Showing papers on "Marangoni effect published in 1988"

Radar signatures of oil films floating on the sea surface and the Marangoni effect

Abstract: Airborne radar backscattering experiments carried out recently by Singh et al. (1986) over sea surfaces covered with mineral oil films show that the radar cross section depression has a maximum as a function of incidence angle. In this paper we show that Singh et al.'s measurements can be explained by the Marangoni effect if the assumption is made that the mineral oil spill contained surface active material as “impurities.” The Marangoni effect causes a resonance-type wave damping in the short gravity wave region when the sea surface is covered with a viscoelastic film. Maximum wave damping was observed by Singh et al. (1986) at frequencies around 8 Hz. Marangoni theory predicts such a maximum for surface active compounds of medium to low wave damping ability with a dilational modulus of the order of 0.01 Nm−1 Furthermore, it is demonstrated that the Ku band and C band radar backscattering depression curves do not contradict each other. It is pointed out that when converting the radar data into information on depression of spectral energy density of short surface waves, one has to take into account that the radar backscattering at low incidence angles is dominated by specular reflection, while at intermediate incidence angles it is dominated by Bragg scattering.

A new interpretation of round embayments in quartz crystals

Abstract: The surfaces of quartz crystals that were partially dissolved in superheated, H2O-saturated rhyolite melt are covered with hemispherical embayments; each embayment is judged to have formed where a gas bubble in the melt approached the crystal ‘Flux-line attack’ and ‘upward-drilling’ of the refractory lining of glass tanks are analogous processes As a bubble nears a dissolving solid it enters a compositional boundary layer in the melt, resulting in melt of variable surface tension surrounding the bubble This unstable situation results in small-scale convection of the melt about the bubble (Marangoni convection) which can cause locally enhanced dissolution rate of the solid It is suggested that this mechanism could cause round embayments to form in quartz phenocrysts in acid volcanic and sub-volcanic rocks Criteria by which embayed phenocrysts formed by dissolution can be distinguished from those formed by unstable growth are reviewed briefly

Temperature and velocity fields due to surface tension driven flow

Abstract: Temperature and velocity fields near an air-bubble in silicon-oil under a heated horizontal wall were investigated. The studies were made with silicon oils of different viscosities so that a wide range of Marangoni numbers was encountered. Schlieren interferograms were taken to analyse the temperature field. For the axisymmetric problem the Abel integral equation was solved numerically by using a coefficient procedure. From the recorded temperature distributions isotherms, radial temperature lines and the temperature along the bubble surface were determined graphically. At low Prandtl- and high Marangoni-numbers an oscillatory instability was observed. The flow field was investigated with neutral-buoyant light scattering glass spheres by observation of the meridian plane of the bubble with a thin light-sheet. The convective mechanism of the flow was recorded photographically by taking pictures with various exposure times. Thus a qualitative and quantitative description of the flow was possible.

Numerical calculation of the thermocapillary motion of a bubble under microgravity

Abstract: The migration of a gas bubble under the action of surface tension and friction is dealt with numerically in some detail. The bubble is immersed in an infinitely extended liquid medium. Within the liquid there exists a constant temperature gradient far away from the bubble. Since surface tension decreases with increasing temperature, the bubble will move in the direction of the higher temperature (Marangoni convection). For creeping convection flow the influence of the convective terms in the energy equation on the flow field and, in particular, on the speed of migration is investigated for higher Marangoni numbers (Mg > 0). For small Marangoni numbers (Mg < 3), analytical results concerning the speed of migration and obtained by Subramanian, as well as the classical solution by Young, Goldstein and Block, for creeping flow, are confirmed. Finally, for non-zero values of the Reynolds numbers and for larger values of the Marangoni numbers, the migration speed of the bubble has been calculated. The nonlinear...

Mass transport effects on the stability of emulsion: emulsion films with acetic acid and acetone diffusing across the interface

Abstract: The stability of emulsions is studied using, as a model of two interacting drops, an aqueous film of a surfactant immersed in an oil phase. It is shown that the mass transfer of a solute across the film changes its life-time. This change depends on several parameters as the nature and concentration of the solute. the direction of mass transfer, the time elapsed after the formation of the film. The destabilizing effect, of the transfer is found to be much less pronounced when the solute is in the continuous water phase. The instability is ascribed to the Marangoni effect and/or to liquid flow from the film drawn by diffusion of the solute.

Utilization of the Second Gradient Theory in Continuum Mechanics to Study the Motion and Thermodynamics of Liquid-Vapor Interfaces

Abstract: A thermomechanical model of continuous media based on second gradient theory has been used to study the motions in liquid-vapor interfaces. In the equilibrium state this model is shown to be fundamentally equivalent to molecular theories. Conservative motions in such fluids verify the first integrals that provide Kelvin’s circulation theorems and potential equations. The dynamic surface tension of a liquid-vapor interface has been deduced from equations written with a viscosity factor. The result provides and explains the Marangoni effect.

On nonlinear doubly-diffusive Marangoni instability

Abstract: Enhancement of simultaneous heat and mass transfer due to surface tension gradient-induced Marangoni convection is studied. The geometry considered is a thin liquid film between a solid phase and a gas phase. In regions where the two Marangoni numbers are of opposite signs (quadrants II and IV), Orr-Sommerfeld analysis reveals the possibility of oscillatory roll cells. A nonlinear bifurcation study of the amplitude equations indicates that the finite-amplitude oscillatory rolls are annihilated by steady rolls slightly beyond the oscillatory stability boundary in a heteroclinic bifurcation. Oscillatory rolls are hence restricted to quadrants II and IV. Finite-amplitude instability due to subcritical bifurcation of the steady rolls is also shown to be limited to these quadrants. In addition to classifying the bifurcations, the numerical study also provides a useful correlation for single-component flux enhancement, Nh* = 1 + 0.001324 Nu0.23Pr0.16(NT − N), which is compared to available experimental data.

Steady Convection caused by the Temperature Inhomogeneity in a Cylindrical Floating Zone

Abstract: Two convections can be induced by a temperature inhomogeneity in a floating zone. One is a buoyancy driven convection caused by a thermal variation in the liquid density. The other is a surface tension driven convection (Marangoni convection) caused by a thermal variation in the liquid surface tension. Combined steady convections in a cylindrical floating zone were computationally obtained for various combinations of the Grashof numbers and Reynolds numbers due to the Marangoni convection. The Marangoni convection was dominant over the buoyancy driven convection if the zone length was shorter than a critical zone length, and vice versa. The critical zone length on earth was calculated for various liquids.

Surface tensions of mixtures at their boiling points

Abstract: Determination de la tension superficielle au point d'ebullition des melanges methanol/eau, ethanol/eau, propanol-1/eau, methanol/propanol-1 et methanol/propanol-1/eau

An extended thermodynamic approach to non-Newtonian fluids and related results in Marangoni instability problem

Abstract: It is shown that extended irreversible thermodynamics provides a simple and coherent modelling of non-Newtonian fluids. The basic hypothesis underlyig the present formalism is to raise the stress tensor to the status of independent variable. Restrictions are placed on the constitutive equations and the material coefficients by the second law of thermodynamics, stability of equilibrium and the objectivity principle. The general procedure is applied to derive the Reiner-Rivlin and Rivlin-Ericksen second-order fluids. As an illustration, Marangoni convection in a thin horizontal layer of a non-Newtonian fluid submitted to a temperature gardient and placed in a microgravity environment is treated.

The effect of gravity and temperature gradients on precipitation in immiscible alloys

Abstract: The structure of immiscible alloys depends on the temperature gradient and the gravity force during solidification. The temperature gradient causes Marangoni movement of the droplets and the gravity force causes sedimentation or flotation of the droplets. In gradient experiments performed under normal and reduced gravity it is possible to distinguish between these two effects. This paper presents and analyses theoretically the results of experiments on Zn-Bi and Cu-Pb alloys, performed on earth and in the reduced gravity of space.

Pool boiling of binary mixtures

Abstract: Nucleate pool boiling heat transfer coefficients for pure water, pure n-propanol and eight of their mixtures including the azeotrope were measured by using a horizontal electrically heated stainless steel tube of 3.35 mm O.D. and 9.8 cm long as heating element. This binary system was chosen for the reason that it is suitable for simultaneous investigation of the mass diffusion effect (the F effect), the effect of slowing down in bubble growth rate caused by the exhaustion of the volatile component near the vapor-liquid interface, the Marangoni effect (the M effect), the effect of surface tension gradient caused by the evaporation of the component of lower surface tension, and the dynamic surface effect (the Y effect), the effect of surface tension gradient caused by the stretching of vapor-liquid interface, on the boiling heat transfer rate in mixture. It is concluded that the mass diffusion effect can qualitatively account for the experimental results of the system. The discrepancy between the experiment...

Interfacial Convection Driven by Surfactant Compounds at Liquid Interfaces: Characterization by a Solutal Marangoni Number

Abstract: We have studied the possibility for Marangoni effects to occur at an oil water interface containing a long chain surfactant compound which is known to damp interfacial convection. This was realized by choosing appropriately the organic solvent. In the range where the instability occurs a solutal Marangoni number recently determined on the basis of an experimental model has been evaluated. Its value is compared to those calculated for oil-water interfaces containing compounds which are known to induce interfacial convection (acetone, acetic acid).

The Influence of Secondary Convection on Axial Segregation in a Floating Zone

Abstract: The redistribution of impurity due to secondary convection in a cylindrical floating zone is studied using a control volume-based numerical technique. The flow is induced by differential rotation of the feed rod and crystal. Attention is focused on the transient stage of segregation. It is found that the transient length increases with the strength of secondary convection. A maximum in radial segregation is found to occur when secondary convection creates radial gradients but is not strong enough to homogenize them. The influence of cellular structure on the transient length is also examined.

Experiments on Steady and Oscillatory Thermocapillary Convection in Space with Application to Crystal Growth

Abstract: The various types of liquid motion (convection) due to inhomogeneities of the interfacial tension in free liquid surfaces are called Marangoni effects. The inhomogeneities of the interfacial tension can be of thermal or chemical origin. Marangoni effects are a common phenomenon for all liquids with a free surface (interface between liquid and gas or between liquid and liquid). Being surface effects, Marangoni effects are of higher importance in small volumina or near the free surface. Marangoni effects are flow phenomena which are independent of gravity. The investigation of Marangoni effects in an earth laboratory meets principle difficulties because they are invariably coupled with buoyant convection under normal gravity. Therefore experiments on Marangoni effects under microgravity in space are useful to investigate the pure surface tension driven flow.

Free-Surface Instability and Stratification of Critical Microemulsions

Abstract: We have studied the influence of temperature gradients on the concentration distribution in a microemulsion near a consolute critical point. Temperature inhomogeneities much smaller than the distance to the critical temperature trigger convective flows along the free surface (liquid-vapour interface) and are the cause of a considerable stratification of the bulk sample. All these phenomena disappear if the vapour phase on top of the sample is replaced by a solid phase. We propose an explanation involving a thermodiffusion-induced superficial concentration gradient and the resulting Marangoni convection.

Marangoni convection in finite cylindrical liquid bridges

Abstract: Etude analytique et numerique de la convection de Marangoni dans une zone flottante finie sous l'effet d'une distribution de temperature lineaire ou quadratique Determination des effets non lineaires

Marangoni Effect in the Process of Steam Absorption into the Falling Film of the Aqueous Solution of LiBr

Abstract: In most commercial machines, absorption enhancement has been done by adding small various kinds of surfactants, thereby reducing the surface tension of the absorbent solution and generating Marangoni convection near the solution surface during the refrigerant vapor-absorption process. This paper clarifies the most effective conditions of surfactant addition for absorption enhancement in the process of refrigerant vapor absorption into a vertical falling absorbent film. H[sub 2]O/LiBr and n-octanol were used as the refrigerant/absorbent combination and additive. Experiments were carried out by using an LiBr aqueous solution with and without n-octanol dissolution. Moreover, n-octanol vapor was supplied to the absorber in order to clarify the effect of an additive on absorption enhancement. The results of these experiments reveal that the presence of dropwise n-octanol in the surface of the absorbent falling film contributed significantly to the generation of violent and irregular Marangoni convection.

Surface Tension Induced Convection in Presence of a Surface Tension Minimum

Abstract: Considerable efforts are accomplished to process materials under reduced gravity conditions. This environment offers the possibility to handle liquid phase without any contact with a container, and leads to the possibility to avoid any contamination due to interactions with the sample holder. It offers also the possibility to handle large stable liquid floating zones.

Influence of the Container Walls in Benard-Marangoni Convection: Experimental Approach

Abstract: In account of its theoretical as experimental complexity the Benard-Marangoni instability has been less studied than Rayleigh-Benard’s one To our knowledge the only study concerning small aspect ratio containers is due to Rosenblat et al1 They considered very small circular or square cylindrical vessels but in the particular case where the Rayleigh number R is zero (no buoyancy forces) They calculated the critical Marangoni number M and the wavenumber but they did not described the geometrical nature of the convective structure The complete theories, that is to say the theories which take into account both buoyancy forces and surface tension forces, use more of less implicitely two hypotheses: (i) the liquid layer is of infinite extension and (ii) the convective structure is perfectly regular, without any defect2 As the experiments are necesseraly performed in finite containers the question is to know what is the influence of walls on the structural disorder and on the selected wavelength in the permanent and in the transient regimes The problem of the selection of the structure will not be examined here It is now admitted, from theoretical analysis and experimental studies, that when M is very different from zero the structure is hexagonal, but when M is zero it is a roll structure3 The experiments described in this paper correspond to cases when M is far from zero and where the aspect ratio is such as there is room for, at least, several hexagonal cells