Showing papers in "Journal of the Indian Institute of Science in 2011"

The Fluid Dynamics of Swimming Microorganisms and Cells

Abstract: In this review, we describe the fluid mechanics of swimming microorganisms, with an emphasis on recent developments. We begin with the mechanics of individual swimmers, and describe the requirement for a non-reciprocal cyclic swimming stroke for net displacement in the absence of inertia. We discuss Purcell's three-link swimmer and other artificial models as simple pedagogical examples. Thereafter, we consider the swimming of real microorganisms, which may be classified into ciliates and the flagellates. In addition to the stroke kinematics, we examine the nature of the fluid velocity field around a swimmer, which governs the hydrodynamic interactions between swimmers. We then consider the large-scale hydrodynamics in a suspension of swimmers, our efforts motivated primarily by experimental observations of coherent motion. The theoretical analyses fall into two categories: the first considers coherent motion that arises from the coupling of gravity with the density difference between the swimmer and the suspending fluid. The second category is more recent, and examines the smaller-scale coherent motion, in the absence of buoyancy forces, that is driven by the anisotropic orientation distribution of the swimmer force-dipoles. We then describe a variety of discrete simulation methods, wherein the motion of every swimmer is tracked in time. The continuum theories and simulations reveal fundamental differences in the collective dynamics between suspensions of pushers, pullers and squirmers; only suspensions of pushers, for instance, are predicted to be linearly unstable. Despite the successes of the theoretical and computational methods, significant issues remain unexplained, some of which are highlighted towards the end of the review.

Electrical Switching and Other Properties of Chalcogenide Glasses

Abstract: | Electrical switching which has applications in areas such as information storage, power control, etc is a scientifically interesting and technologically important phenomenon exhibited by glassy chalcogenide semiconductors. The phase change memories based on electrical switching appear to be the most promising next generation non-volatile memories, due to many attributes which include high endurance in write/read operations, shorter write/read time, high scalability, multi-bit capability, lower cost and a compatibility with complementary metal oxide semiconductor technology.Studies on the electrical switching behavior of chalcogenide glasses help us in identifying newer glasses which could be used for phase change memory applications. In particular, studies on the composition dependence of electrical switching parameters and investigations on the correlation between switching behavior with other material properties are necessary for the selection of proper compositions which make good memory materials.In this review, an attempt has been made to summarize the dependence of the electrical switching behavior of chalcogenide glasses with other material properties such as network topological effects, glass transition & crystallization temperature, activation energy for crystallization, thermal diffusivity, electrical resistivity and others.

The Role of Porous Medium Modeling in Biothermofluids

Abstract: Biothermology or Bio- fluid flow and heat transfer is an important and developing subdivision of bioengineering. Seeking simplifications for biological processes that are inherently complex, through porous medium models, is an exciting and useful multidisciplinary pursuit. This review presents an overview of the post-2002 research on the modelling aspects of several biothermofluid processes based primarily on the porous medium approach. Beginning with a definition for porous medium suited for analysing transport phenomena, concepts of volume averaging, momentum and energy conservation statements are briefly discussed to motivate the ensuing review discussions. Porous medium modelling of several biomedical processes pertaining to human physiology is then discussed under two broad categories of bio-mass and bio-heat transport. The bio-mass transport section discusses LDL transport in arteries, drug delivery, drug eluting stents, functions of organs modelled as porous medium, porous medium modelling of microbial transport. Under the bioheat transport section, porous medium approach based bio-heat equations are described accompanied by a literature review. A final subsection discusses non-Fourier type bio-heat conduction phenomena. Requirement of analysis and computational efforts in the future using the generalized porous medium momentum equation and the local thermal non-equilibrium based two energy equations are highlighted.

Fluid Mechanics of Aquatic Locomotion at Large Reynolds Numbers

Abstract: | There exist a huge range of fish species besides other aquatic organisms like squids and salps that locomote in water at large Reynolds numbers, a regime of flow where inertial forces dominate viscous forces. In the present review, we discuss the fluid mechanics governing the locomotion of such organisms. Most fishes propel themselves by periodic undulatory motions of the body and tail, and the typical classification of their swimming modes is based on the fraction of their body that undergoes such undulatory motions. In the angulliform mode, or the eel type, the entire body undergoes undulatory motions in the form of a travelling wave that goes from head to tail, while in the other extreme case, the thunniform mode, only the rear tail (caudal fin) undergoes lateral oscillations. The thunniform mode of swimming is essentially based on the lift force generated by the airfoil like crosssection of the fish tail as it moves laterally through the water, while the anguilliform mode may be understood using the “reactive theory” of Lighthill. In pulsed jet propulsion, adopted by squids and salps, there are two components to the thrust; the first due to the familiar ejection of momentum and the other due to an over-pressure at the exit plane caused by the unsteadiness of the jet. The flow immediately downstream of the body in all three modes consists of vortex rings; the differentiating point being the vastly different orientations of the vortex rings. However, since all the bodies are self-propelling, the thrust force must be equal to the drag force (at steady speed), implying no net force on the body, and hence the wake or flow downstream must be momentumless. For such bodies, where there is no net force, it is difficult to directly define a propulsion efficiency, although it is possible to use some other very different measures like “cost of transportation” to broadly judge performance.

A Review of Photo-Induced Inter-Diffusion in Nano-Layered Chalcogenide Films

Abstract: A growing interest in the research of chalcogenide glasses can be currently witnessed, which to a large extent is caused by newly opened fields of applications for these materials. Applications in the field of micro- and opto-electronics, xerography and lithography, acousto-optic and memory switching devices and detectors for medical imaging seem to be most remarkable. Accordingly, photo induced phenomena in chalcogenide glasses are attracting much interest. These phenomena can be found both in uniform thin films as well as multilayered films. Among amorphous multilayers, chalcogenide multilayers are attractive because of the potential it has for tailoring the optical properties. I will be presenting some basic idea of photoinduced effects followed by the diffusion mechanisms of Se, Sb and Bi in to As2S3 films.

Utilizing Model Nanostructured Porous Inorganic Compounds Such as Silica to Beneficially Confine “Bio”-Relevant Molecules and Demonstrate their Potential in Therapeutics

Abstract: The importance of well-defined inorganic porous nanostructured materials in the context of biotechnological applications such as drug delivery and biomolecular sensing is reviewed here in detail. Under optimized conditions, the confinement of “bio”- relevant molecules such as pharmaceutical drugs, enzymes or proteins inside such inorganic nanostructures may be remarkably beneficial leading to enhanced molecular stability, activity and performance. From the point of view of basic research, molecular confinement inside nanostructures poses several formidable and intriguing problems of statistical mechanics at the mesoscopic scale. The theoretical comprehension of such non-trivial issues will not only aid in the interpretation of observed phenomena but also help in designing better inorganic nanostructured materials for biotechnological applications.

Dynamic Equilibrium: Is it an Important Concept in Chemical Biology and Drug Discovery?

Abstract: Dynamic equilibrium is one of the important aspects of study, because its existence can be observed in the infinitesimally small cells within the body to the huge adversities of the nature. In organic chemistry, a single compound having dynamic equilibrium states gives a driving force to create diversity-oriented synthesis of library of products from the same single compound. In chemical biology, the protein/nucleic acids are in constant equilibrium with their changing conformations/folds, which is responsible for the biological activity with a very small level of energy barrier for the conformational/folds inter-conversion. In medicinal/ pharmaceutical chemistry, a drug having dynamic equilibrium states plays an important role in the delivery of the drug on the active site across the cell membrane in a dynamic fashion and also acts as self-protection for the active drug molecule. Herein, we present a brief account on the existence and application of dynamic equilibrium states in chemical and biological chemistry as well as its existence in other inorganic complexes. Information regarding the existence of exact 1:1 ratio of the two dynamic equilibrium forms of chemical entities in the chemical reaction from organic, inorganic and biological perspectives have been discussed. We believe that this review is the first of its kind to discuss the importance of dynamic equilibrium states in chemical and biological systems, addressing the question to the scientific community as and the importance of the concept for further study.