Showing papers on "Marangoni effect published in 1970"

Thermal instabilities in two-fluid horizontal layers

Abstract: An analytical and experimental study of thermally induced instability in horizontal two-fluid layers is reported. A liner stability analysis for two initially motionless, viscous immiscible fluids confined between horizontal isothermal solid surface and subject to both density (Benard) and surface-tension-grandient (Marangoni) drving mechanisms is presented. Calculations for the labortory configuaration reported below predict instability for heating from above or below. Response is strongly department on the ratios of the properties of the fluids, the total depth of the layer and the depth fraction of one fluid. Three different response modes occur(interfacial-tension-gradient dominated, buoyancy dominated and surface-deflexion dominated) depending on the fluid depth fractions when the heating is from above, the buoyanvy mechanism is stabilizing for most wavenumbers, including the xritical one. Heating from below lowere the critical Marangoni number and adds a buoyancy driven response mode. Results of experimental measurement of the critical Marangoni number exceeds the predicted critical value by as much as five times. The critical Rayleigh number observe for heating from below fallos between the critical values predicted with and without the Marangoni effect. The presence of surface contamination is belived to be responsible for the apparent lack of convection when heating is from above and for the different between the predicted and measured critical Rayleigh number when heating is from below.

The spreading of liquids on liquids

Abstract: The surface velocity field induced by the Marangoni effect is obtained experimentally for the steady dissolving of a low surface tension liquid at the free surface of a higher surface tension one. Aliphatic alcohols C2C4 were used as the dissolving liquids, and water or aqueous solutions as the supporting liquids. The effects of surface tension and concentration differences as well as that of viscosity are examined. The experimental velocities are correlated as a function of the product Δ c( ∂ γ ∂c ) . Some qualitative explanations of the mechanism of the dissolving process under the action of surface forces are given.

Thermocapillary flow around hemispherical bubble

Abstract: A hemispherical bubble, attached to a plate, is surrounded by an initially quiescent and isothermal liquid. By suddenly heating the plate, a thermal gradient over the bubble surface results. Because surface tension is temperature dependent, tangential stresses arise at the bubble surface. The liquid is viscous, and motion in the liquid phase begins. Such motion is an example of thermocapillary flow. This problem, besides being of interest from a fundamental point of view, is of possible concern in the design of space vehicles capable of storing cryogenic fluids for long periods of time in a weightless condition. Solutions to the problem are developed by numerical treatment of the governing equations. Flow and temperature fields, which depend upon the Prandtl and Marangoni numbers, were obtained for Prandtl numbers 1 and 5 and Marangoni numbers from 0 to 100,000. Results show that liquid is pulled toward the intersection of the bubble and the plate, then flows around the bubble surface, and leaves the bubble as a jet. The extent of the jet increases with increasing Marangoni number and decreases with increasing Prandtl number. Thermocapillary flow increases heat transfer (Nusselt number) over that obtained from conduction, but the increase is modest. The Nusselt number increases with the Marangoni number and is insensitive to the Prandtl number. At a Marangoni number of 40,000, the local Nusselt number was increased by a factor of 2. In order for thermocapillary flow to become a dominant heat transfer mechanism, the Marangoni number must exceed 100,000.

The effect of surface active agents on interphase mass transfer

Abstract: The effect of various soluble and insoluble surface active agents on the absorption of ammonia into a static aqueous system was studied. Saturated straight-chain hydrocarbons with four to twenty-two carbon atoms and polar end groups were selected as the surface active agents to be studied. Alcohol, amine, and amide end groups were investigated. Most of the insoluble surface active agents, which were studied as films, were found to decrease the ammonia absorption rate. There was a definite correlation between the amount of mass transfer reduction and the hydrocarbon chain length, while the effect of the various end groups appeared to depend on the chain length. Surface mass transfer coefficients were computed for each surface active agent that retarded mass transfer. Most of the soluble surface active agents were found to increase the ammonia absorption rates. For all cases of enhanced mass transfer, movements in the interface could be observed. It was concluded that the interfacial movements were caused by the Marangoni effect. In general, as the chain length of the surface active agent decreased, the mass transfer enhancement increased. A mathematical model based on a surface renewal theory was fitted to the experimental data.