347 research outputs found
Dynamics of Bound and Free Water in an Aqueous Micellar Solution : Analysis of the Lifetime and Vibrational Frequencies of Hydrogen Bonds at a Complex Interface
In order to understand the nature and dynamics of interfacial water molecules
on the surface of complex systems, large scale molecular dynamics simulations
of an aqueous micelle of cesium perfluorooctanoate surfactant molecules have
been carried out. The lifetime and the intermolecular vibrational frequencies
of the hydrogen bonds that the water molecules form with the polar headgroups
of the surfactants, are calculated. Our earlier classification of the
interfacial water molecules, based on structural and energetic considerations,
into bound and free type is further validated by their dynamics. Lifetime
correlation functions of the water-surfactant hydrogen bonds show the long
lived nature of the bound water species. The water molecules that are singly
hydrogen bonded to the surfactants have longer lifetime than those that form
two such hydrogen bonds. The free water molecules that do not form any such
hydrogen bonds behave similar to bulk water in their reorientational dynamics.
A few water molecules that form two such hydrogen bonds are orientationally
locked in for durations of the order of few hundreds of picoseconds. The
intermolecular vibrational frequencies of these interfacial water molecules
shows a significant blue shift in the librational band apart from a similar
shift in the near neighbor bending modes, relative to water molecules in bulk.
These blue shifts suggest an increase in rigidity in the structure around
interfacial water molecules. This is in good agreement with recent incoherent,
inelastic neutron scattering data on macromolecular solutions. The results of
the present simulations should be relevant to the understanding of dynamics of
water near any hydrophilic surface.Comment: 36 Pages including 7 Figures; Submitted to Phys. Rev.
Constraining the relativistic mean-field model equations of state with gravitational wave observations
The first detection of gravitational waves from the binary neutron star
merger event GW170817 has started to provide important new constraints on the
nuclear equation of state at high density. The tidal deformability bound of
GW170817 combined with the observed two solar mass neutron star poses a serious
challenge to theoretical formulations of realistic equations of state. We
analyze a fully comprehensive set of relativistic nuclear mean-field theories
by confronting them with the observational bounds and the measured neutron-skin
thickness. We find that only a few models can withstand these bounds which
predict a stiff overall equation of state but with a soft neutron-proton
symmetry energy. Two possible indications are proposed: Circumstantial evidence
of hadron-quark phase transition inside the star and new parametrizations that
are consistent with ground state properties of finite nuclei and observational
bounds. Based on extensive analysis of these sets, an upper limit on the radius
of a neutron star of km is deduced.Comment: Matches with the published versio
Evidence for Bound and Free Water Species in the Hydration Shell of an Aqueous Micelle
Our atomistic molecular dynamics simulations reveal the existence of bound
and free water molecules in the hydration layer of an aqueous micelle. The
bound water molecules can be either singly or doubly hydrogen bonded to the
polar head group on the surface of the micelle. The ratio of bound to free
water is found to be approximately equal to 9:1 at 300 K.Comment: 7 pages and 3 figures. Submitted to Curr. Sci.(Ind. Acad. Sci.
Shear viscosity to entropy density ratio in nuclear multifragmentation
Nuclear multifragmentation in intermediate energy heavy ion collisions has
long been associated with liquid-gas phase transition. We calculate the shear
viscosity to entropy density ratio eta/s for an equilibrated system of nucleons
and fragments produced in multifragmentation within an extended statistical
multifragmentation model. The temperature dependence of eta/s exhibits
surprisingly similar behavior as that for water. In the coexistence phase of
fragments and light particles, the ratio eta/s reaches a minimum of comparable
depth as that for water in the vicinity of the critical temperature for
liquid-gas phase transition. The effects of freeze-out volume and surface
symmetry energy on eta/s in multifragmentation are studied.Comment: 5 pages, 5 figures, to appear in PR
Nonlinear hydrodynamic response confronts LHC data
Higher order harmonic flow (with ) in heavy-ion collisions can
be measured either with respect to their own plane, or with respect to a plane
constructed using lower-order harmonics. By assuming that higher flow harmonics
are the superposition of medium nonlinear and linear responses to initial
anisotropies, we propose a set of nonlinear response coefficients 's,
which are independent of initial state by construction. In experiments,
's can be extracted as the ratio between higher order harmonic flow
measured in the plane constructed by and , and moments of lower
order harmonic flow. Simulations with single-shot hydrodynamics and AMPT model
lead to results of these nonlinear response coefficients in good agreement with
the experimental data at the LHC energy. Predictions for and
measured with respect to plane of lower order harmonics are given accordingly.Comment: 4 pages and 2 figures. Proceedings of Quark Matter 2015, Kobe, Japa
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