arXiv subjects

Apratim Chatterji

Publications and source records attributed to Apratim Chatterji.

21 records · Page 2Linked to original sources

arXiv2013

Self assembly of monodisperse CdS nano-cylinders with a pore

We present investigate the self assembly and growth of an array of $CdS$ nanotubes: a consequence of a fine balance of directed motion, diffusion and aggregation of reacting ${\rm Cd^{+2}}$ and ${\rm S^{-2}}$ ions. In a previous communication [J. Kiruthiga, A. Chatterji, J. Chem. Phys., {\bf 138} 024905 (2013)], we identified the mechanism of a unexpected growth of a very uniform $CdS$ nano-cylinder from the end of a nano-channel. Furthermore, the cylinder had a pore along the axis but were closed at one end. This unique phenomenon of self assembly of {\em monodisperse} CdS nano-cylinders had been observed in a rather simple experiment where two chambers containing 0.1 M ${\rm Cd Cl_2}$ and 0.1 M ${\rm Na_2 S}$ solutions were joined by an array of anodized aluminium oxide (AAO) nano-channels [A. Varghese, S. Datta, Phys. Rev. E., {\bf 85}, 056104 (2012)]. Our previous study identified the principles governing the growth of a single nano-tube at the exit point of a single AAO-nano-channel. In this communication, we identify factors affecting the self-assembly process for nano-tubes growing out an array of closely spaced AAO nano-channel exits. Our model is not $Cd^{+2}$ or $S^{2-}$ specific, the experimental scheme can be extended to self assemble a general class of reacting-diffusing A and B ions with A (${\rm Cd^{+2}}$) selectively migrating out from a nano-channel. In particular, we note that after the initial prolonged growth of nanotubes, there can arise a severe deficiency of B-ions (${\rm S^{-2}}$) ions near the AAO-nano-channel exits, the points where the reaction and aggregation occurs to form the $CdS$ nanotube, thus impeding further growth of uniform CdS nano-tubes. Thereby we predict the necessary characteristics of reacting systems which can be self assembled using suitable adaptations of previous experiments.

cond-mat.soft↗

arXiv2006

Electrophoretic Properties of Highly Charged Colloids: A Hybrid MD/LB Simulation Study

Using computer simulations, the electrophoretic motion of a positively charged colloid (macroion) in an electrolyte solution is studied in the framework of the primitive model. Hydrodynamic interactions are fully taken into account by applying a hybrid simulation scheme, where the charged ions (i.e. macroion and electrolyte), propagated via molecular dynamics (MD), are coupled to a Lattice Boltzmann (LB) fluid. In a recent experiment it was shown that, for multivalent salt ions, the mobility $μ$ initially increases with charge density $σ$, reaches a maximum and then decreases with further increase of $σ$. The aim of the present work is to elucidate the behaviour of $μ$ at high values of $σ$. Even for the case of monovalent microions, we find a decrease of $μ$ with $σ$. A dynamic Stern layer is defined that includes all the counterions that move with the macroion while subject to an external electrical field. The number of counterions in the Stern layer, $q_0$, is a crucial parameter for the behavior of $μ$ at high values of $σ$. In this case, the mobility $μ$ depends primarily on the ratio $q_0/Q$ (with $Q$ the valency of the macroion). The previous contention that the increase in the distortion of the electric double layer (EDL) with increasing $σ$ leads to the lowering of $μ$ does not hold for high $σ$. In fact, we show that the deformation of the EDL decreases with increase of $σ$. The role of hydrodynamic interactions is inferred from direct comparisons to Langevin simulations where the coupling to the LB fluid is switched off. Moreover, systems with divalent counterions are considered. In this case, at high values of $σ$ the phenomenon of charge inversion is found.

cond-mat.soft↗

arXiv2005

Combining Molecular Dynamics with Lattice-Boltzmann: A Hybrid Method for the Simulation of (Charged) Colloidal Systems

We present a hybrid method for the simulation of colloidal systems, that combines molecular dynamics (MD) with the Lattice-Boltzmann (LB) scheme. The LB method is used as a model for the solvent in order to take into account the hydrodynamic mass and momentum transport through the solvent. The colloidal particles are propagated via MD and they are coupled to the LB fluid by viscous forces. With respect to the LB fluid, the colloids are represented by uniformly distributed points on a sphere. Each such point (with a velocity V(r) at any off-lattice position r is interacting with the neighboring eight LB nodes by a frictional force F=ξ_0(V(r)-u(r)) with ξ_0 being a friction force and u(r) being the velocity of the fluid at the position r. Thermal fluctuations are introduced in the framework of fluctuating hydrodynamics. This coupling scheme has been proposed recently for polymer systems by Ahlrichs and D"unweg [J. Chem. Phys. 111, 8225 (1999)]. We investigate several properties of a single colloidal particle in a LB fluid, namely the effective Stokes friction and long time tails in the autocorrelation functions for the translational and rotational velocity. Moreover, a charged colloidal system is considered consisting of a macroion, counterions and coions that are coupled to a LB fluid. We study the behavior of the ions in a constant electric field. In particular, an estimate of the effective charge of the macroion is yielded from the number of counterions that move with the macroion in the direction of the electric field.

cond-mat.soft↗