Shankar Ghosh

A high-precision study of hindered diffusion near a wall

A high-precision study of hindered diffusion of a sphere due to its proximity to a solid interface was performed using an optical tweezer combined with digital holography microscopy and a phase-sensitive detection technique. The study provides a confirmation, with high accuracy and no adjustable parameters, of Faxen’s law which describes the variation of the diffusivity of a sphere as a function of its distance from the wall in a Newtonian liquid due to hydrodynamic effects. This general technique is useful for application in microfluidics and lubrication of small devices with moving parts.

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 = ) 32 K. 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.

Self organization of exotic oil-in-oil phases driven by tunable electrohydrodynamics

Self organization of large-scale structures in nature - either coherent structures like crystals, or incoherent dynamic structures like clouds - is governed by long-range interactions. In many problems, hydrodynamics and electrostatics are the source of such long-range interactions. The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self organization in situations where both electrostatic and hydrodynamic interactions are present. We present a minimal two-component oil-in-oil model system where we can control the strength and lengthscale of the electrohydrodynamic interactions by tuning the amplitude and frequency of the imposed electric field. As a function of the hydrodynamic lengthscale, we observe a rich phenomenology of exotic structure and dynamics, from incoherent cloud-like structures and chaotic droplet dynamics, to polyhedral droplet phases, to coherent droplet arrays.

Weak adhesion at the mesoscale: particles at an interface

Adhesion of colloidal particles (synthetic or biological) to one another or to other surfaces, typically immersed in a fluid medium, is frequently a mesoscale phenomenon, intermediate between microscopic and macroscopic scales. Similar to other mesoscale phenomena, gaining a useful understanding based on ideas from either continuum or atomistic concepts alone is limited. Combination of experimental techniques like optical trapping and scanning probe techniques with simultaneous imaging provides direct insight into the space- and time-dependent dynamics of the adhesion process in the relevant scales. The strength of adhesion typically evolves with time; the details of the processes depend on the overall strength of attractive interaction among the adhering objects as well as its spatial fluctuations governed by the randomness of the surfaces involved and their own time-dependent reconstruction due to varying local stresses. This review focuses on situations where the adhesive inter-object interactions are “weak” compared to the cohesive intra-object interactions whereby all processes occur, to a good approximation, in the interfacial region. In these situations, the dynamics of adhesion is affected by both the “quenched” spatial disorder of the surfaces as well as the thermal fluctuations. A local microrheological characterization of the stress-coupling in the interfacial region provides a heuristic description of the adhesion process. The discrete, thermally assisted “punctuated descent” of the effective adhering system down a rough energy landscape through a hierarchy of finite metastable minima reflects the mesoscopic nature of adhesion. We review these experimental observations of particle adhesion in the weak limit and explore possible common mechanisms underlying apparently disparate phenomena and processes.

Surface enhanced Raman scattering from multiwalled carbon nanotubes at low temperatures

We report temperature dependent Raman spectra of multiwalled carbon nanotubes deposited as dilute dispersions on surface enhanced Raman scattering active substrate. At low temperature, i.e, below 30 K both the tangential and the disorder mode acquire additional spectral features which are otherwise absent in the room temperature spectra. Further, we observe that the surface enhancement effect is more for modes involving motion of atoms in the direction perpendicular to the substrate which is consistent with the surface selection rules. These experiments provide access to certain spectral features of multiwalled carbon nanotubes which are otherwise very difficult to see.

Implications of magnetic and magnetodielectric behavior of GdCrTiO5

We have carried out dc magnetization, heat-capacity, and dielectric studies down to 2 K 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 with that in isostructural Nd counterpart, NdCrTiO5. This finding on the Gd compound suggests that Nd 4f hybridization plays an uncommon role in 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 150–400 cm−1...

Three-dimensional real-time imaging of bi-phasic flow through porous media.

We present a scanning laser-sheet video imaging technique to image bi-phasic flow in three-dimensional porous media in real time with pore-scale spatial resolution, i.e., 35 μm and 500 μm for directions parallel and perpendicular to the flow, respectively. The technique is illustrated for the case of viscous fingering. Using suitable image processing protocols, both the morphology and the movement of the two-fluid interface, were quantitatively estimated. Furthermore, a macroscopic parameter such as the displacement efficiency obtained from a microscopic (pore-scale) analysis demonstrates the versatility and usefulness of the method.

Ultra-low breakdown voltage and origin of 1/f 2 noise in metallic nanorod arrays

The application of a dc voltage to an array of copper nanorods causes field evaporation of atoms from the tips, resulting in their progressive sharpening and a further increase in the local field. The process is self-limited by the build-up of space charge on the nanorod tips. From an analysis of the conductance noise recorded across the nanorod array, we show that the conduction mechanism bears a strong analogy with the stick-slip problem in sliding friction. The in situ sharpening results in an unprecedented lowering of the breakdown voltage of air by over 90%, as compared to plane parallel electrodes.

Clustered copper nanorod arrays: a new class of adhesive hydrophobic materials

Clustered copper nanorod arrays constitute single-component, two-level hierarchical structures that mimic rose petals by showing high contact angles as well as considerable contact angle hysteresis (CAH). The cluster morphology of the nanorod arrays – electrochemically grown within porous alumina templates – is determined by the template geometry and the drying conditions. The CAH (a measure of the adhesion) is a function of the nanorod length and the cluster size to inter-cluster separation ratio. The measured pinning force was 176 μN, among the highest reported. The first level of the hierarchical nano-microstructure consists of the nanorod clusters and the air trapped between them – leading to a Cassie–Baxter type hydrophobicity. The second level comprises the tips of the clustered copper nanorods which pin the water droplets due to the inherently hydrophilic nature of metals. This conjecture is supported by fluorescence imaging and surface polymerization experiments. Thus, a clustered metal nanorod array provides a simple model system for studying the coexistence and complex interplay of hydrophobicity and adhesion, while also providing some degree of control over these mutually contradictory properties.

Raman study of Ca3Co2O6 single crystals

We report Raman scattering experiments on single crystals of Ca3Co2O6. The polarization dependent scattering was used to assign symmetry of the observed vibrational modes. The low temperature experiments show a subtle change in peak widths at 75 K. The increase in the peak widths is also accompanied by a gain in spectral weight for wave numbers greater than 1000 cm−1. We have associated the increase in peak widths to lattice distortion and the high frequency spectral feature to the inelastic light scattering processes involving two magnons. These observations are consistent with the reported extended x-ray absorption fine structure [Bindu et al., Phys. Rev. B 79, 094103 (2009)] and Mossbauer measurements [Paulose et al., Phys. Rev. B 77, 172403 (2008)] which suggests simultaneous presence of lattice distortion and short range magnetic order in the system well above the Neel temperature (24 K). Thus, our experiments highlight the usage of Raman spectroscopy as a tool to study systems which show precursor e...