44,904 research outputs found
Symmetric Diblock Copolymers in Thin Films (I): Phase stability in Self-Consistent Field Calculations and Monte Carlo Simulations
We investigate the phase behavior of symmetric AB diblock copolymers confined
into a thin film. The film boundaries are parallel, impenetrable and attract
the A component of the diblock copolymer. Using a self-consistent field
technique [M.W. Matsen, J.Chem.Phys. {\bf 106}, 7781 (1997)], we study the
ordered phases as a function of incompatibility and film thickness in
the framework of the Gaussian chain model. For large film thickness and small
incompatibility, we find first order transitions between phases with different
number of lamellae which are parallel oriented to the film boundaries. At high
incompatibility or small film thickness, transitions between parallel oriented
and perpendicular oriented lamellae occur. We compare the self-consistent field
calculations to Monte Carlo simulations of the bond fluctuation model for chain
length N=32. In the simulations we quench several systems from to
and monitor the morphology into which the diblock copolymers
assemble. Three film thicknesses are investigated, corresponding to parallel
oriented lamellae with 2 and 4 interfaces and a perpendicular oriented
morphology. Good agreement between self-consistent field calculations and Monte
Carlo simulations is found.Comment: to appear in J.Chem.Phy
Equation of state and critical behavior of polymer models: A quantitative comparison between Wertheim's thermodynamic perturbation theory and computer simulations
We present an application of Wertheim's Thermodynamic Perturbation Theory
(TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones
monomers. We use both the Reference Hyper-Netted-Chain (RHNC) and Mean
Spherical approximation (MSA) integral equation theories to describe the
properties of the reference fluid. The equation of state, the density
dependence of the excess chemical potential, and the critical points of the
liquid--vapor transition are compared with simulation results and good
agreement is found. The RHNC version is somewhat more accurate, while the MSA
version has the advantage of being almost analytic. We analyze the scaling
behavior of the critical point of chain fluids according to TPT1 and find it to
reproduce the mean field exponents: The critical monomer density is predicted
to vanish as upon increasing the chain length while the critical
temperature is predicted to reach an asymptotic finite temperature that is
attained as . The predicted asymptotic finite critical temperature
obtained from the RHNC and MSA versions of TPT1 is found to be in good
agreement with the point of our polymer model as obtained from the
temperature dependence of the single chain conformations.Comment: to appear in J.Chem.Phy
Driving steady-state visual evoked potentials at arbitrary frequencies using temporal interpolation of stimulus presentation
Date of Acceptance: 29/10/2015 We thank Renate Zahn for help with data collection. This work was supported by Deutsche Forschungsgemeinschaft (AN 841/1-1, MU 972/20-1). We would like to thank A. Trujillo-Ortiz, R. Hernandez-Walls, A. Castro-Perez and K. BarbaRojo (Universidad Autonoma de Baja California) for making Matlab code for non-sphericity corrections freely available.Peer reviewedPublisher PD
Visual adaptation to convexity in macaque area V4
Aftereffects are perceptual illusions caused by visual adaptation to one or more stimulus attribute, such as orientation, motion, or shape. Neurophysiological studies seeking to understand the basis of visual adaptation have observed firing rate reduction and changes in tuning of stimulus-selective neurons following periods of prolonged visual stimulation. In the domain of shape, recent psychophysical work has shown that adaptation to a convex pattern induces a subsequently seen rectangle to appear slightly concave. In the present study, we investigate the possible contribution of V4 neurons of rhesus monkeys, which are thought to be involved in the coding of convexity, to shape-specific adaptation. Visually responsive neurons were monitored during the brief presentation of simple shapes varying in their convexity level. Each test presentation was preceded by either a blank period or several seconds of adaptation to a convex or concave stimulus, presented in two different sizes. Adaptation consistently shifted the tuning of neurons away from the convex or concave adapter, including shifting response to the neutral rectangle in the direction of the opposite convexity. This repulsive shift resembled the known perceptual distortion associated with adaptation to such stimuli. In addition, adaptation caused a nonspecific response decrease, as well as a specific decrease for repeated stimuli. The latter effects were observed whether or not the adapting and test stimuli matched closely in their size. Taken together, these results provide evidence for shape-specific adaptation of neurons in area V4, which may contribute to the perception of the convexity aftereffect
New mechanism of membrane fusion
We have carried out Monte Carlo simulation of the fusion of bilayers of
single chain amphiphiles which show phase behavior similar to that of
biological lipids. The fusion mechanism we observe is very different from the
``stalk'' hypothesis. Stalks do form on the first stage of fusion, but they do
not grow radially to form a hemifused state. Instead, stalk formation
destabilizes the membranes and results in hole formation in the vicinity of the
stalks. When holes in each bilayer nucleate spontaneously next to the same
stalk, an incomplete fusion pore is formed. The fusion process is completed by
propagation of the initial connection, the stalk, along the edges of the
aligned holes.Comment: 4 pages, 3 figure
Technical note: Absorption aerosol optical depth components from AERONET observations of mixed dust plumes
© Author(s) 2019.Absorption aerosol optical depth (AAOD) as obtained from sun–sky photometer measurements provides a measure of the light-absorbing properties of the columnar aerosol loading. However, it is not an unambiguous aerosol-type-specific parameter, particularly if several types of absorbing aerosols, for instance black carbon (BC) and mineral dust, are present in a mixed aerosol plume. The contribution of mineral dust to total aerosol light absorption is particularly important at UV wavelengths. In this study we refine a lidar-based technique applied to the separation of dust and non-dust aerosol types for the use with Aerosol Robotic Network (AERONET) direct sun and inversion products. We extend the methodology to retrieve AAOD related to non-dust aerosol (AAODnd) and BC (AAODBC). We test the method at selected AERONET sites that are frequently affected by aerosol plumes that contain a mixture of Saharan or Asian mineral dust and biomass-burning smoke or anthropogenic pollution, respectively. We find that aerosol optical depth (AOD) related to mineral dust as obtained with our methodology is frequently smaller than coarse-mode AOD. This suggests that the latter is not an ideal proxy for estimating the contribution of mineral dust to mixed dust plumes. We present the results of the AAODBC retrieval for the selected AERONET sites and compare them to coincident values provided in the Copernicus Atmosphere Monitoring System aerosol reanalysis.We find that modelled and AERONET AAODBC are most consistent for Asian sites or at Saharan sites with strong local anthropogenic sources.Peer reviewe
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