DYNAMIC INSTABILITY AT INTERFACES

Non-linear phenomena such as temporal oscillations and chemical waves in the case of chemical reactions are governed by the autocatalytic reaction (positive feedback) and reaction where the product of autocatalysis is destroyed by some other species (negative feedback). Of course in the case of chemical waves (spatio-temporal oscillations), diffusion does play a role, and the concept of reaction diffusion equation is evoked to predict the dependence of velocity of chemical waves on different parameters. In this chapter, we propose to discuss electrical potential oscillations generated due to coupling of volume flux, solute flux and electric current through solid-liquid interface (membrane systems), liquid-liquid interface, solid-liquid-liquid interface (density oscillator) and liquid-liquid-vapour interface. 

Oscillatory transport phenomena under discussion are quite complex where departures from distribution, ionic and adsorption equilibrium occur. Role of Helmholtz electrical double layer becomes important in such cases. 

Two types of cases generally occur in the above type of oscillatory phenomena  

Self-excited oscillations are quite relevant in the context of biological oscillator. Typical self-excited oscillations observed in the case of (i) density oscillator, (ii) liquid-liquid interface oscillator and (iii) liquid-vapour interface oscillator, the mechanism of 

The above phenomena have a bearing on various physiological processes, e.g. membrane transport, oscillatory phenomena in cells and living organism, models for sense of taste and smell and flow system associated with cardiac rhythms. 

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The above potentials have various applications in electro-physiology as pointed out in Chapter 11 on dynamic instability at interfaces. 

Dynamic instability at solid-liquid interface 11.2.1. Electro-kinetic oscillators... 

Dynamic instability at liquid-liquid interface along with solid-liquid interface... 

Dynamic instability at liquid-vapour interface 11.4.1. Introduction... 

Earlier, it was difficult to produce a clean surface and to characterize its surface structure. However, with the development of electronic industry, techniques have been developed to produce clean surface with well-defined properties. It has been possible to investigate catalytic oxidation on metal surface in depth. Example of dynamic instability at gas-liquid interface is provided by such studies. Studies on chemical oscillations during oxidation of CO over surface of platinum group metals have attracted considerable interest [62-68]. 

An interdisciplinary team of leading experts from around the world discuss recent concepts in the physics and chemistry of various well-studied interfaces of rigid and deformable particles in homo- and hetero-aggregate dispersed systems, including emulsions, dispersoids, foams, fluosols, polymer membranes, and biocolloids. The contributors clearly elucidate the hydrodynamic, electrodynamic, and thermodynamic instabilities that occur at interfaces, as well as the rheological properties of interfacial layers responsible for droplets, particles, and droplet-particle-film structures in finely dispersed systems. The book examines structure and dynamics from various angles, such as relativistic and non-relativistic theories, molecular orbital methods, and transient state theories. 

Mass-transport (i.e., diffusion or electromigration) effects are particularly acute in the cases of cracking, pitting, and crevice corrosion, whereby occlusion effects can create highly concentrated solutions that move an otherwise stable system into regions of thermodynamic instability at the local level [13,14,41 3, 79-82]. When porous films or particular solution flow conditions exist, mass-transport effects should also be taken into account [83, 84]. Molecular dynamics and Monte Carlo simulations of interfaces over the past few decades have provided some insight into the concentration gradients that occur close to the electrochemical interface [85-91], and these, coupled with computational fluid dynamics simulations, can indicate the extent to which mass-transport effects can dominate an overall corrosion scenario [92]. 

Velarde, M. G., Rednikov, A. Ye., and Linde, H. (1999) Waves generated by surface tension gradients and Instability, in Fluid Dynamics at Interfaces, edited by W. Shyy, University Press, Cambridge, 43-56. 

In the course of interfacial mass transfer, from molecular point of view, the process is stochastic, that means some local molecules may undergo the mass transfer in advance than the others, so that small concentration gradient (where i — x, y, z) is established at the interface. As the surface tension a is function of concentration, it follows that the surface tension gradient is also created at the interface. If is increased up to a critical point, the fluid dynamic instability will appear to induce the interfacial convection as well as the formation of orderly structure at the interface. At the same time, the rate of mass transfer may be enhanced or suppressed depending on the properties of the mass transfer system concerned such phenomena is generally regarded as interfacial effect. 

Wang, S.-Q. Molecular Transitions and Dynamics at Polymer/Wall Interfaces Origins of Flow Instabilities and Wall Slip. VoL 138, pp. 227-276. 

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