Showing papers on "Marangoni effect published in 1995"

Nonlinear excitation of capillary waves by the Marangoni motion induced with a modulated laser beam.

Abstract: We have studied the amplitude and phase relationships of capillary waves produced by modulated laser light on the surface of strongly absorbing liquids. For increasing laser fluences a strong nonlinear behavior is observed. This is connected with the convective motion of the liquid created by surface tension gradients known as the Marangoni motion. The abrupt changes in both the capillary wave amplitude and the phase are found to be due to an autoblocking effect of the heat flux from the region of the laser absorption as the result of the development of a closed circulating flow of the liquid.

Moving zone Marangoni drying of wet objects using naturally evaporated solvent vapor

Abstract: A surface tension gradient driven flow (a Marangoni flow) is used to remove the thin film of water remaining on the surface of an object following rinsing. The process passively introduces by natural evaporation and diffusion of minute amounts of alcohol (or other suitable material) vapor in the immediate vicinity of a continuously refreshed meniscus of deionized water or another aqueous-based, nonsurfactant rinsing agent. Used in conjunction with cleaning, developing or wet etching application, rinsing coupled with Marangoni drying provides a single-step process for 1) cleaning, developing or etching, 2) rinsing, and 3) drying objects such as flat substrates or coatings on flat substrates without necessarily using heat, forced air flow, contact wiping, centrifugation or large amounts of flammable solvents. This process is useful in one-step cleaning and drying of large flat optical substrates, one-step developing/rinsing and drying or etching/rinsing/drying of large flat patterned substrates and flat panel displays during lithographic processing, and room-temperature rinsing/drying of other large parts, sheets or continuous rolls of material.

Marangoni effects of adsorption—desorption controlled surfactants on the leading end of an infinitely long bubble in a capillary

Abstract: The leading end of an infinitely long gas bubble which displaces a viscous surfactant solution in a capillary tube is studied. The surfactant is present at elevated concentration and has sorption controlled mass transfer. The displaced fluid wets the wall, forming a continuous liquid film between the bubble and the capillary wall. Both the thickness of this film and the additional pressure required to aspirate the bubble depend upon the Marangoni stresses caused by non-uniform surfactant adsorption along the interface. The equations governing this flow are solved at asymptotically small capillary number for the case where the balances of momentum and mass transfer are coupled to leading order. As the Marangoni effect is increased over several orders of magnitude, the additional pressure and the wetting-layer thickness increase above the stress-free interface values found by Bretherton (1961) and approach an upper bound of 42/3 times the Bretherton values. Accompanying changes in the surface tension, Marangoni stress and surface velocity profiles as this upper bound is approached are described. Finally, surface viscosities that are intrinsic to the interfacial region are incorporated in the analysis. When small departures from surface equilibrium states are considered, the terms representing surface viscous effects have the same functional form as the Marangoni stresses and result in thicker films and higher additional pressures.

The Interfacial Thermodynamics of Micro Heat Pipes

Abstract: Successful analysis and modeling of micro heat pipes requires a complete understanding of the vapor-liquid interface. A thermodynamic model of the vapor-liquid interface in micro heat pipes has been formulated that includes axial pressure and temperature differences, changes in local interfacial curvature, Marangoni effects, and the disjoining pressure. Relationships were developed for the interfacial mass flux in an extended meniscus, the heat transfer rate in the intrinsic meniscus, the 'thermocapillary' heat-pipe limitation, as well as the nonevaporating superheated liquid film thickness that exists between adjacent menisci and occurs during liquid dry out in the evaporator. These relationships can be used to define quantitative restrictions and/or requirements necessary for proper operation of micro heat pipes. They also provide fundamental insight into the critical mechanisms required for proper heat pipe operation. 29 refs., 6 figs.

Marangoni instabilities in liquid mixtures with Soret effects

Abstract: The stability of a binary liquid mixture heated from above is analysed. The heat transfer is driven by the imposed temperature difference between the horizontal bottom plate and the ambient gas. The mass flux in the layer is induced by the Soret effect. The gravitational effects are ignored, and the instability is driven by solutocapillarity and retarded by thermocapillarity. The interface is allowed to deform, and both the small-wavenumber and the Pearson-type instabilities are studied. Oscillatory instability can exist when the thermocapillary is destabilizing and the solutocapillarity is stabilizing.

Effects of soluble and insoluble surfactant on laminar interactions of vortical flows with a free surface

Abstract: We study the two-dimensional, laminar interactions between a contaminated free surface and a vortical flow below. Two canonical vortical flows are considered: a pair of vortex tubes impinging onto the free surface; and an unstable shear wake behind a body operating on the surface. A quantitative model for free-surface viscous flows in the presence of soluble or insoluble surfactants is developed. For the low to moderate Froude numbers considered here, for which weakly nonlinear free-surface boundary conditions are valid, the surface boundary layer and vorticity production are weak for clean water and the vortical flow evolution does not differ qualitatively from that under a free-slip boundary. When even a small amount of contamination is present, the flow can be dramatically affected. The vortical flow creates gradients in the surfactant surface concentration which leads to Marangoni stresses, strong surface vorticity generation, boundary layers, and even separation. These significantly influence the underlying flow which itself affects surfactant transport in a closed-loop interaction. The resulting flow features are intermediate between but qualitatively distinct from those under either a free- or no-slip boundary. Surfactant effects are most prominent for insoluble surface contamination with likely development of surfactant shocks and associated surface features such as Reynolds ridges. For soluble surfactant with initially uniform bulk concentration, surface concentration variations are moderated by sorption kinetics between the surface and bulk phases, and the overall effects are generally diminished. For initially stratified bulk concentrations, however, the evolution dynamics becomes more varied and surfactant effects may be amplified relative to the insoluble case. The dependence of these results on the properties of the contamination is studied.

The effect of surfactant on bursting gas bubbles

Abstract: A numerical technique, based on the boundary integral method, is developed to allow the modelling of unsteady free-surface flows at large Reynolds numbers in cases where the surface is contaminated by some surface-active compound. This requires the method to take account of the tangential stress condition at the interface and is achieved through a boundary-layer analysis. The constitutive relation that forms the surface stress condition is assumed to be of the Boussinesq type and allows the incorporation of surface shear and dilatational viscous forces as well as Marangoni effects due to gradients in surface tension. Sorption kinetics can be included in the model, allowing calculations for both soluble and insolube surfactants. Application of the numerical model to the problem of bursting gas bubbles at a free surface shows the greatest effect to be due to surface dilatational viscosity which drastically reduces the amount of surface compression and can slow and even prevent the information of a liquid jet. Surface tension gradients give dilatational elasticity to the surface and thus also significantly prevent surface compression. Surface shear viscosity has a smaller effect on the interface motion but results in initially increased surface concentrations due to the sweeping up of surface particles ahead of the inward-moving surface wave.

Mass transfer across liquid-phase boundaries

Abstract: A literature review on various analyses referring to the Marangoni effect is given The stability behaviour during mass transfer has been analyzed in the toluene/acetone/water system by holographic interferometry at a flat moving and quiescent phase boundary Concentration differences between the bulk phases were varied and the influence of the direction of mass transfer and of the density profile in the mass exchanging phases have been considered The experiments show the development of strong eddies in the area of the phase boundary as a result of growing concentration differences, with equilibrium being attained more rapidly A limiting influence of diffusive transport in the bulk phases was observed

Casting process for hypermonotectic alloys under terrestrial conditions

Abstract: Bearing materials are generally heterogeneous materials, containing hard as well as soft, phases. Hypermonotectic AlPb and AlBi alloys, especially, are considered as exceptionally qualified bearing materials if they also contain additional hard phases to decrease wear. Based on the considerable differences in the density of the decomposed fluid phases at high temperatures and the high velocity of separation, such alloys cannot, to date, be manufactured under terrestrial conditions. The results of microgravity experiments for the manufacturing of suitable sample material with a fine phase dispersion of the monotectic phases were rather disappointing. The cause of the rapid phase separation and local enrichment under microgravity conditions was found to be the Marangoni convection, the effects of which, to date, have been underestimated. The results of these space experiments are now utilized in a terrestrial casting process, whereby a comparatively high Marangoni convection is superposed in the opposite direction to the sedimentation action of gravity, thereby partially compensating the effects of gravity. Thus, cast strips of AlSiPb and AlSiBi alloys could be manufactured, the lead and bismuth phases being present in a characteristic fine dispersion overthe length of the cast strips. The first tribological laboratory tests give an indication of the excellent suitability of such advanced bearing material for the future.

Spontaneous motion of two droplets caused by mass transfer

Abstract: Spontaneous motion of two droplets driven by surface tension gradients which result from mass transfer between the droplets and the continuous fluid phase is considered It is shown that mass flux of a weak surfactant from the dispersed phase into the continuous one generates the droplet motion toward each other ; the oppositely directed mass transfer makes the droplets move apart from each other In the approximation of a quasistationary Stokes flow and a low Peclet number in the outer fluid, the velocities of the droplets' spontaneous motion are calculated as functions of the separation distance between the drops and the Sherwood number characterizing the intensity of mass transfer The mass transfer driven interaction of two drops significantly influences their motion and coalescence in the gravity field The velocities of the droplets in this case are calculated as functions of the Marangoni and Archimedean numbers, as well as the ratios of the droplet radii The results obtained account for a large number of experimental data on the influence of mass transfer direction on the rate of coalescence of drops and bubbles

Coarsening of liquid AlPb dispersions under reduced gravity conditions

Abstract: In the multifurnace assembly (MFA) of the free-flying satellite EURECA, experiments were performed with molten AlPb alloys to investigate Ostwald ripening of liquid-liquid dispersions under diffusive conditions free from any sedimentation effect. AlPb alloys with very low volume fractions of dispersed phase (0.75 and 1.1 vol.%) were processed by annealing cylindrical samples within the liquid miscibility gap at 1073 K for 1, 5 and 20 h. The initial droplet size distribution was centered around 0.5 μm diameter. After the annealing treatments in the MFA a coarsening of the droplets to sizes around 5–10 μm was observed. The droplets were, however, not evenly distributed in the samples. There was some drift which can be attributed to Marangoni motion caused by small temperature gradients (0.5-3 K cm−1) during the annealing treatment and also the heating and cooling periods. The microstructural observations agree with similar results obtained in Spacelab 1 with ZnPb alloys. The coarsening is a superposition of diffusional Ostwald ripening and coagulation of droplets by Marangoni motion and discussed with a model based on population dynamics.

Effects of interaction between Marangoni and double-diffusive instabilities

Abstract: The effect of surface tension on the onset of convection in horizontal double-diffusive layer was studied both experimentally and by linear stability analysis. The experiments were conducted in a rectangular tank with base dimension of 25×13 cm and 5 cm in height. A stable solute (NaCl) stratification was first established in the tank, and then a vertical temperature gradient was imposed. Vertical temperature and concentration profiles were measured using a thermocouple and a conductivity probe and the flow patterns were visualized by a schlieren system. Two types of experiments were carried out which illustrate the effect of surface tension on the onset of convection. In the rigid–rigid experiments, when the critical thermal Rayleigh number, RT, is reached, large double-diffusive plumes were seen simultaneously to rise from the heated bottom and descend from the cooled top. In the rigid–free experiments, owing to surface tension effects, the first instability onset was of the Marangoni type. Well-organized small plumes were seen to emerge and persist close to the top free surface at a relatively small RTM (where subscript M denotes ‘Marangoni’). At larger RTt > RTM (where subscript t denotes ‘top’) these plumes evolved into larger double-diffusive plumes. The onset of double-diffusive instability at the bottom region occurred at a still higher RTb > RTt (where subscript b denotes ‘bottom‘). A series of stability experiments was conducted for a layer with an initial top concentration of 2 wt% and different concentration gradients. The stability map shows that in the rigid–free case the early Marangoni instability in the top region reduces significantly the critical RT for the onset of double-diffusive convection. Compared with the rigid–rigid case, the critical RT in the top region is reduced by about 60% and in the bottom region by about 30%. The results of the linear stability analysis, which takes into account both surface tension and double-diffusive effects, are in general agreement with the experiments. The analysis is then applied to study the stability characteristics of such a layer as gravity is reduced to microgravity level. Results show that even at 10 −4g0, where g0 is the gravity at sea level, the double-diffusive effect is of equal importance to the Marangoni effect.

Marangoni Effects on Wave Structure and Liquid Film Breakdown along a Heated Vertical Tube

Abstract: This study provides the effects of heating at the tube surface on the flow instability and the onset of a dry patch in a subcooled liquid film. A simplified approach to analyze the wave structure was used by adding the shear stress due to the Marangoni effects to the isothermal film-flow equation. Theoretical analysis shows the liquid film for low flow rate conditions becomes unstable at a constant Marangoni number. Thermography technique was employed for determining the temperature distribution along the liquid surface. The dynamic process to the liquid film breakdown was discussed in relation to the wave structure.

Viscous flow at infinite Marangoni number.

Abstract: We formulate the theory of surface tension driven Stokes flow set up by an active scalar with zero diffusivity. The 3D hydrodynamic problem can be reduced to a 2D nonlinear evolution equation involving only free surface quantities. For a semi-infinite layer it can be rigorously demonstrated that the solutions to this equation blow up in finite time and develop singular forms. The new type of nonlinearity plays a universal role in the description of interfacial turbulence.

Time-growth and correlation length of fluctuations in thermocapillary convection with surface deformation

Abstract: Thermocapillary instabilities with surface deformation in a fluid layer heated from below are studied; both gravity and capillary waves are considered. The aim of the present work is to determine the dependence of the time growth of fluctuations and their correlation length in supercritical conditions with respect to the crispation, Bond, Prandtl and Biot numbers. Two kinds of instabilities are analysed: the «classical» Marangoni mode correspondind to wave-numher approximately equal to two and the long-wave mode for which the wave-numher is zero

On Bénard–Marangoni instability in the presence of a magnetic field

Abstract: Explicit asymptotic expressions are derived for the first unstable mode of surface tension driven instability in an electrically conducting fluid subjected to a strong magnetic field. The spatial structure of the velocity, temperature, and electric current density is characterized in terms of Hartmann boundary layers—a concept that permits a physical explanation of the role of the magnetic field and an understanding of scaling laws derived in previous work.

Finite element simulation of combined buoyancy and thermocapillary driven convection in open cavities

Abstract: Thermocapillary-induced and buoyancy-driven convective flows that commonly occur in crystal growth are numerically simulated using Galerkin finite element method. The physical domain comprises of a open cavity with aspect ratio one and differentially heated vertical walls. The top gas–melt interface is free to deform subject to 90° contact angle boundary conditions at the two vertical walls. The unsteady two-dimensional Navier–Stokes equations are discretized in time using Chorin-type splitting scheme and pressure is determined from the Poisson's equation. The free surface is taken to be resting on vertical spines and its evolution in time is determined from the kinematic free surface equation. The governing equations for heat and momentum are solved in the Arbitrary Lagrangian Eulerian frame of reference to handle the moving boundary. The influence of Grashof number, Marangoni number, Bond number, Ohnesorge number and Prandtl number on the flow field and heat transfer is investigated.

The role of time-varying gravity on the motion of a drop induced by Marangoni instability

Abstract: We study the influence of a time‐dependent force on the translatory convective instability of a drop due to surface stresses (Marangoni effect). This effect and its associate flows inside and outside the drop are induced by solute transfer from within the drop to the drop surface and its consumption there in an isothermal chemical reaction. In particular, we show that for a gravity field sinusoidally varying in time, the drop rather than aligning with the direction of the force tends to move in a direction orthogonal to it.