Dillip K Satapathy

Evaporation of fluid from a pinned drop placed on solid surface proceeds via constant contact radius (CCR) mode, with a continuous reduction in the contact angle. The reduction of contact angle leads to an imbalance of interfacial tensions at the three-phase contact line. When the unbalanced force is sufficiently strong, the drop slips from the pinned contact line and slides inward. Depinning of the drop alters the mode of evaporation to constant contact angle (CCA) mode till it repins onto the surface. The change in evaporation mode from CCR to CCA is usually achieved by tuning the pinning energy barrier by controlling the surface properties of the substrate. Here, we demonstrate that the evaporation mode can be controlled by solely tailoring the surface tension of the drop, which is achieved in microgel particle-laden sessile drops that show spontaneous adsorption of microgels to the air/water interface, leading to a decrease in the interfacial tension. We show that droplets containing a sufficient number of microgels evaporate predominantly in CCR mode even on a hydrophobic surface, and the contact line remains pinned throughout the evaporation of the drop. Interestingly, the contact line dynamics can be controlled by tuning the softness of the microgels and the particle concentration in the drops.

The orientation and morphology of cracks in coffee-ring like particulate deposit obtained by drying sessile drops containing anisotropic magnetic particles strongly depends on the magnitude of the applied magnetic field and its direction. This opens up the possibility of tuning the micro-structure of cracks via suitable manipulation of magnetic and hydrodynamic torques on the particles which has potential applications in nano-fabrication and field driven self-assembly. We report a systematic study of magnetic field driven self-assembly of hematite ellipsoids in sessile drops dried on solid substrates and resulting crack patterns. The experiments are carried out over a wide range of applied magnetic field strength (|B→|) varying from 0 to 400 G and ellipsoids of two different aspect ratios. Dried coffee-ring deposits of ellipsoids in absence of the external applied magnetic field and at low field strength, |B→|≤20G exhibit circular cracks. However, at |B→|⩾30G, the cracks are observed to be linear and perpendicular to the direction of the applied magnetic field. Random cracks are observed in the intermediate field range of 20G<|B→|<30G. Thus our experiments reveal that there exists a critical magnetic field at which the orientation of cracks change from circular to linear. The knowledge of the critical field is exploited to measure the hydrodynamic torque experienced by nano-ellipsoids and fluid velocities during evaporation, which are challenging to measure experimentally.

We report the densification phenomena inunentangled and low-molecular weight polystyrene (PS) thin films supported on solid substrates having thickness from 25 nm to 230 nm. The mass density of the thin polymer films were extracted from X-ray reflectivity profiles and also from the refractive index by using Clausius and Mossotti equation. The mass densityof polymeris found to increasesignificantly with decrease in film thickness below ten times the radius of gyration of the polymer. The net increase in mass density of the polymer film upon reduction in thickness is discussed in terms of three-layer model and the presence of unentangled polymer chains. We conjecture that, the densification of ultra-thin polymer films can strongly alter the polymer conformations at film/substrate interface.

We report the effect of cobalt ferrite (CoFe O 2 4) nanoparticles on dielectric and ferroelectric properties of poly(vinylidene fluoride)(PVDF). Large enhancements in dielectric constant and remanent polarization upon addition of 5 wt.% CoFe O 2 4 nanoparticles to PVDF have been observed. Nearly 17% increase of dielectric constant and 31% increase in remanent polarization was observed in PVDF-CoFe O 2 4 nanocomposite films as compared to pristine PVDF films prepared under identical conditions. These enhancements were accompanied by a decrease in crystallinity and an increase in electroactive β-phase content as evidenced by x-ray diffraction and fourier transform infrared spectroscopy, respectively. A 5% increase in the amount of β-phase content is also observed in nanocomposite films. These enhancements in dielectric constant and remanent polarization are attributed to the influence of CoFe O 2 4 nanoparticles on nucleation, morphology, dipole aligning and dipole pinning behavior of PVDF.

We show that loosely packed homogeneous monolayers of soft colloidal particles can be fabricated via simple evaporative drying of aqueous dispersion containing colloidal particles. Owing to the inherent amphiphilicity of the soft microgels, they are preferentially adsorbed onto the water-vapor interface of the drying droplet, resulting in a uniform deposition of the particles throughout the fluid-substrate contact area. Distinct order-to-disorder transition is also identified within the dried deposits on changing spatial locations from edge to center of the circular stain. Such monolayer deposits exhibit vibrant colors on irradiating with white light due to interference.

The advantage of secondary-phase induced carrier filtering on the thermoelectric properties has paved the way for developing cost-effective, highly efficient thermoelectric materials. Here, we report a very high thermoelectric figure-of-merit of skutterudite nanocomposites achieved by tailoring interface carrier filtering. The single-filled skutterudite nanocomposites are prepared by dispersing rare-earth oxides nanoparticles (Yb2O3, Sm2O3, La2O3) in the skutterudite (Dy0.4Co3.2Ni0.8Sb12) matrix. The nanoparticles/skutterudite interfaces act as efficient carrier filters, thereby significantly enhancing the Seebeck coefficient without compromising the electrical conductivity. As a result, the highest power factor of ∼6.5 W/mK2 is achieved in the skutterudite nanocomposites. The nonuniform strain distribution near the nanoparticles due to the local lattice misfit and concentration fluctuations affect the heat carriers and thereby reduce the lattice thermal conductivity. Moreover, the three-dimensional atom probe analysis reveals the formation of Ni-rich grain boundaries in the skutterudite matrix, which also facilitates the reduction of lattice thermal conductivity. Both the factors, i.e., the reduction in lattice thermal conductivity and the enhancement of the power factor, lead to an enormous increase in ZT up to ∼1.84 at 723 K and an average ZT of about 1.56 in the temperature range from 523 to 723 K, the highest among the single-filled skutterudites reported so far.

Using X-ray reflectivity, spectroscopic ellipsometry and Raman spectroscopy, we have studied the stratified structure and the two glass-like thermal transitions in sufficiently aged glassy polystyrene films. We find that favorable interaction between the solid substrate and the polymer film induces stratification within the film resulting in different densities across the film thickness. Existence of two glass-like thermal transitions (one at 70 °C and the other at 95 °C) is independently confirmed by temperature dependent spectroscopic ellipsometry and Raman spectroscopy measurements. Interestingly, the thermal coefficient of expansion of the polymer film displays anomalous behavior with temperature and is found to have the lowest value over the temperature range 70-95 °C, i.e. between the two observed glass-like thermal transition temperatures.

We report an interplay between the desorption of intrinsic water and relaxation of polymer chains resulting in an unusual thermomechanical response of a hydrogel, wherein the elastic modulus increases in a certain temperature range followed by a sharp decrease with a further increase in temperature. We establish that, in a hydrogel, the desorption of disparate water types having distinct binding energy affects the consolidation and relaxation behaviour of the matrix, which in turn affects the mechanical properties at different temperature ranges. Using temperature-dependent dielectric relaxation spectroscopy and nanoindentation techniques, the chain dynamics and mechanical properties are investigated.

Cracks in a colloidal film formed by evaporation induced drying can be controlled by changing drying conditions. We show, for the first time that the crack morphologies in colloidal films are dependent on shape of constituting particles apart from the microstructure and particle assembly. In order to investigate the particle shape effect on crack patterns, monodispered spherical and ellipsoidal particles are used in sessile drop experiments. On observing the dried sessile drop we found cracks along the radial direction for spherical particle dispersions and circular crack patterns for ellipsoidal particle dispersions. The change in crack pattern is a result of self assembly of shape anisotropic particles and their ordering. The ordering of particles dictate the crack direction and the cracks follow the path of least resistance to release the excess stress stored in the particle film. Ellipsoids having different aspect ratio (∼3 to 7) are used and circular crack patterns are repeatedly observed in all experiments.

Intermolecular hydrogen bonds play a crucial role in determining the unique characteristics of liquid water. We present low-frequency (1 Hz-40 MHz) dielectric spectroscopic investigations on water in the presence and absence of added solutes at different temperatures from 10 °C to 60 °C. The intersection points of temperature dependent permittivity contours at the vicinity of isopermitive frequency (IPF) in water are recorded and its properties are presumed to be related to the extent of hydrogen bond networks in water. IPF is defined as the frequency at which the relative permittivity of water is almost independent of temperature. The set of intersection points of temperature dependent permittivity contours at the vicinity of IPF are characterized by the mean (MIPF) and root-mean-square deviation/standard deviation (σIPF) associated with IPF. The tunability of MIPF by the addition of NaCl and MgCl2 salt emphasizes the strong correlation between the concentration of ions in water and the MIPF. The σIPF is surmised to be related to the orientational correlations of water dipoles as well as to the intermolecular hydrogen bond networks in water. Further, alterations in σIPF is observed with the addition of kosmotropic and chaotropic solutes into water and are thought to arise due to the restructuring of hydrogen bond networks in water in presence of added solutes. Notably, the solute induced reconfiguration of hydrogen bond networks in water or often-discussed structure making/breaking effects of the added solutes in water can be inferred, albeit qualitatively, by examining the MIPF and σIPF. Further, the Gaussian deconvoluted OH-stretching modes present in the Raman spectra of water and aqueous solutions of IPA and DMF strongly endorses the structural rearrangements occurring in water in presence of kosmotropes and chaotropes and are in line with the results derived from the root-mean-square deviation in IPF.