arXiv subjects

Shankar Ghosh

Publications and source records attributed to Shankar Ghosh.

36 records · Page 2Linked to original sources

arXiv2020

Tuning of friction noise by accessing the rolling-sliding option

Variable power transmission in mechanical systems is often achieved by devices, e.g., clutches and brakes, that use dry friction. In these systems, the variability in power transmission is brought about by engaging and disengaging the friction plates. Though commonly used, this method of making the coupling noisy is not as versatile as their electrical analog. An alternative method would be to intermittently vary the frictional force. In this paper, we demonstrate a self-organized way to tune the noise in the frictional coupling between two surfaces which are in relative motion with each other. This is achieved by exploiting the complexity that arises from the frictional interaction of the balls which are placed in a circular groove between the surfaces. The extent of floppiness in the coupling is related to the rate at which the balls make transitions between their rolling and sliding states. If the moving surface is soft and the static surface is hard we show that with increasing filling fraction of the balls the transitions between rolling and sliding against the static surface give way to the transitions between rolling and sliding against the moving surface. As a consequence, the noise in the coupling is large for both small and large filling fraction with a dip in the middle. In contrast, the sliding with the static surface is suppressed if the moving pate is hard and the noise in the coupling decreases monotonically with the filling fraction of the balls.

cond-mat.soft↗

arXiv2019

Molding 3D curved structures by selective heating

It is of interest to fabricate curved surfaces in three dimensions from easily available homogeneous material in the form of flat sheets. The aim is not just to obtain a surface $M$ in $\mathbb{R}^3$ which has a desired intrinsic Riemannian metric, but to get the desired embedding $M \subset \mathbb{R}^3$ up to translations and rotations (the Riemannian metric alone need not uniquely determine this). In this paper, we demonstrate three generic methods of molding a flat sheet of thermo-responsive plastic by selective contraction induced by targeted heating. These methods do not involve any cutting and gluing, which is a property they share with origami. The first method is inspired by tailoring, which is the usual method for making garments out of plain pieces of cloth. Unlike usual tailoring, this method produces the desired embedding in $\mathbb{R}^3$, and in particular, we get the desired intrinsic Riemannian metric. The second method just aims to bring about the desired new Riemannian metric via an appropriate pattern of local contractions, without directly controlling the embedding. The third method is based on triangulation, and seeks to induce the desired local distances. This results in getting the desired embedding in $\mathbb{R}^3$, in particular, it also gives us the target Riemannian metric. The second and the third methods, and also the first method for the special case of surfaces of revolution, are algorithmic in nature. We give a theoretical account of these methods, followed by illustrated examples of different shapes that were physically molded by these methods.

cond-mat.soft↗

arXiv2017

Relaxor ferroelectric behavior and intrinsic magnetodielectric behavior near room temperature in Li2Ni2Mo3O12, a compound with distorted honeycomb and spin-chains

Keeping current interests to identify materials with intrinsic magnetodielectric behavior near room temperature and with novel pyroelectric current anomalies, we report temperature and magnetic-field dependent behavior of complex dielectric permittivity and pyroelectric current for an oxide, Li2Ni2Mo3O12, containing magnetic ions with (distorted) honey-comb and chain arrangement and ordering magnetically below 8 K. The dielectric data reveal the existence of relaxor ferroelectricity behavior in the range 160-240 K and there are corresponding Raman mode anomalies as well in that temperature range. Pyrocurrent behavior is also consistent with this interpretation, with the pyrocurrent peak-temperature interestingly correlating with the poling temperature. 7Li NMR offer an evidence for crystallographic disorder intrinsic to this compound and we therefore conclude that such a disorder is apparently responsible for the randomness of local electric field leading to relaxor ferroelectric property. Another observation of emphasis is that there is a notable decrease in the dielectric constant with the application of magnetic field to the tune of about -2.4% at 300 K, with the magnitude varying mariginally with temperature. Small loss factor values validate intrinsic behavior of the magnetodielectric effect at room temperature.

cond-mat.str-el↗

arXiv2017

On the geometric phenomenology of static friction

In this note we introduce a hierarchy of phase spaces for static friction, which give a graphical way to systematically quantify the directional dependence in static friction via subregions of the phase spaces. We experimentally plot these subregions to obtain phenomenological descriptions for static friction in various examples where the macroscopic shape of the object affects the frictional response. The phase spaces have the universal property that for any experiment in which a given object is put on a substrate fashioned from a chosen material with a specified nature of contact,the frictional behavior can be read off from a uniquely determined classifying map on the control space of the experiment which takes values in the appropriate phase space.

cond-mat.soft↗

arXiv2017

Mechanics of a granular skin

Magic Sand, a hydrophobic toy granular material, is widely used in popular science instructions because of its non-intuitive mechanical properties. A detailed study of the failure of an underwater column of magic sand shows that these properties can be traced to a single phenomenon: the system self-generates a cohesive skin that encapsulates the material inside. The skin, consists of pinned air-water-grain interfaces, shows multi-scale mechanical properties: they range from contact-line dynamics in the intra-grain roughness scale, plastic flow at the grain scale, all the way to the sample-scale mechanical responses. With decreasing rigidity of the skin, the failure mode transforms from brittle to ductile (both of which are collective in nature) to a complete disintegration at the single grain scale.

cond-mat.soft↗

arXiv2017

Rheology of hydrating cement paste: crossover between two aging processes

The roles of applied strain and temperature on the hydration dynamics of cement paste are uncovered in the present study. We find that the system hardens over time through two different aging processes. The first process dominates the initial period of hydration and is characterized by the shear stress $σ$ varying sub-linearly with the strain-rate $\dotγ$; during this process the system is in a relatively low-density state and the inter-particle interactions are dominated by hydrodynamic lubrication. At a later stage of hydration the system evolves to a high-density state where the interactions become frictional, and $σ$ varies super-linearly with $\dotγ$; this is identified as the second process. An instability, indicated by a drop in $σ$, that is non-monotonic with $\dotγ$ and can be tuned by temperature, separates the two processes. Both from rheology and microscopy studies we establish that the observed instability is related to fracture mechanics of space-filling structure.

cond-mat.soft↗

arXiv2016

Curvilinear polyhedra as dynamical arenas, illustrated by an example of self-organized locomotion

Experiment shows that dumbbells, placed inside a tilted hollow cylindrical drum that rotates slowly around its axis, climb uphill by forming dynamically stable pairs, seemingly against the pull of gravity. Analysis of this experiment shows that the dynamics takes place in an underlying space which is a curvilinear polyhedron inside a six dimensional manifold, carved out by unilateral constraints that arise from the non-interpenetrability of the dumbbells. The energetics over this polyhedron localizes the configuration point within the close proximity of a corner of the polyhedron. This results into a strong entrapment, which provides the configuration of the dumbbells with its observed shape that leads to its functionality -- uphill locomotion. The stability of the configuration is a consequence of the strong entrapment in the corner of the polyhedron.

nlin.AO↗

arXiv2015

Implications of magnetic and magnetodielectric behavior of GdCrTiO5

We have carried out dc magnetization (M), heat-capacity (C) and dielectric studies down to 2K for the compound GdCrTiO5, crystallizing in orthorhombic Pbam structure, in which well-known multiferroics RMn2O5 (R= Rare-earths) form. The points of emphasis are: (i) The magnetic ordering temperature of Cr appears to be suppressed compared to that in isostructural Nd counterpart, NdCrTiO5, for which the Neel temperature is about 21 K. This finding on the Gd compound suggests that Nd 4f orbital plays a role on the magnetism of Cr in contrast to a proposal long ago. (ii) Dielectric constant does not exhibit any notable feature below about 30 K in the absence of external magnetic field, but a peak appears and gets stronger with the application of external magnetic fields, supporting the existence of magnetodielectric coupling. (iii) The dielectric anomalies appear even near 100 K, which can be attributed to short-range magnetic-order. We also observe a gain in spectral weight below about 150 K in Raman spectra in the frequency range 150 to 400 cm-1, which could be magnetic in origin supporting short-range magnetic order. It is of interest to explore whether geometrically frustration plays any role on the dielectric properties of this family, as in the case of RMn2O5.

cond-mat.mtrl-sci↗

arXiv2014

Electric-field-induced turbulent energy cascade in an oil-in-oil emulsion

We observe electro-hydrodynamically driven turbulent flows at low Reynolds numbers in a two-fluid emulsion consisting of micron-scale droplets. In the presence of electric fields, the droplets produce interacting hydrodynamic flows which result in a dynamical organization at a spatial scale much larger than the size of the individual droplets. We characterize the dynamics associated with these structures by both video imaging and a simultaneous, in situ, measurement of the time variation of the bulk Reynolds stress with a rheometer. The results display scale invariance in the energy spectra in both space and time.

cond-mat.soft↗

arXiv2014

Displacive-type ferroelectricity from magnetic correlations within spin-chain

Observation of ferroelectricity among non-d0 systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN=) 32K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d0-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

cond-mat.str-el↗

arXiv2014

Spreading of triboelectrically charged granular matter

We report on the spreading of triboelectrically charged glass particles on an oppositely charged surface of a plastic cylindrical container in the presence of a constant mechanical agitation. The particles spread via sticking, as a monolayer on the cylinder's surface. Continued agitation initiates a sequence of instabilities of this monolayer, which first forms periodic wavy-stripe-shaped transverse density modulation in the monolayer and then ejects narrow and long particle-jets from the tips of these stripes. These jets finally coalesce laterally to form a homogeneous spreading front that is layered along the spreading direction. These remarkable growth patterns are related to a time evolving frictional drag between the moving charged glass particles and the countercharges on the plastic container. The results provide insight into the multiscale time-dependent tribolelectric processes and motivates further investigation into the microscopic causes of these macroscopic dynamical instabilities and spatial structures.

cond-mat.soft↗

arXiv2013

Transition of a particle between adjacent optical traps: A study using catastrophe theory

In spite of the widespread use of optical tweezers as a quantitative tool to measure small forces, there exists no unambiguous and simple experimental method for either validating its theoretically predicted form or empirically parameterizing it over the entire range. This problem is addressed by studying the transition of a colloidal particle between two spatially separated optical traps. The transition as a function of the relative intensity of the traps and the separation between them reveals a formal resemblance to the `butterfly catastrophe' which also maps onto to phase transitions observed, for example in ferroelectrics, on a phenomenological level. The method has been used to experimentally determine the force-displacement curve for an optical trap over its entire range.

cond-mat.soft↗

arXiv2012

Amorphous to amorphous transition in particle rafts

Space-filling assemblies of athermal hydrophobic particles floating at an air-water interface, called particle rafts, are shown to undergo an unusual phase transition between two i.e., a low density `less-rigid and a high density `more-rigid' amorphous states as a function of particulate number density ($Φ$). The former is shown to be a capillary-bridged solid and the later a frictionally-coupled one. Simultaneous studies involving direct imaging as well as measuring its mechanical response to longitudinal and shear stresses show that the transition is marked by a subtle structural anomaly and a weakening of the shear response. The structural anomaly is identified from the variation of the mean coordination number, mean area of the Voronoi cells and the spatial profile of the displacement field with $Φ$. The weakened shear response is related to local plastic instabilities caused by the depinning of the contact-line of the underlying fluid on the rough surfaces of the particles.

cond-mat.dis-nn↗

arXiv2011

Dynamical instabilities of two-fluid interfaces in a porous medium: A three-dimensional video imaging study

Two-fluid interfaces in porous media, an example of driven disordered systems, were studied by a real time three-dimensional imaging technique with pore scale resolution for a less viscous fluid displacing a more viscous one. With increasing flow rate the interface transforms from flat to fingers and thence to droplets for both drainage and imbibition. The results compare and contrast the effects of randomness, both physical (geometry of the pore space) and chemical (wettability of the fluids), on the dynamical instability and identify the origin of the pore-scale processes that govern them.

cond-mat.soft↗

arXiv2009

Sticking dynamics of deformable colloids

The sticking of a soft polystyrene colloidal particle to a planar glass plate was studied by a microrheological technique using an optical tweezer to trap the particle and a piezoelectric-stage to position the plate and to sinusoidally drive it in-planeThe sticking process was well-described by a crossover of the mechanical coupling between the plate and the particle from being dominantly viscous to becoming dominantly elastic. By varying three parameters in the experiment, namely, the strength of the tweezer potential, the strength of the particle-plate interaction and the strength of the oscillatory drive, three different regimes of dynamics - stuck, non-stuck and ageing were observed, corresponding to the coupling that is elastic, viscous and viscoelastic, respectively. The observations of [1] can be illustrated better by a parametric Nyquist plot of the real and imaginary parts of the effective rigidity modulus of the coupling medium. Upon contact between the particle and the plate, these parameters change abruptly from their bulk values, regardless of whether the particle is stuck or non-stuck in the terminal steady state. In the ageing regime, where sticking evolves gradually in time, the system shows a parametric plot significantly different from what would be expected from a single-relaxation process. A tendency of clustering of the data in the Nyquist plot provides an interesting contrast between truly many-body systems and one with a few degrees of freedom.

cond-mat.soft↗

arXiv2009

Low temperature Raman study of Ca3Co2O6

We present low temperature Raman measurements on Ca3Co2O6. Shell model lattice dynamics calculations were done to determine the vibrational pattern of the expected Raman modes. In the temperature window 300K to 52K the mode frequencies and the full width at half maxima (FWHM) of the modes vary in an expected fashion, i.e., the mode frequencies shift to higher values and the FWHM decreases with lowering of temperature. At about 52K (T *) there happens an abrupt change in both the peak position and FWHM. The peak positions show a sudden softening while FWHM increases abruptly. To explain these spectral features we postulate that below T * the degeneracy of the high symmetry Eg is lifted due to the possible distortion of the CoOO6 octahedra.

cond-mat.mtrl-sci↗

arXiv2008

Non-monotonic size dependence of the elastic modulus of nanocrystalline ZnO embedded in a nanocrystalline silver matrix

We present the first high pressure Raman study on nanocrystalline ZnO films with different average crystallite sizes. The problem of low Raman signals from nano sized particles was overcome by forming a nanocomposite of Ag and ZnO nanoparticles. The presence of the nanodispersed Ag particles leads to a substantial surface enhancement of the Raman signal from ZnO. We find that the elastic modulus of nanocrystalline ZnO shows a non-monotonic dependence on the crystallite size. We suggest that the non-monotonicity arises from an interplay between the elastic properties of the individual grains and the intergranular region.

cond-mat.mtrl-sci↗

arXiv2006

Motion of a sphere in an oscillatory boundary layer: an optical tweezer based study

The drag forces acting on a single polystyrene sphere in the vicinity of an oscillating glass plate have been measured using an optical tweezer. The phase of the sphere is found to be a sensitive probe of the dynamics of the sphere. The evolution of the phase from an inertially-coupled regime to a purely velocity-coupled regime is explored. Moreover, the frequency dependent response is found to be characteristic of a damped oscillator with an effective inertia which is several orders of magnitude greater than that of the particle.

cond-mat.soft↗