Surface-tension induced convection

As was stressed by Professor Ubbelohde, in the process of cell recognition not only the lateral diffusion of the binding sites has to be considered, but also the mechanical effects resulting from the local change of surface tension, inducing convection at the cell surface. It is well known, in the cell-to-cell contact inhibition of motion, in tissue culture, that a cell approaches another cell by touching it by means of microvilli and that this process can be affected when adding surfactants to the culture. Now the point is, What is the relative importance of both diffusion and convection Well, in binary surface films, it was observed that the transport process induced by two-dimensional convection is much more rapid than the two-dimensional diffusion. 

Figure 9.11 The relative effects of reaction-diffusion, buoyancy and Marangoni convection were demonstrated by Simoyi and colleagues in the chlorite-thiourea-barium chloride reaction system (Hauser and Simoyi, 1994b). The front rises along the tilted side of the container faster than it propagates horizontally. When the front reaches the interface, its horizontal propagation accelerates because of surface-tension-induced convection. (Courtesy of R. Simoyi.)...

If a system lacks an interface between different fluids, such a monomer/air interface, then only buoyancy-driven convection will occur. If a free interface exists, then we will see that gradients in the interfacial tension can cause fluid motion — a process called Surface-Tension Induced Convection or Marangoni convection. This will be especially important in "microgravity". (How the condition of apparently zero gravity is achieved is discussed in chapter 2.)... 

Li et al. 48) studied how 1.0 pm particles of polystyrene and poly(methyl methacrylate) interacted when they were melt processed at 180 °C. They observed by confocal microscopy on a hot stage that there was a preferential motion for particles, which they attributed to a buoyancy-driven flow because of the 10% density difference between the polymers. Jang et al. had made a similar observation 49), Li et al. did not consider possible surface-tension induced convection or that droplets could migrate in a temperature gradient, as has been observed by Balasubramaniam et al. for the thermocapillary migration of bubbles 50),... 

The factors inducing anisotropy into the ambient phase flows in solution, such as laminar or turbulent flow, convection induced by temperature difference, concentration difference, or difference in surface tension. 

Convection in Melt Growth. Convection in the melt is pervasive in all terrestrial melt growth systems. Sources for flows include buoyancy-driven convection caused by the solute and temperature dependence of the density surface tension gradients along melt-fluid menisci forced convection introduced by the motion of solid surfaces, such as crucible and crystal rotation in the CZ and FZ systems and the motion of the melt induced by the solidification of material. These flows are important causes of the convection of heat and species and can have a dominant influence on the temperature field in the system and on solute incorporation into the crystal. Moreover, flow transitions from steady laminar, to time-periodic, chaotic, and turbulent motions cause temporal nonuniformities at the growth interface. These fluctuations in temperature and concentration can cause the melt-crystal interface to melt and resolidify and can lead to solute striations (25) and to the formation of microdefects, which will be described later. 

Marangoni streaming — A -> surface tension gradient (due to local temperature or composition variation) induces a convection effect. Marangoni effects [i, ii] occur at liquid-gas or at liquid-liquid interfaces and result in convection or streaming in the liquid adjacent to the interface. See also -> polarographic maximum. 

Pearson, J.R.A., On convection cells induced by surface tension, J. Fluid Mech., 4, 489, 1958. 

In the spirit of our restriction to the laminar regime, we shall only briefly touch on natural convection—that is, flows produced by buoyancy forces acting on fluids in which there are density differences. A common example is buoyant motion in a gravitational field where the density difference arises from heat exchange. Even in weakly buoyant motions generated by small density differences, turbulence is ubiquitous. We shall, however, consider convection induced by surface tension gradients. 

Cellular Convection Induced by Surface Tension Gradients... 

Figure 10.6.1 Plan photograph taken by Henri Benard of hexagonal cells in a thin film of molten spermaceti from his original experiments on convection cells induced by surface tension gradients. [Courtesy of Prof. Simon Ostrach. From Benard 1900.1...

The observed stabilizing effect of surfactants toward convection induced by surface tension has been confirmed theoretically in a recent paper by Berg and Acrivos (B13), in which the stability analysis technique and the physical model were the same as Pearson s except that the free-surface boundary condition [(iii) of Table III] took into account the presence of surface active agents. Critical values for the Thompson number were computed as functions of two dimensionless parameters, one embodying the surface viscosity and the other the surface elasticity. ... 

Berg, J.C. and Acrivos, A., The effect of surface active agents on convection cells induced by. surface tension, Chem. Eng. Sci., 20, 737, 1965. 

Smith, K.A., On convective instability induced by surface-tension gradients, J. Fluid Meek, 24, 401, 1966. 

Indeed, the Benard (1900), famous convective cells are primarily induced by the surface tension gradients resulting from temperature variations across the free surface (the so-called, Marangoni effect). For this, it seems justified to use the term of Benard-Marangoni (B-M) thermocapillary instability problem when, as in Benard s experiments, the dominant acting driving force is the surface tension gradient on the deformable free surface (without the influence of the buoyancy force). 

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