arXiv subjects

Arun Roy

Publications and source records attributed to Arun Roy.

9 recordsLinked to original sources

arXiv2026

Investigating the properties of nearby young moving groups using GaiaDR3

Moving groups, which are gravitationally unbound collections of stars spread over large portions of the sky, pose challenges to their identification. Analyzing the three-dimensional spatial motion of stars is one method to identify their members. Despite several studies in the past, a reliable and conclusive catalog of moving group stars is currently lacking. Our objective is to present a most recent and updated catalog of nearby young moving group (NYMG) candidates in the solar neighborhood and investigate their properties using updated science data from the Gaia Data Release 3 (DR3). We searched for candidates of twelve NYMGs within a distance of 150 pc using Gaia DR3. To determine the membership, we employed the Bayesian algorithm BANYAN-Sigma. We compiled a total list of 3,153 NYMG candidates, of which 1,651 were new candidates. We also assessed the credibility of the literature defined 'Good-box' criterion with the latest comprehensive catalog of NYMGs from our study. We homogeneously estimated the ages of the NYMG candidates using Gaia DR3 photometry data. The analysis revealed that our NYMG candidates display a large scatter on the CMDs resulting in a large range of estimated ages from isochrones. Additionally, we conducted an infrared (IR) excess analysis to identify disk candidates among our final sample. Our spectral energy distribution (SED) analysis found 51 stars with IR excess. We present a largely inclusive list of all the NYMG candidates within 150 pc of Solar neighbourhood using Gaia DR3. The wide range of ages obtained from isochrone fitting underscores the need for more robust age analysis techniques to accurately determine the ages of NYMG members. The presence of IR excess in the 51 stars confirms the existence of disks indicating that they could be potential candidates for exoplanet detections.

astro-ph.SR↗

arXiv2026

Experimental evidence of an Apolar Biaxial Smectic-A Phase Comprised of Bent-Core Molecules

We report structural and physical investigations on the biaxial smectic-A phase exhibited by a compound consisting of asymmetric bent-core molecules. Upon cooling from the isotropic phase, the compound exhibits the following phase sequence: Isotropic (403.9 K) $\rightarrow$ biaxial Smectic-A (359.8 K) $\rightarrow$ Crystal. The polarized optical microscopy, X-ray diffraction, and polarization reversal current measurements clearly establish the biaxial nature of the smectic-A phase without any layer polarization. The measured layer spacing in the entire temperature range of the smectic-A phase is close to the molecular length. The schlieren texture in a homeotropically aligned sample shows both $\pm \frac{1}{2}$ and $\pm 1$ defects which indicate the biaxial nature of this smectic-A phase. The dielectric spectroscopy studies on the samples revealed Debye-type relaxation processes with the relaxation time following the Arrhenius equation with temperature. Interestingly, the observed biaxial smectic-A phase exhibits a remarkable electro-optic response for a planar-aligned sample without any reorganization of the smectic layer structure.

cond-mat.soft↗

arXiv2024

Experimental studies on collision between self-propelling liquid crystal droplets in quasi-two-dimensional geometry

Active droplets undergoing micellar solubilization can self-propel themselves by emitting filled micelles from their surface and are by nature anti-chemotactic. These droplets get repelled by their own trail of filled micelles or by other neighbouring droplets. We experimentally study the various types of scattering events between two such active droplets. We define different collision parameters to characterize the scattering events and monitor the time evolution of these parameters about the closest approach between the droplets. For generic oblique collisions of the droplets, we find four major types of scattering events in our experimental studies depending on the initial angle of approach, the initial Peclet numbers, the delay time, and the initial distance between the droplets. For higher Peclet number droplets, the existence of a bound state was also found.

cond-mat.soft↗

arXiv2023

A simple model for self-propulsion of microdroplets in surfactant solution

We propose a simple active hydrodynamic model for the self-propulsion of a liquid droplet suspended in micellar solutions. The self-propulsion of the droplet occurs by spontaneous breaking of isotropic symmetry and is studied using both analytical and numerical methods. The emergence of self-propulsion arises from the slow dissolution of the inner fluid into the outer micellar solution as filled micelles. We propose that the surface generation of filled micelles is the dominant reason for the self-propulsion of the droplet. The flow instability is due to the Marangoni stress generated by the non-uniform distribution of the surfactant molecules on the droplet interface. In our model, the driving parameter of the instability is the excess surfactant concentration above the critical micellar concentration which directly correlates with the experimental observations. We consider various low-order modes of flow instability and show that the first mode becomes unstable through a supercritical bifurcation and is the only mode contributing to the swimming of the droplet. The flow fields around the droplet for these modes and their combined effects are also discussed.

physics.flu-dyn↗

arXiv2023

Glassy relaxation in de Vries smectic liquid crystal consisting of bent-core molecules

We report the experimental investigations on a liquid crystal comprised of thiophene-based achiral bent-core banana shaped molecules. The results reveal the presence of a short range nematic phase at high temperatures and a long-range SmA phase at lower temperatures, which transits to a SmC phase on further cooling the sample. Practically no layer contraction was observed across the SmA to SmC transition, indicating the de Vries nature of the SmA phase. Interestingly, the crystallization does not occur on cooling the sample till 223 K; instead, a glass transition at 271 K was observed. The dielectric spectroscopy studies carried out on the sample reveal the presence of a dielectric mode whose relaxation process is of the Cole-Cole type. The relaxation frequency of the mode was found to drop rapidly with decreasing temperature, further confirming the glassy behavior. The variation of relaxation frequency with temperature follows the Vogel-Fulcher-Tammann equation indicating the fragile glassy nature of the sample.

cond-mat.soft↗

arXiv2022

Optical and structural properties of starch films formed on drying droplets of gelatinized starch solutions

We report the optical and structural properties of starch films formed on drying droplets of starch solutions on a flat substrate. The starch films formed after drying are circular in shape and have an azimuthally symmetric "M-shaped" height profile along their diameter. These films are found to be semi-crystalline in nature and transparent in visible light. Our experimental results show that these films are optically uniaxial at their center but biaxial away from the center. The variation of birefringence along the radial direction is studied for the films formed after drying droplets with different volumes and starch concentrations. The cryo-SEM studies on the drying droplets and the films reveal the micro-structures and the possible origin of birefringence of these films.

cond-mat.soft↗

arXiv2022

Observation of banded spherulite in a pure compound by rhythmic growth

Banded spherulitic growth of crystal is observed in some materials with spherically symmetric growth front and periodic radial variation of birefringence. This variation of birefringence in quasi two dimensional geometry produces concentric interference colour bands when viewed through crossed polarisers. In most materials, the banded spherulites are found to be formed by radially oriented periodically twisted fibrillar crystallites. Here, we report the formation of banded spherulites due to the rhythmic growth of concentric crystallite-rich and crystallite-poor bands for a pure compound consisting of strongly polar rod like molecules. The compound exhibits coexistence of untwisted fibrillar crystallites and an amorphous phase in its most stable solid state. On sufficient supercooling of the sample from its melting point, the banded spherulites are formed with a periodic variation of composition of untwisted radially aligned fibrillar crystallites and an amorphous solid phase. We have developed a time dependent Ginzburg-Landau model to account for the observed banded spherulitic growth.

cond-mat.mtrl-sci↗

arXiv2020

Stability of topological wall defects on spheres with n-atic order

Topological point defects on orientationally ordered spheres, and on deformable fluid vesicles have been partly motivated by their potential applications in creating super-atoms with directional bonds through functionalization of the "bald-spots" created by topological point defects, thus paving the way for atomic chemistry at micron scales. We show that singular wall defects, topologically unstable "bald lines" in two dimensions, are stabilized near the order-disorder transition on a sphere. We attribute their stability to free-energetic considerations, which override those of topological stability.

cond-mat.soft↗

arXiv2008

Experimental studies of the transient fluctuation theorem using liquid crystals

In a thermodynamical process, the dissipation or production of entropy can only be positive or zero according to the second law of thermodynamics. However the laws of thermodynamics are applicable to large systems in the thermodynamic limit. Recently a fluctuation theorem known as the Transient Fluctuation Theorem (TFT) which generalizes the second law of thermodynamics even for small systems has been proposed. This theorem has been tested in small systems such as a colloidal particle in an optical trap. We report for the first time an analogous experimental study of TFT in a spatially extended system using liquid crystals.

cond-mat.stat-mech↗