1,067 research outputs found
Effect of water-wall interaction potential on the properties of nanoconfined water
Much of the understanding of bulk liquids has progressed through study of the
limiting case in which molecules interact via purely repulsive forces, such as
a hard-core potential. In the same spirit, we report progress on the
understanding of confined water by examining the behavior of water-like
molecules interacting with planar walls via purely repulsive forces and compare
our results with those obtained for Lennard-Jones (LJ) interactions between the
molecules and the walls. Specifically, we perform molecular dynamics
simulations of 512 water-like molecules which are confined between two smooth
planar walls that are separated by 1.1 nm. At this separation, there are either
two or three molecular layers of water, depending on density. We study two
different forms of repulsive confinements, when the interaction potential
between water-wall is (i) and (ii) WCA-like repulsive potential. We
find that the thermodynamic, dynamic and structural properties of the liquid in
purely repulsive confinements qualitatively match those for a system with a
pure LJ attraction to the wall. In previous studies that include attractions,
freezing into monolayer or trilayer ice was seen for this wall separation.
Using the same separation as these previous studies, we find that the crystal
state is not stable with repulsive walls but is stable with WCA-like
repulsive confinement. However, by carefully adjusting the separation of the
plates with repulsive interactions so that the effective space
available to the molecules is the same as that for LJ confinement, we find that
the same crystal phases are stable. This result emphasizes the importance of
comparing systems only using the same effective confinement, which may differ
from the geometric separation of the confining surfaces.Comment: 20 pages, 10 figure
Real-time phase-shift detection of the surface plasmon resonance
We investigate a method to directly measure the phase of a laser beam
reflected from a metallic film after excitation of surface plasmon polaritons.
This method permits real time access to the phase information, it increases the
possible speed of data acquisition, and it may thus prove useful for increasing
the sensitivity of surface plasmon based sensors
Surface plasmon resonance under conditions of electromagnetically induced transparency
A scheme for a surface plasmon resonance system under conditions of
electromagnetically induced transparency (EIT) is proposed. The system is
composed of three layers: a prism, a thin metal film, and a hybrid dielectric
consisting of EIT atoms and a background substance. A probe and a coupling
laser beam are input. Corresponding analytical formulas are derived for the
cases when one or both of the laser beams excite surface plasmon polaritons at
the metal/dielectric interface. Under resonance conditions, an extremely sharp
dip appears in the reflectivity-frequency spectrum of the probe field,
revealing new properties of two-dimensional EIT. The reflectivity is extremely
sensitive to shifts in the laser frequencies and atomic levels, and to
variations of permittivity of the substrate. This EIT-SPR system may to be used
for novel magnetometers and biosensors
Influence of monomer architecture on the shear properties of molecularly thin polymer melts
The effective viscosity and shear behavior of ultrathin films of two structurally different perfluoropolyether fluid have been investigated. The materials used were Fomblin Z, a linear random copolymer of fluoroethylene oxide and fluoromethylene oxide, and Fomblin Y, a branched random copolymer with fluoropropylene oxide and fluoromethylene oxide monomer units. The shearing experiments were conducted with the fluids confined between molecularly smooth surfaces at shear rates ranging from 200 to 4 x 10(3) s-1. It was found that when the thickness of the fluid films decreases from 10 to 2 nm, both perfluoropolyethers exhibited a sharp increase in viscosity, from bulk values to "surface viscosity" values that are many orders of magnitude larger. With increasing shear rates, the Z-type fluid showed a gradual decrease in the shear stress indicating an apparent ordering of the molecules due to the applied shear forces. On the other hand, with the Y-type copolymer, the shear stresses were significantly lower and were proportional to the shear rate resembling Newtonian fluids. The results are explained in terms of the differences in the molecular architecture of the fluids and suggest a close relation between the molecular structure of the polymer melt near a solid wall and its frictional and lubricating properties
Boundary lubrication properties of materials with expansive freezing
We have performed molecular dynamics simulations of solid-solid contacts
lubricated by a model fluid displaying many of the properties of water,
particularly its expansive freezing. Near the region where expansive freezing
occurs, the lubricating film remains fluid, and the friction force decreases
linearly as the shear velocity is reduced. No sign of stick-slip motion is
observed even at the lowest velocities. We give a simple interpretation of
these results, and suggest that in general good boundary lubrication properties
will be found in the family of materials with expansive freezing.Comment: Version to appear in Phys. Rev. Let
Adsorption-desorption kinetics in nanoscopically confined oligomer films under shear
The method of molecular dynamics computer simulations is employed to study oligomer melts confined in ultra-thin films and subjected to shear. The focus is on the self-diffusion of oligomers near attractive surfaces and on their desorption, together with the effects of increasing energy of adsorption and shear. It is found that the mobility of the oligomers near an attractive surface is strongly decreased. Moreover, although shearing the system forces the chains to stretch parallel to the surfaces and thus increase the energy of adsorption per chain, flow also promotes desorption. The study of chain desorption kinetics reveals the molecular processes responsible for the enhancement of desorption under shear. They involve sequences of conformations starting with a desorbed tail and proceeding in a very fast, correlated, segment-by-segment manner to the desorption of the oligomers from the surfaces.
Increased Finger-Tapping Related Cerebellar Activation in Cervical Dystonia, Enhanced by Transcranial Stimulation: An Indicator of Compensation?
Background: Cervical dystonia is a movement disorder causing abnormal postures and movements of the head. While the exact pathophysiology of cervical dystonia has not yet been fully elucidated, a growing body of evidence points to the cerebellum as an important node.Methods: Here, we examined the impact of cerebellar interference by transcranial magnetic stimulation on finger-tapping related brain activation and neurophysiological measures of cortical excitability and inhibition in cervical dystonia and controls. Bilateral continuous theta-burst stimulation was used to modulate cerebellar cortical excitability in 16 patients and matched healthy controls. In a functional magnetic resonance imaging arm, data were acquired during simple finger tapping before and after cerebellar stimulation. In a neurophysiological arm, assessment comprised motor-evoked potentials amplitude and cortical silent period duration. Theta-burst stimulation over the dorsal premotor cortex and sham stimulation (neurophysiological arm only) served as control conditions.Results: At baseline, finger tapping was associated with increased activation in the ipsilateral cerebellum in patients compared to controls. Following cerebellar theta-burst stimulation, this pattern was even more pronounced, along with an additional movement-related activation in the contralateral somatosensory region and angular gyrus. Baseline motor-evoked potential amplitudes were higher and cortical silent period duration shorter in patients compared to controls. After cerebellar theta-burst stimulation, cortical silent period duration increased significantly in dystonia patients.Conclusion: We conclude that in cervical dystonia, finger movements—though clinically non-dystonic—are associated with increased activation of the lateral cerebellum, possibly pointing to general motor disorganization, which remains subclinical in most body regions. Enhancement of this activation together with an increase of silent period duration by cerebellar continuous theta-burst stimulation may indicate predominant disinhibitory effects on Purkinje cells, eventually resulting in an inhibition of cerebello-thalamocortical circuits
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