1,024 research outputs found
Dimensionality effects in dipolar fluids
Using classical density functional theory (DFT) in a modified mean-field
approximation we investigate the fluid phase behavior of quasi-two dimensional
dipolar fluids confined to a plane. The particles carry three-dimensional
dipole moments and interact via a combination of hard-sphere, van-der-Waals,
and dipolar interactions. The DFT predicts complex phase behavior involving
first- and second-order isotropic-to-ferroelectric transitions, where the
ferroelectric ordering is characterized by global polarization within the
plane. We compare this phase behavior, particularly the onset of ferroelectric
ordering and the related tricritical points, with corresponding
three-dimensional systems, slab-like systems (with finite extension into the
third direction), and true two-dimensional systems with two-dimensional dipole
moments.Comment: 7 pages, 2 figure
Capillary Condensation, Freezing, and Melting in Silica Nanopores: A Sorption Isotherm and Scanning Calorimetry Study on Nitrogen in Mesoporous SBA-15
Condensation, melting and freezing of nitrogen in a powder of mesoporous
silica grains (SBA-15) has been studied by combined volumetric sorption
isotherm and scanning calorimetry measurements. Within the mean field model of
Saam and Cole for vapor condensation in cylindrical pores a liquid nitrogen
sorption isotherm is well described by a bimodal pore radius distribution. It
encompasses a narrow peak centered at 3.3 nm, typical of tubular mesopores, and
a significantly broader peak characteristic of micropores, located at 1 nm. The
material condensed in the micropores as well as the first two adsorbed
monolayers in the mesopores do not exhibit any caloric anomaly. The
solidification and melting transformation affects only the pore condensate
beyond approx. the second monolayer of the mesopores. Here, interfacial melting
leads to a single peak in the specific heat measurements. Homogeneous and
heterogeneous freezing along with a delayering transition for partial fillings
of the mesopores result in a caloric freezing anomaly similarly complex and
dependent on the thermal history as has been observed for argon in SBA-15. The
axial propagation of the crystallization in pore space is more effective in the
case of nitrogen than previously observed for argon, which we attribute to
differences in the crystalline textures of the pore solids.Comment: 10 pages, 7 figure
Crystal structures and freezing of dipolar fluids
We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential . A crossover between qualitatively
different sequences of phases occurs near the exponent . The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.Comment: submitted to Phys. Rev.
Surface-charge-induced freezing of colloidal suspensions
Using grand-canonical Monte Carlo simulations we investigate the impact of
charged walls on the crystallization properties of charged colloidal
suspensions confined between these walls. The investigations are based on an
effective model focussing on the colloids alone. Our results demonstrate that
the fluid-wall interaction stemming from charged walls has a crucial impact on
the fluid's high-density behavior as compared to the case of uncharged walls.
In particular, based on an analysis of in-plane bond order parameters we find
surface-charge-induced freezing and melting transitions
Non-equilibrium condensation and coarsening of field-driven dipolar colloids
In colloidal suspensions, self-organization processes can be easily fueled by
external fields. One particularly interesting class of phenomena occurs in
monolayers of dipolar particles that are driven by rotating external fields.
Here we report results from a computer simulation study of such systems
focusing on the clustering behavior also observed in recent experiments. The
key result of this paper is a novel interpretation of this pattern formation
phenomenon: We show the clustering to be a by-product of a vapor-liquid first
order phase transition. In fact, the observed dynamic coarsening process
corresponds to the spindodal demixing that occurs during such a transitionComment: 6 pages, 5 figure
The effect of distance on reaction time in aiming movements
Target distance affects movement duration in aiming tasks but its effect on reaction time (RT) is poorly documented. RT is a function of both preparation and initiation. Experiment 1 pre-cued movement (allowing advanced preparation) and found no influence of distance on RT. Thus, target distance does not affect initiation time. Experiment 2 removed pre-cue information and found that preparing a movement of increased distance lengthens RT. Experiment 3 explored movements to targets of cued size at non-cued distances and found size altered peak speed and movement duration but RT was influenced by distance alone. Thus, amplitude influences preparation time (for reasons other than altered duration) but not initiation time. We hypothesise that the RT distance effect might be due to the increased number of possible trajectories associated with further targets: a hypothesis that can be tested in future experiments
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