Biomedical subjects

V Kumaran

Publications and source records attributed to V Kumaran.

At least 19 recordsLinked to original sources

PubMed2006

Velocity autocorrelations and viscosity renormalization in sheared granular flows.

The decay of the velocity autocorrelation function in a sheared granular flow is analyzed in the limit where the wavelength of fluctuations is larger than the "conduction length," so that energy is a nonconserved variable. The decay of the velocity autocorrelation function is much faster than that in a fluid at equilibrium for which energy is a conserved variable. Specifically, the autocorrelation function in a sheared granular flow decays proportional to t-3 in 2D and t-9/2 in 3D, in contrast with the decay proportional to t-1 in 2D and t-3/2 in 3D for a fluid at equilibrium. The renormalization of the viscosity due to mode coupling is evaluated using this form of the decay of the autocorrelation function. It is found that the logarithmic divergence in the viscosity in 2D, and the divergence of the Burnett coefficients in 3D, which is characteristic of a fluid of elastic particles at equilibrium, is absent in a sheared granular flow.

Journal Article↗

PubMed2006

Does patent ductus arteriosus affect feed tolerance in preterm neonates?

Patent ductus arteriosus (PDA), especially PDA with sepsis, has been reported as a risk factor for feed intolerance in preterm neonates. In this study, the start to full feeds interval was found to be longest in preterm neonates (<or=28 weeks' gestation) with sepsis, followed by that in preterm neonates with sepsis and PDA, and in those with PDA alone.

Age Factors↗

PubMed2006

Bond-orientational analysis of hard-disk and hard-sphere structures.

We report the bond-orientational analysis results for the thermodynamic, random, and homogeneously sheared inelastic structures of hard-disks and hard-spheres. The thermodynamic structures show a sharp rise in the order across the freezing transition. The random structures show the absence of crystallization. The homogeneously sheared structures get ordered at a packing fraction higher than the thermodynamic freezing packing fraction, due to the suppression of crystal nucleation. On shear ordering, strings of close-packed hard-disks in two dimensions and close-packed layers of hard-spheres in three dimensions, oriented along the velocity direction, slide past each other. Such a flow creates a considerable amount of fourfold order in two dimensions and body-centered-tetragonal (bct) structure in three dimensions. These transitions are the flow analogs of the martensitic transformations occurring in metals due to the stresses induced by a rapid quench. In hard-disk structures, using the bond-orientational analysis we show the presence of fourfold order. In sheared inelastic hard-sphere structures, even though the global bond-orientational analysis shows that the system is highly ordered, a third-order rotational invariant analysis shows that only about 40% of the spheres have face-centered-cubic (fcc) order, even in the dense and near-elastic limits, clearly indicating the coexistence of multiple crystalline orders. When layers of close-packed spheres slide past each other, in addition to the bct structure, the hexagonal-close-packed (hcp) structure is formed due to the random stacking faults. Using the Honeycutt-Andersen pair analysis and an analysis based on the 14-faceted polyhedra having six quadrilateral and eight hexagonal faces, we show the presence of bct and hcp signatures in shear ordered inelastic hard-spheres. Thus, our analysis shows that the dense sheared inelastic hard-spheres have a mixture of fcc, bct, and hcp structures.

Journal Article↗

PubMed2006

Voronoi neighbor statistics of homogeneously sheared inelastic hard disks and hard spheres.

In this work we extend our earlier Voronoi neighbor analysis [J. Chem. Phys. 123, 074502 (2005)] to homogeneously sheared inelastic hard-particle structures, the simplest model for rapid granular matter. The pair distribution function is partitioned into the nth neighbor coordination number (Cn), and the nth neighbor position distribution [fn(r)]. The distribution of the number of Voronoi faces (Pn) is also considered since C1 is its mean. We report the Cn and Pn for the homogeneously sheared inelastic hard-disk and hard-sphere structures. These statistics are sensitive to shear ordering transition, the nonequilibrium analogue of the freezing transition. In the near-elastic limit, the sheared fluid statistics approach that of the thermodynamic fluid. On shear ordering, due to the onset of order, the Cn for sheared structures approach that of the thermodynamic solid phase. The suppression of nucleation by the homogeneous shear is evident in these statistics. As inelasticity increases, the shear ordering packing fraction increases. Due to the topological instability of the isotropically perturbed face-centered cubic lattice, polyhedra with faces 12 to 18, with a mean at 14, coexist even in the regular close packed limit for the thermodynamic hard-sphere solid. In shear ordered inelastic hard-sphere structures there is a high incidence of 14-faceted polyhedra and a consequent depletion of polyhedra with faces 12, 13, 15-18, due to the formation of body-centered-tetragonal (bct) structures. These bct structures leave a body-centered-cubic-like signature in the Cn and Pn data. On shear ordering, close-packed layers slide past each other. However, with a velocity-dependent coefficient of restitution, at a critical shear rate these layers get disordered or amorphized. We find that the critical shear rate for amorphization is inversely proportional to the particle diameter, as compared to the inverse square scaling observed in dense colloidal suspensions.

Journal Article↗

PubMed2005

Voronoi cell volume distribution and configurational entropy of hard-spheres.

The Voronoi cell volume distributions for hard-disk and hard-sphere fluids have been studied. The distribution of the Voronoi free volume vf, which is the difference between the actual cell volume and the minimal cell volume at close packing, is well described by a two-parameter (2gamma) or a three-parameter (3gamma) gamma distribution. The free parameter m in both the 2gamma and 3gamma models is identified as the "regularity factor." The regularity factor is the ratio of the square of the mean and the variance of the free volume distribution, and it increases as the cell volume distribution becomes narrower. For the thermodynamic structures, the regularity factor increases with increasing density and it increases sharply across the freezing transition, in response to the onset of order. The regularity factor also distinguishes between the dense thermodynamic structures and the dense random or quenched structures. The maximum information entropy (max-ent) formalism, when applied to the gamma distributions, shows that structures of maximum information entropy have an exponential distribution of vf. Simulations carried out using a swelling algorithm indicate that the dense random-packed states approach the distribution predicted by the max-ent formalism, though the limiting case could not be realized in simulations due to the structural inhomogeneities introduced by the dense random-packing algorithm. Using the gamma representations of the cell volume distribution, we check the numerical validity of the Cohen-Grest expression [M. H. Cohen and G. S. Grest, Phys. Rev. B 20, 1077 (1979)] for the cellular (free volume) entropy, which is a part of the configurational entropy. The expression is exact for the hard-rod system, and a correction factor equal to the dimension of the system, D, is found necessary for the hard-disk and hard-sphere systems. Thus, for the hard-disk and hard-sphere systems, the present analysis establishes a relationship between the precisely defined Voronoi free volume (information) entropy and the thermodynamic entropy. This analysis also shows that the max-ent formalism, when applied to the free volume entropy, predicts an exponential distribution which is approached by disordered states generated by a swelling algorithm in the dense random-packing limit.

Journal Article↗

PubMed2005

Kinetic model for sheared granular flows in the high Knudsen number limit.

The sheared granular flow of rough inelastic granular disks is analyzed in the high Knudsen number limit, where the frequency of particle-wall collisions is large compared with particle-particle collisions, using a kinetic theory approach. An asymptotic expansion is used in the small parameter epsilon =(nsigmaL), which is the ratio of the frequencies of particle-particle and particle-wall collisions, where n is the number of disks per unit area, sigma is the disk diameter, and L is the channel width. The collisions are specified using a normal coefficient of restitution e(n) and a tangential coefficient of restitution e(t). The analysis identifies two regions in the e(t) - e(n) parameter space, one where the final steady state is a static one in which the translational velocities of all particles decrease to zero, and the second where the final steady state is a dynamic one in which the mean square velocities scale as a power of epsilon in the limit epsilon --> 0. Both of these predictions are shown to be in quantitative agreement with computer simulations.

Journal Article↗

PubMed2005

Brazil nut effect and excluded volume attraction in vibrofluidized granular mixtures.

A two dimensional bi-disperse vibrofluidized granular mixture is studied in the rapid flow regime, where particle interactions occur due to instantaneous collisions. Both experiments and simulations are carried out, and these show the existence of two phenomena which have been observed only in very dense granular flows or in equilibrium systems. The Brazil nut phenomenon, which involves the rise of larger particles in a granular mixture upon vibration, has been observed in dense systems due to the percolation of small particles though the interstitial spaces between the large particles, or due to convection rolls. In the present case, where neither effect is present, it is observed that the fluidization of the smaller particles by vibration results in an exponentially decaying density profile, at heights large compared to the particle diameter, and thereby a pressure field that decreases with height. The larger particles, suspended in this decaying pressure field, experience a larger pressure at the bottom and a smaller pressure on top, and they rise to a height where the net force caused by the decreasing pressure is balanced by the weight of the particle. An attractive force between the large particles, similar to the entropic attraction effect in mixtures of colloids and polymers, is also observed in this nonequilibrium system, because when the distance between the large particles is less than the small particle diameter, the pressure between the large particles is smaller than that on the outside. Analytical results are derived for each of these effects, and these are in agreement with the experimental and simulation results.

Journal Article↗

PubMed2005

Voronoi neighbor statistics of hard-disks and hard-spheres.

The neighbor distribution in hard-sphere and hard-disk fluids is analyzed using Voronoi tessellation. The statistical measures analyzed are the nth neighbor coordination number (Cn), the nth neighbor distance distribution [fn(r)], and the distribution of the number of Voronoi faces (Pn). These statistics are sensitive indicators of microstructure, and they distinguish thermodynamic and annealed structures. A sharp rise in the hexagon population marks the onset of hard-disk freezing transition, and Cn decreases sharply to the hexagonal lattice values. In hard-disk random structures the pentagon and heptagon populations remain significant even at high volume fraction. In dense hard-sphere (three-dimensional) structures at the freezing transition, C1 is close to 14, instead of the value of 12 expected for a face-centered-cubic lattice. This is found to be because of a topological instability, where a slight perturbation of the positions in the centers of a pair of particles transforms a vertex in the Voronoi polyhedron into a Voronoi surface. We demonstrate that the pair distribution function and the equation-of-state obtained from Voronoi tessellation are equal to those obtained from thermodynamic calculations. In hard-sphere random structures, the dodecahedron population decreases with increasing density. To demonstrate the utility of the neighbor analysis, we estimate the effective hard-sphere diameter of the Lennard-Jones fluid by identifying the diameter of the spheres in the hard-sphere fluid which has C1 equal to that for the Lennard-Jones fluid. The estimates are within 2% deviation from the theoretical results of Barker-Henderson and Weeks-Chandler-Andersen.

Journal Article↗

PubMed2005

Transplantation of endothelial cells corrects the phenotype in hemophilia A mice.

BACKGROUND: The deficiency of factor VIII, a co-factor in the intrinsic coagulation pathway results in hemophilia A. Although FVIII is synthesized largely in the liver, the specific liver cell type(s) responsible for FVIII production is controversial. OBJECTIVE: This study aimed to determine the cellular origin of FVIII synthesis and release in mouse models. METHODS: We transplanted cells into the peritoneal cavity of hemophilia A knockout mice. Plasma FVIII activity was measured using a Chromogenix assay 2-7 days after cell transplantation, and phenotypic correction was determined with tail-clip challenge 7 days following cell transplantation. Transplanted cells were identified by histologic and molecular assays. RESULTS: Untreated hemophilia A mice, as well as mice treated with the hepatocyte-enriched fraction, showed extensive mortality following tail-clip challenge. In contrast, recipients of unfractionated liver cells (mixture of hepatocytes, liver sinusoidal endothelial cells (LSEC), Kupffer cells, and hepatic stellate cells) or of the cell fraction enriched in LSECs survived tail-clip challenge (P < 0.001). FVIII was secreted in the blood stream in recipients of unfractionated liver cells, LSECs and pancreatic islet-derived MILE SVEN 1 (MS1) endothelial cells. Although transplanted hepatocytes maintained functional integrity in the peritoneal cavity, these cells did not produce detectable plasma FVIII activity. CONCLUSIONS: The assay of cell transplantation in the peritoneal cavity showed that endothelial cells but not hepatocytes produced phenotypic correction in hemophilia A mice. Therefore, endothelial cells should be suitable additional targets for cell and gene therapy in hemophilia A.

Animals↗

PubMed2004

Velocity distribution for a two-dimensional sheared granular flow.

The velocity distribution for a two-dimensional collection of disks of number density n per unit area and radius a in a channel of width L is studied. The particle-particle collisions are considered to be inelastic with a coefficient of restitution e, while the particle-wall coefficients of restitution are inelastic with a tangential and normal coefficients of restitution, e(t) and e(n), respectively. The Knudsen number, which is the ratio of the channel width and the mean free path of the particles, is varied from Kn<<1 to Kn>>1. In the limit of high Knudsen number, the distribution function for the streamwise velocity is bimodal, as predicted by theory [J. Fluid Mech. 340, 3l9 (1997)]], and the scalings of the moments of the velocity distribution with the Knudsen number are in agreement with the theory. In the low Knudsen number limit, the distribution function for the streamwise velocity is a Gaussian if the coefficient of restitution is close to 1, but assumes the form of a "composite Gaussian" if the coefficient of restitution is not close to 1. The distribution function has a complex structure in the intermediate regime, where there are three maxima in the distribution function near the wall, while the distribution function is bimodal at the center. The granular temperature is accurately predicted by kinetic theory at the center of the channel, but there is dissipation at the wall due to inelastic particle-wall collisions, which results in a significant decrease in the temperature even when the coefficient of restitution is 0.9; this finding is in agreement with previous results with bumpy wall boundary conditions and with specular reflection conditions. The slip velocity at the wall has a power law dependence on the Knudsen number, and the exponent in this power law depends on the coefficients of restitution.

Journal Article↗

PubMed2003

Successful surgical management of a prenatally diagnosed intrapericardial teratoma.

Congenital intrapericardial teratoma is a rare, usually benign tumour frequently associated with massive pericardial effusion, cardiac compression, and severe cardiorespiratory distress shortly after birth. Surgical removal is not only curative but also potentially lifesaving because these lesions often become fatal if not promptly excised. We present a case of a newborn with a huge intrapericardial teratoma diagnosed in utero. After normal delivery the infant underwent surgical removal and has had clinical follow-up for more than 11 months.

Adult↗

PubMed2002

Weakly nonlinear analysis of the electrohydrodynamic instability of a charged membrane.

The effect of nonlinear interactions on the linear instability of shape fluctuations of a flat charged membrane immersed in a fluid is analyzed using a weakly nonlinear stability analysis. There is a linear instability when the surface tension reduces below a critical value for a given charge density, because a displacement of the membrane surface causes a fluctuation in the counterion density at the surface, resulting in an additional Maxwell normal stress at the surface which is opposite in direction to the stress caused by surface tension. The nonlinear analysis shows that at low surface charge densities, the nonlinear interactions saturate the growth of perturbations resulting in a new steady state with a fluctuation amplitude determined by the balance between the destabilizing electrodynamic force and surface tension. As the surface charge density is increased, the nonlinear terms destabilize the perturbations, and the bifurcation is subcritical. There is also a significant difference in the predictions of the approximate Debye-Huckel and more exact Poisson-Boltzmann equations at high charge densities, with the former erroneously predicting that the bifurcation is supercritical at all charge densities.

Biophysical Phenomena↗

PubMed2001

Effect of surface charges on the curvature moduli of a membrane.

The modification of the curvature moduli due to surface charges in lipid bilayers is analyzed using the nonlinear Poisson-Boltzmann equation for the relationship between the charge density and surface potential. An expansion in a small parameter epsilon, which is the ratio of the Debye length and the radius of curvature, is used. At low charge densities, previous results obtained from the Debye-Huckel approximation are recovered. At high charge densities, the corrections to the mean and Gaussian curvature approach constant values. The total energy of curvature for a symmetrically charged membrane becomes negative when the charge density is increased beyond a critical value, indicating that the membrane spontaneously forms vesicles. An asymmetry in the charge densities on the two monolayers that form the bilayer results in a spontaneous curvature, and the radius of curvature could be large compared to the Debye length when the asymmetry is small. The case of adsorbed charges is also considered, where there is a reduction in the chemical energy when a charge is adsorbed on the surface. At low charge density, the mean and Gaussian curvature are equal in magnitude and opposite in sign to that for fixed charges, while at high charge density, the mean and Gaussian curvature approach values identical to that for a surface with fixed charges. Numerical calculations of the change in the curvature moduli with realistic parameter values indicate that these effects are likely to be of importance in the spontaneous formation of vesicles.

Journal Article↗

PubMed2001

Characterization of the stationary states of a dilute vibrofluidized granular bed.

This paper reports two phenomena in an event driven simulation of a dilute vibrofluidized granular material in two dimensions. Both phenomena show inhomogeneity in the horizontal direction. They are convection rolls similar to the Rayleigh-Bénard thermal convection in fluids, and a clustering instability, where the bed spontaneously phase separates into coexisting dense and dilute regions. Detailed investigations show that these are different from the known instabilities in a vibrated granular medium. Characterization of these instabilities is carried out with a phase diagram using suitable parameters from the kinetic theory of vibrofluidized beds.

Journal Article↗

PubMed2001

Electrohydrodynamic instability of a charged membrane.

The stability of shape fluctuations of a flat charged membrane immersed in a fluid is analyzed using a linear stability analysis. A displacement of the membrane surface causes a fluctuation in the conterion density at the surface. This in turn causes an additional contribution to the force density in the momentum equation for the fluid, which results in a normal stress at the surface which is opposite in direction to the stress caused by surface tension. This electrohydrodynamic effect destabilizes fluctuations when the surface potential exceeds a critical value.

Biomechanical Phenomena↗

PubMed2000

Instabilities due to charge-density-curvature coupling in charged membranes.

Instabilities are caused by the reduction in the electrostatic energy when the membrane is curved with the higher charge density on the bilayer which is stretched by the curvature. In a bilayer where the charges can flip from one lipid layer to the other, there is a thermodynamic instability to a spontaneously curved state with different charge densities on the two sides. If the charges are not permitted to flip, there is a dynamical instability due to the correlated modulation of the charge density and curvature fields. Numerical estimates show that these effects are present in parameter regimes relevant to biological systems.

Animals↗

PubMed2000

Spontaneous growth of fluctuations in the viscous flow of a fluid past a soft interface

The flow induced instability in the flow past a soft material is studied in the limit of low Reynolds number where inertial effects are insignificant. A transition from laminar flow to a more complicated flow profile is observed when the strain rate of the base flow increases beyond a critical value; the transition is found to be reproducible. The experimental results are compared with theoretical predictions and quantitative agreement is found with no adjustable parameters.

Journal Article↗

PubMed1999

An evaluation of the no-scalpel vasectomy technique.

OBJECTIVE: To evaluate the magnitude and range of complications encountered during no-scalpel vasectomy (NSV) and to identify any limiting factor or contraindication to NSV. SUBJECTS AND METHODS: A total of 4255 men visiting a family welfare clinic of a city hospital in Delhi, India, or outpatients of the surgery department, opted for NSV; 4253 procedures were performed from September 1989 to December 1997, with two not performed because of technical difficulties. The success and complication rates were recorded. RESULTS: The complication rates with NSV were surprisingly low; seven complications were encountered, comprising two small haematomas (0.047%), three painful nodules (0.07%), three wound infections (0.07%) and two vasal fistulae, a complication not previously described. A meticulous procedure should avoid this last complication. The mean duration of NSV was 9.5 min. CONCLUSION: NSV is a refined and rapid technique of vasectomy with an extremely low complication rate. Being a minimally invasive procedure, it allays the fear of incision commonly encountered in men choosing vasectomy.

Contraindications↗