Dillip K. Satapathy

Confined water layers in graphene oxide probed with spectroscopic ellipsometry

The confinement of water in quasi two-dimensional layers is intriguing because its physical properties can be significantly different when compared to those of the bulk fluid. This work describes spectroscopic ellipsometry study of confined water layers trapped between sheets of graphene oxide at varied thermal annealing temperatures. The wavelength-dependent refractive index of graphene oxide changes abruptly with annealing temperatures for Tann ≈ 125–160 °C, and we demonstrate that these changes are primarily governed by the expulsion of trapped water. This expulsion is associated with the decrease of interlayer separation of graphene oxide sheets from 7.8 A to 3.4 A. Graphene oxide annealed at high temperatures lacks trapped water layers and robust estimates of refractive index can be obtained within a Lorentz oscillator model. The trends in oscillator parameters are extended to lower annealing temperatures, where trapped water is present, in order to estimate the refractive index of confined water, wh...

Role of particle shape anisotropy on crack formation in drying of colloidal suspension.

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.

Cracks in dried deposits of hematite ellipsoids: Interplay between magnetic and hydrodynamic torques

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 400G 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.

Influence of Microstructure on the Nanomechanical Properties of Polymorphic Phases of Poly(vinylidene fluoride)

Poly(vinylidine fluoride) (PVDF) is a semicrystalline polymer which is known to exist in several polymorphic phases, namely, α, β, and γ. Each one of these polymorphic phases is characterized by unique features such as spherulite formation in the case of the α and γ phases and the presence of large piezoelectric and ferroelectric activity in the β phase. Despite being widely used as thin coatings in sensors, lack of reports on nanomechanical properties suggests that investigation of mechanical properties of PVDF, let alone those of its polymorphic phases, seems to have evaded the sight of the research community. Herein, we report the nanomechanical properties of the α, β, and γ phases of PVDF. The modulus and hardness values were evaluated from nanoindentation experiments; it was found that the electroactive β phase is the softest among the three polymorphic phases. This result was further confirmed by scratch experiments. We have attempted to establish a correlation between the microstructure and nanomechanical properties of these phases. This work sheds light on the mechanisms responsible for the observed mechanical behavior and the role of tie molecules and amorphous content in providing flexibility to the polymer.