Now showing 1 - 10 of 131
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    A semianalytical model to study the effect of cortical tension on cell rolling
    (15-12-2010)
    Bose, Suman
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    Karp, Jeffrey M.
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    Karnik, Rohit
    Cell rolling on the vascular endothelium plays an important role in trafficking of leukocytes, stem cells, and cancer cells. We describe a semianalytical model of cell rolling that focuses on the microvillus as the unit of cell-substrate interaction and integrates microvillus mechanics, receptor clustering, force-dependent receptor-ligand kinetics, and cortical tension that enables incorporation of cell body deformation. Using parameters obtained from independent experiments, the model showed excellent agreement with experimental studies of neutrophil rolling on P-selectin and predicted different regimes of cell rolling, including spreading of the cells on the substrate under high shear. The cortical tension affected the cell-surface contact area and influenced the rolling velocity, and modulated the dependence of rolling velocity on microvillus stiffness. Moreover, at the same shear stress, microvilli of cells with higher cortical tension carried a greater load compared to those with lower cortical tension. We also used the model to obtain a scaling dependence of the contact radius and cell rolling velocity under different conditions of shear stress, cortical tension, and ligand density. This model advances theoretical understanding of cell rolling by incorporating cortical tension and microvillus extension into a versatile, semianalytical framework. © 2010 by the Biophysical Society.
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    Analyses of drag on viscoelastic liquid infused bio-inspired patterned surfaces
    (01-02-2016)
    Rajagopal, Manjunath C.
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    The villi structures found in intestinal tract, infused with mucus, can attain different configurations by elongation and lashing movements. The present work analyzes the different configurations of villi separately and the associated drag behavior that may aid in movement of food through the tract. Using numerical simulations, the variation of drag in intestinal tract with respect to different configurations, especially the inclination of villi, has been studied, with quasi-steady approximations that consider the villi as solid objects. Extending this intestinal tract surface with villi and mucus, to a normal ridge textured surface infused with a viscoelastic fluid, significant changes in drag, owing to the viscoelastic nature of the infused fluid, have been found. The extent of drag reduction or drag enhancement, which is dependent on texture of the surface, is also found to be contingent on rate of deformation of the viscoelastic fluid along the patterned surface, and the density of ridges or solid fraction. Such control over drag on these surfaces, through Weissenberg number and texture of the surface, indicates the possibility of using viscoelastic liquid infused engineered surfaces for bio-medical and industrial applications.
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    Effects of nanostructure permittivity and dimensions on the increased dielectric strength of nano insulating oils
    (20-11-2016)
    Katiyar, Ajay
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    Dhar, Purbarun
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    Nandi, Tandra
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    The influence of nanostructures concentration, morphology, permittivity and size on the augmentation of the dielectric breakdown characteristics of nano insulating oils has been experimentally examined and demonstrated in detailed for the first time. Various dielectric nanoparticles/structures, viz. zinc oxide (ZnO), zirconium oxide (ZrO2) and aluminium oxide (Al2O3), of different sizes over a range of 25–125 nm have been employed to investigate the influence of nanoparticles concentration as well as size on the dielectric performance of nano insulating oils. Bismuth oxide (Bi2O3), magnesium oxide (MgO) and copper oxide (CuO) have been utilized to investigate the effect of nanoparticles concentration and all the nanoparticles contribute to the detailed study on the effects of morphology on the breakdown characteristics. Experimental findings reveal that particle size, permittivity as well as concentration affects the dielectric performance of nano insulating oils to large extents. Particles with smaller size offer higher enhancements in the dielectric breakdown voltage compared to the bigger size particles and its mechanism and role in hampering streamer development and growth has been deliberated. The influence of temperature and moisture content has been found to have major effects on the BD performance and experimentally examined and reported. The present work reveals the potential of nanomaterials towards designing more robust power systems employing liquid dielectrics by engineering their breakdown characteristics.
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    Scaling analysis for the investigation of slip mechanisms in nanofluids
    (01-12-2011)
    Savithiri, S.
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    The primary objective of this study is to investigate the effect of slip mechanisms in nanofluids through scaling analysis. The role of nanoparticle slip mechanisms in both water- and ethylene glycol-based nanofluids is analyzed by considering shape, size, concentration, and temperature of the nanoparticles. From the scaling analysis, it is found that all of the slip mechanisms are dominant in particles of cylindrical shape as compared to that of spherical and sheet particles. The magnitudes of slip mechanisms are found to be higher for particles of size between 10 and 80 nm. The Brownian force is found to dominate in smaller particles below 10 nm and also at smaller volume fraction. However, the drag force is found to dominate in smaller particles below 10 nm and at higher volume fraction. The effect of thermophoresis and Magnus forces is found to increase with the particle size and concentration. In terms of time scales, the Brownian and gravity forces act considerably over a longer duration than the other forces. For copper-water-based nanofluid, the effective contribution of slip mechanisms leads to a heat transfer augmentation which is approximately 36% over that of the base fluid. The drag and gravity forces tend to reduce the Nusselt number of the nanofluid while the other forces tend to enhance it. © 2011 Fang et al.
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    Experimental and numerical investigation of flow of nanofluids in microchannels
    (01-12-2010)
    Singh, Pawan K.
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    Harikrishna, P. V.
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    Sundararajan, T.
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    The current study investigates the flow of nanofluids in microchannels experimentally and numerically. For this purpose, two microchannels of hydraulic diameters of 211 and 300 μm are used with alumina(45nm)-water nanofluids. The nanofluids with the concentrations 0.25, 0.50 and 1 vol% are used to observe the effect of volume fraction in the present analysis. With regard to the numerical simulation of nanofluids in microchannels, two approaches have been chosen in the current work. First one considers the nanofluids as single phase fluid and applies the mixture rule for evaluating properties for the simulation. The second type of modeling is done using the discrete phase approach which involves Eulerian-Lagrangian considerations. The fluid phase is assumed to be continuous and governed by Navier-Stokes equation. The movement of discrete nanoparticles is determined by the Newton's second law which takes into account the body force, hydrodynamic forces, the Brownian and thermophoresis forces. The predictions are validated against experimental results obtained for nanofluid flow in a chemically etched silicon wafer channel. It is found that the discrete phase modeling is more accurate with regard to the prediction of nanofluids behavior in microchannels, as compared to the single phase model. The results also show the non-uniformity of nanoparticle distribution across the channel cross-section. This non-uniformity in distribution can be attributed to the shear induced particle migration. This can also be the reason for the difference in pressure drop and heat transfer from the single phase model. The pressure drop with 0.25 and 0.5 vol% of alumina is more or less same as that of water and thus, makes it a suitable cooling liquid. However, an enhancement in heat transfer is observed from the computational predictions. © 2010 by ASME.
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    Triggering of flow asymmetry by anisotropic deflection of lamella during the impact of a drop onto superhydrophobic surfaces
    (01-07-2018)
    Regulagadda, Kartik
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    A water drop impacting a superhydrophobic surface (SHS) rebounds completely with remarkable elasticity. For a given drop size, the time of contact on a flat SHS remains constant. However, recent studies show that the contact time can be reduced further by triggering an asymmetry in the hydrodynamics of impact. This can be achieved in different ways; an example being the impact on a cylindrical SHS with a curvature comparable to the drop. Here, the anisotropic flow generated from the tangential momentum and elliptical footprint of the drop before the crash leads to the formation of lobes. In the present work, we perform drop impact experiments on a bathtub-like SHS and show that the radial anisotropy can be triggered even in the absence of both the tangential momentum and non-circular footprint. This is shown to be a consequence of lamella deflection during the drop spreading. The reduction in contact time is quite clearly evident in this experimental regime.
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    Effect of Interaction of Nanoparticles and Surfactants on the Spreading Dynamics of Sessile Droplets
    (31-10-2017)
    Harikrishnan, A. R.
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    Dhar, Purbarun
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    While a body of literature on the spreading dynamics of surfactants and a few studies on the spreading dynamics of nanocolloids exist, to the best of the authors' knowledge, there are no reports on the effect of presence of surfactants on the spreading dynamics of nanocolloidal suspensions. For the first time the present study reports an extensive experimental and theoretical study on the effect of surfactant impregnated nanocolloidal complex fluids in modulating the spreading dynamics. A segregation analysis of the effect of surfactants alone, nanoparticle alone, and the combined effect of nanoparticle and surfactants in altering the spreading dynamics have been studied in detail. The spreading dynamics of nanocolloidal solutions alone and of the surfactant impregnated nanocolloidal solutions are found to be grossly different, and particle morphology is found to play a predominant role. For the first time the present study experimentally proves that the classical Tanner's law is disobeyed by the complex fluids in the case of particle alone and combined particle and surfactant case. We also discuss the role of imbibitions across the particle wedge in the precursor film in tuning spreading dynamics. We propose an analytical model to predict the nature of dependency of contact radius on time for the complex colloids. A detailed theoretical examination of the governing factors, the interacting forces at the three phase contact line, and the effects of interplay of surfactants and the nanoparticles at the precursor film in modulating the spreading dynamics has been presented for such complex colloids.
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    An analytical solution to predict the inception of two-phase flow in a proton exchange membrane fuel cell
    (01-12-2010)
    Bansode, A. S.
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    Sundararajan, T.
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    The presence of liquid water at the cathode of proton exchange membrane fuel cell hinders the reactant supply to the electrode and is known as electrode flooding. The flooding at the cathode due to the presence of two-phase flow of water is one of the major performance limiting conditions. A pseudo-two-dimensional analytical model is developed to predict the inception of two-phase flow along the length of the cathode channel. The diffusion of the water is considered to take place only across the gas diffusion layer (GDL). The current density corresponding to the inception of two-phase flow, called the threshold current density, is found to be a function of the channel length and height, GDL thickness, velocity, and relative humidity of the air at the inlet and cell temperature. Thus, for given design and operating conditions, the analytical model is capable of predicting the inception of two-phase flow, and therefore a flooding condition can be avoided in the first place. © 2010 American Society of Mechanical Engineers.
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    E.coli DH5α cell response to a sudden change in microfluidic chemical environment
    (04-11-2015)
    Murugesan, Nithya
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    Motile bacteria respond to changing chemical environment by moving towards or away from a particular location. Bacterial migration under chemical gradient is one of the most studied areas in biomedical field. In this work we looked into how bacterial cells respond to sudden change in the microfluidic chemical environment. E.coli DH5α cells were subjected to an attractant gradient (0.1 mM sorbitol - attractant to E.coli cells) and after 120 min the same cells were exposed to an inhibitor (0.1 mM NiSO4) gradient in the same microfluidic device. Our studies revealed that when the E.coli DH5α cells were exposed to 0.1 mM sorbitol, they showed faster chemotaxis towards the attractant (0.1 mM sorbitol) and achieved steady state by 60 min. When we replaced 0.1 mM sorbitol with 0.1 mM NiSO4 in the device we found that that the E.coli DH5α cells started responding to change in chemical environment within 10 min and achieved steady state at the end of 60 min. This shows that the bacterial cells respond to change in local chemical environment is within few minutes.
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    Effect of flow maldistribution on the thermal performance of parallel microchannel cooling systems
    (01-01-2014)
    Manoj Siva, V.
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    This paper brings out the phenomenon of the influence of flow maldistribution on temperature distribution in parallel microchannel system that is supposed to have an adverse effect on hot spot formation in microelectronic devices. An extensive experimental study is carried out where in the parameters affecting the flow maldistribution such as channel hydraulic diameter, channel flow configurations (U, Z, I type) and chip power are varied to study their effect on the pressure drop and temperature distribution across the parallel channels designed for liquid cooling of a CPU using distilled water. It is observed that the flow distribution among the channels improves significantly with a decrease in the channel hydraulic diameter due to higher pressure drop offered by each individual channels simultaneously. This results in a considerable reduction in both the peak temperature and the average temperature of the device with decrease in channel diameter and better temperature distribution. It is observed that a higher pressure drop in d = 88 μm induces more uniform distribution compared to d = 176 μm resulting in a 3 C improvement in the standard deviation of temperature on the chip surface and a reduction in maximum surface temperature. Higher heat fluxes induce a reduction in viscosity of the fluid resulting in higher flow maldistribution. © 2014 Elsevier Ltd. All rights reserved.