Papers by Debdip Bhandary
Soft matter, Jan 16, 2016
In this work, the effect of temperature on the contact angle of a water droplet on grafted thermo... more In this work, the effect of temperature on the contact angle of a water droplet on grafted thermo-responsive poly-(N-isopropylacrylamide) (PNIPAAm) polymer brushes is studied using all-atom molecular dynamics simulations in the temperature range of 270-330 K. A shift from 55° to 65° in contact angle values is observed as the temperature increases from 300 K to 310 K, which is in line with the experimental reports. The behavior of a water droplet on PNIPAAm brushes is analyzed using hydrogen bond analysis, water diffusion, radial distribution functions, the potential of mean force, excess entropy and the second virial coefficient (B2). The thermo-responsive behavior of PNIPAAm brushes, quantified using the excess entropy and B2 of PNIPAAm-water and water-water interactions, is mainly governed by polymer-water interactions. In particular, the excess entropy and B2 of PNIPAAm resulting from the PNIPAAm-water interactions are found to increase with increasing temperature. The dehydratio...
The Journal of Physical Chemistry C, 2015
In this work, we performed atomistic simulations to study the structural properties of mixed self... more In this work, we performed atomistic simulations to study the structural properties of mixed self-assembled monolayers (SAM) of hydrophilic and hydrophobic alkylthiols, with two different chain lengths (C5 and C11), on gold nanoparticles (NPs) at three different arrangements, namely: random, patchy, and Janus domains. In particular, we report the effect of mixing of thiols with unequal carbon chain lengths (C5 and C11) at three different arrangements on the structural properties and hydration of SAMs. Our simulation study reveals that the arrangement of thiols having unequal carbon chains in mixed SAMs is a key parameter in deciding the hydrophilicity of the coated gold NPs. Thus, our findings suggest that the hydration of the SAMs-protected gold NPs is not only dependent on the molecular composition of the thiols, but also on the organization of their mixing. In addition, our results show that the bending of longer thiols, when these are mixed with shorter thiols, depends on the arrangement of thiols as well as the chemical nature of their terminal groups.
Journal of Chemical & Engineering Data, 2014
ABSTRACT The effects of the electric field on the vapor–liquid equilibria of methanol and ethanol... more ABSTRACT The effects of the electric field on the vapor–liquid equilibria of methanol and ethanol confined in a graphitic slit pore of width 4 nm using molecular dynamics simulations are reported. The vapor–liquid critical temperature of methanol gets suppressed under confinement. The external electrical field further decreases the critical temperature with increasing electric field strength up to E = 1.5 V·nm–1. Surprisingly, a further increase in the electric field strength reverses the critical temperature behavior and is seen to increase with increasing electric field. The reversible behavior of the critical temperature with the electric field is also seen for nanoconfined ethanol at approximately 1.5 V·nm–1. The critical density, on the other hand, is found to continuously decrease with increasing electric field strength. Application of an external electric field results in the decrease in vapor and liquid densities. The coordination number in the liquid phase is found to decrease first with increasing electric field until E = 1.5 V·nm–1 and then increases with a further increase in the electric field, confirming the observed trend in the critical temperature according to the mean field theory. Orientational order of nanoconfined methanol and ethanol, on the other hand, is found to increase with increasing electric field.
The Journal of Physical Chemistry C, 2014
ABSTRACT All-atom molecular dynamics simulations are conducted to understand the structural and d... more ABSTRACT All-atom molecular dynamics simulations are conducted to understand the structural and dynamical behavior of self-assembled monolayer of n-alkanols on a mica surface. In particular, we report the effect of increasing carbon chain length (C1–C10) on the self-assembly, surface diffusion, and preferential tilting of n-alkanol monolayer, for monolayer surface coverage ranging from 6 × 10–5 to 3.54 × 10–3 mol/m2. The adsorption phenomena typically follow the Langmuir adsorption isotherm. However, the maximum adsorption is observed for n-hexanol, and it drops with further increase in the chain length. The surface diffusion coefficient, Ds, within monolayer, is nonmonotonic in nature. The maximum value of Ds decreases with increasing carbon chain length, with an exception of methanol owing to its preferential attachment with the cage of mica due to the presence of K+. The behavior of Ds is clearly explained using instantaneous autocorrelation function of hydrogen bonds with the surface. Further, Ds, is found to vary inversely proportional to the lifetime of hydrogen bond of alkanols with the surface. Most probable tilt angle of molecules with increasing alkyl group (C1, C2, C4, and C6) is in the order 71° > 38° > 29° > 19°. However, for octanol we observed molecules to attain a preferential tilt angle of 80°. The self-assembly behavior of lower alkanols, i.e., C1–C6 is contrary to that seen for higher alkanols.
Molecular Physics, 2012
Surface phase transitions of Lennard–Jones (LJ) based two-and four-site associating fluids have b... more Surface phase transitions of Lennard–Jones (LJ) based two-and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid ...
Molecular Physics, 2012
Surface phase transitions of Lennard-Jones (LJ) based two-and four-site associating fluids have b... more Surface phase transitions of Lennard-Jones (LJ) based two-and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor-liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor-liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.
The Journal of Physical Chemistry C, Jan 2014
All-atom molecular dynamics simulations are conducted to understand the structural and dynamical ... more All-atom molecular dynamics simulations are conducted to understand the structural and dynamical behavior of self-assembled monolayer of n-alkanols on a mica surface. In particular, we report the effect of increasing carbon chain length (C1–C10) on the self-assembly, surface diffusion, and preferential tilting of n-alkanol monolayer, for monolayer surface coverage ranging from 6 × 10–5 to 3.54 × 10–3 mol/m2. The adsorption phenomena typically follow the Langmuir adsorption isotherm. However, the maximum adsorption is observed for n-hexanol, and it drops with further increase in the chain length. The surface diffusion coefficient, Ds, within monolayer, is nonmonotonic in nature. The maximum value of Ds decreases with increasing carbon chain length, with an exception of methanol owing to its preferential attachment with the cage of mica due to the presence of K+. The behavior of Ds is clearly explained using instantaneous autocorrelation function of hydrogen bonds with the surface. Further, Ds, is found to vary inversely proportional to the lifetime of hydrogen bond of alkanols with the surface. Most probable tilt angle of molecules with increasing alkyl group (C1, C2, C4, and C6) is in the order 71° > 38° > 29° > 19°. However, for octanol we observed molecules to attain a preferential tilt angle of 80°. The self-assembly behavior of lower alkanols, i.e., C1–C6 is contrary to that seen for higher alkanols.
Molecular Physics, Jan 1, 2012
Surface phase transitions of Lennard-Jones (LJ) based two-and four-site associating fluids have b... more Surface phase transitions of Lennard-Jones (LJ) based two-and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor-liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor-liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.
We examine here sub-surface fracture of a thin metallic foil sandwiched between two elastomeric l... more We examine here sub-surface fracture of a thin metallic foil sandwiched between two elastomeric layers under impact. In particular we generate a vertical stack consisting of alternate layers of soft elastomers and thin aluminum foils and place it on a rigid substrate; we then allow a rigid sphere to impact the stack from a small vertical height. We show that under impact the foil at the top of the stack undergoes buckling deformation; however the foil sandwiched between the two elastic layers undergoes both deformation and fracture. We show that because of friction at the contacting interfaces with the elastomer, the sandwiched foil is subjected to in-plane stretching which when exceeds a threshold limit causes fracture. Experiments show that this threshold condition is reached within a range of critical thicknesses of the top and bottom elastomeric layers, for a given height of impact of the rigid spherical indenter. We present a theoretical analysis to predict the critical thickness of the stack below which the foil is expected to undergo fracture and also the critical heights within this stack at which the foil would fracture.
Journal of Adhesion, 2011
Damping against moderate to high impact is important for variety of engineering and technological... more Damping against moderate to high impact is important for variety of engineering and technological applications. While, conventionally, damping has been achieved by using soft, elastic, and viscoelastic composite materials of different kinds, here we show that thin elastic layers embedded with fluid-filled micro-channels can have several advantages not found in the conventional ones. These layers are either smooth, homogeneous, or are embedded with micro-channels of different diameters, either open to atmosphere or are filled with oil of desired viscosity. We sandwich these layers between a rigid substrate and a flexible aluminum foil and subject them to moderate impacting load by colliding with rigid spheres of steel of different diameter. These experiments show that the kinetic energy dissipates more for layers of smaller thickness and for the ones embedded with the fluid filled micro-channels. Furthermore, the dissipation increases with viscosity of the oil inside the channels until a critical viscosity is reached beyond which it decreases. These results suggest that the damping occurs via frictional dissipation at interfaces and the viscous dissipation of liquid inside the channels.
International Journal of Solids and Structures
We examine here sub-surface fracture of a thin metallic foil sandwiched between two elastomeric l... more We examine here sub-surface fracture of a thin metallic foil sandwiched between two elastomeric layers under impact. In particular we generate a vertical stack consisting of alternate layers of soft elastomers and thin aluminum foils and place it on a rigid substrate; we then allow a rigid sphere to impact the stack from a small vertical height. We show that under impact the foil at the top of the stack undergoes buckling deformation; however the foil sandwiched between the two elastic layers undergoes both deformation and fracture. We show that because of friction at the contacting interfaces with the elastomer, the sandwiched foil is subjected to in-plane stretching which when exceeds a threshold limit causes fracture. Experiments show that this threshold condition is reached within a range of critical thicknesses of the top and bottom elastomeric layers, for a given height of impact of the rigid spherical indenter. We present a theoretical analysis to predict the critical thickness of the stack below which the foil is expected to undergo fracture and also the critical heights within this stack at which the foil would fracture.
Damping against moderate to high impact is important for variety of engineering and technological... more Damping against moderate to high impact is important for variety of engineering and technological applications. While, conventionally, damping has been achieved by using soft, elastic, and viscoelastic composite materials of different kinds, here we show that thin elastic layers embedded with fluid-filled micro-channels can have several advantages not found in the conventional ones. These layers are either smooth, homogeneous, or are embedded with micro-channels of different diameters, either open to atmosphere or are filled with oil of desired viscosity. We sandwich these layers between a rigid substrate and a flexible aluminum foil and subject them to moderate impacting load by colliding with rigid spheres of steel of different diameter. These experiments show that the kinetic energy dissipates more for layers of smaller thickness and for the ones embedded with the fluid filled micro-channels. Furthermore, the dissipation increases with viscosity of the oil inside the channels until a critical viscosity is reached beyond which it decreases. These results suggest that the damping occurs via frictional dissipation at interfaces and the viscous dissipation of liquid inside the channels.
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Papers by Debdip Bhandary