven by combined buoyancy and surface tension forces in a horizontal layer of an electrically conducting Boussinesq fluid with suspended particles confined between an upper free/adiabatic and a lower rigid/isothermal boundary is considered in 1g and μg situations in this article.
TL;DR: In this article, orderly roll and polygonal convection patterns in liquid films were observed using schlieren photography, and the results indicated that the mass-transfer rate was largely enhanced by interfacial turbulence.
Abstract: Interfacial turbulence caused by both Rayleigh and Marangoni instablity in the physical absorption and desorption of carbon dioxide into and from nonaqueous solvents was studied experimently. Orderly roll and polygonal convection patterns in liquid films were observed using schlieren photography. Liquid mass-transfer coefficients were measured in a quiescent gas−liquid contactor and in a gas−liquid channel for stratified, laminar, and cocurrent flows, and the results were compared with values calculated by penetration theory. The measured data indicate that the mass-transfer rate is largely enhanced by interfacial turbulence. Correlations for liquid-phase mass-transfer enhancement factors were developed.
TL;DR: In this article, a set of global analyses of momentum, heat and mass transfer in small Cz furnaces (crucible diameter: 7.2 cm, crystal diameter: 3.5 cm, operated in a 10 Torr argon flow environment) is carried out using the finite element method.
TL;DR: In this paper, the authors report on convection experiments with a fluid with Prandtl number Pr=7 in a Czochralski model system and in annular gaps at normal gravity (12g) and under microgravity (mg).
TL;DR: In this article, a NASA KC-135 aircraft was used to assess the validity of two hypotheses proposed for the growth of macrovoid (MV) pores formed during the dry-casting of cellulose acetate (CA)/acetone/water casting solutions.
TL;DR: Quantitative measurements of both wave number selection and defect motion in nonequilibrium hexagonal patterns are reported, using a novel optical technique used to imprint initial patterns with selected characteristics in a Bénard-Marangoni convection experiment.
Abstract: We report quantitative measurements of both wave number selection and defect motion in nonequilibrium hexagonal patterns. A novel optical technique ("thermal laser writing") is used to imprint initial patterns with selected characteristics in a Benard-Marangoni convection experiment. Initial patterns of ideal hexagons are imposed to determine the band of stable pattern wave numbers while initial patterns containing an isolated penta-hepta defect are imprinted to study defect propagation directions and velocities. The experimental results are compared to recent theoretical predictions.