Interface-mediated oscillatory phenomena
Introduction
A good number of biological transport phenomena, which are rhythmical in nature are interface mediated. The exotic oscillatory phenomena, particularly those relating to excitable tissues and membranes, occur in the non-linear far from equilibrium region. A quantitative study of the oscillatory phenomena in actual biological systems, throwing light on the physics and physical chemistry of the processes is prohibitively difficult due to the chemical and structural complexities of the biological materials. This is why well-characterizable artificial systems capable of mimicking the events of excitable tissues and membranes have been experimented with. In the present article we have focused attention on physical phenomena of biological relevance. We plan to discuss some of the crucial oscillatory phenomena mediated by: (i) solid–liquid interfaces; and (ii) liquid–liquid interfaces. Although the major motivation for this article is the biological context, a few studies, which do not have an apparent biological relevance, have also been included. For example recent studies from our group have thrown new light on the phase transfer catalytic system; a discussion of these studies has also been included in this article.
There is another inherent interest in the study of oscillatory phenomena. Non-equilibrium thermodynamics have proven most useful in the study of coupled flow processes in the steady state when the linear relationships between fluxes and forces are obeyed. Since the domain of validity of Gibb's entropy equation, which is utilized for the proper choice of fluxes and forces, is larger than those of both linear phenomenological relations and Onsagar's relations, attempts have been made [1] to explore the non-equilibrium region by considering non-linear relationships between fluxes and forces. In the non-linear region beyond the domain of validity of Gibb's equation, the thermodynamics of irreversible processes cease to be applicable. The oscillatory phenomena are not obtained in the linear region; these are obtained in the non-linear region which lies in the far from equilibrium regime. Dynamics and stability theory, together with the subservient role of the thermodynamics of irreversible processes has helped in exploring the far from equilibrium region. The far from equilibrium region where the phenomena like bistability and oscillations are observed has been subjected to intense investigations in chemically reacting systems [2]. A chemically reacting system can be held at a desired distance from equilibrium and experimented with using a continuously stirred tank reactor (CSTR). The interface mediated transport processes, e.g. membrane transport, electrokinetics, etc., are the examples amongst the non-reacting systems, where also the system can be maintained at a desired distance from equilibrium by controlling the magnitude of fluxes and forces and the non-linear far from equilibrium region explored experimentally. In fact, experimental investigation of the far from equilibrium region in chemically non-reacting systems has been hindered due to non-availability of suitable experimental systems. Above all the study of oscillatory transport processes has assumed great significance from the viewpoint of science of complexity, which is considered to be the science of the 21st century [3].
Section snippets
Teorell's oscillator
The phenomenon of neuronal excitations (action potentials) is essentially an electrokinetic phenomenon. This is, prima facie, obvious from the fact that the membranes of neuronal axons separating electrolytic solutions of differing concentrations maintain a very high electric field across them; obviously the membranes have hydrophilic pathways present in them. This probably led Teorell [4] to fabricate his oscillator. Teorell not only demonstrated the rhythmic phenomena but also tried to give a
Marangoni instability
Marangoni instability is related with the onset of interfacial agitation by local variation of interfacial tension. The difference between dynamic and equilibrium behavior of interface is supposed to be responsible for the paradoxical motion under buoyancy of small bubbles and droplets in various liquids. Several experiments with surface movements are documented [135], [136], [137]. The following types of surface effects have been observed: (i) movement in a fluid surface, caused by local
Future projections
Although we have mentioned all along in the text the tasks for future, we would like to focus on them once again before closing this article:
- 1.
The theoretical framework for Teorell type oscillators given by different authors is far from satisfactory. Stationary state methods are applied to non-stationary state phenomena on the grounds that the flows are slow enough. This indeed is a difficult task. The non-equilibrium thermodynamics of dynamical systems is still in a state of infancy. The main
Acknowledgements
Thanks are due to The Council of Scientific and Industrial Research, New Delhi for support.
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