152 research outputs found
Isotropic-nematic transition in hard-rod fluids: relation between continuous and restricted-orientation models
We explore models of hard-rod fluids with a finite number of allowed
orientations, and construct their bulk phase diagrams within Onsager's second
virial theory. For a one-component fluid, we show that the discretization of
the orientations leads to the existence of an artificial (almost) perfectly
aligned nematic phase, which coexists with the (physical) nematic phase if the
number of orientations is sufficiently large, or with the isotropic phase if
the number of orientations is small. Its appearance correlates with the
accuracy of sampling the nematic orientation distribution within its typical
opening angle. For a binary mixture this artificial phase also exists, and a
much larger number of orientations is required to shift it to such high
densities that it does not interfere with the physical part of the phase
diagram.Comment: 4 pages, 2 figures, submitted to PR
Dynamic Stern layers in charge-regulating electrokinetic systems: three regimes from an analytical approach
We present analytical solutions for the electrokinetics at a charged surface
with both non-zero Stern-layer conductance and finite chemical reaction rates.
We have recently studied the same system numerically [Werkhoven {\em et al.},
Phys. Rev. Lett. {\bf 120}, 264502 (2018)], and have shown that an applied
pressure drop across the surface leads to a non-trivial, laterally
heterogeneous surface charge distribution at steady state. In this work, we
linearise the governing electrokinetic equations to find closed expressions for
the surface charge profile and the generated streaming electric field. The main
results of our calculations are the identification of three important length
and time scales that govern the charge distribution, and consequently the
classification of electrokinetic systems into three distinct regimes. The three
governing time scales can be associated to (i) the chemical reaction, (ii)
diffusion in the Stern layer, and (iii) conduction in the Stern layer, where
the dominating (smallest) time scale characterises the regime. In the
reaction-dominated regime we find a constant surface charge with an edge
effect, and recover the Helmholtz-Smoluchowski equation. In the other two
regimes, we find that the surface charge heterogeneity extends over the entire
surface, either linearly (diffusion-dominated regime) or nonlinearly
(conduction-dominated regime).Comment: Accepted for publication in European Physical Journal: Special Topic
Coupled water, charge and salt transport in heterogeneous nano-fluidic systems
We theoretically study the electrokinetic transport properties of
nano-fluidic devices under the influence of a pressure, voltage or salinity
gradient. On a microscopic level the behaviour of the device is quantified by
the Onsager matrix , a generalised conductivity matrix relating the
local driving forces and the induced volume, charge and salt flux. Extending
from a local to a global linear-response relation is trivial for
homogeneous electrokinetic systems, but in this manuscript we derive a
generalised conductivity matrix from that applies also to
heterogeneous electrokinetic systems. This extension is especially important in
the case of an imposed salinity gradient, which gives necessarily rise to
heterogeneous devices. Within this formalism we can also incorporate a
heterogeneous surface charge due to, for instance, a charge regulating boundary
condition, which we show to have a significant impact on the resulting fluxes.
The predictions of the Poisson-Nernst-Planck-Stokes theory show good agreement
with exact solutions of the governing equations determined using the Finite
Element Method under a wide variety of parameters. Having established the
validity of the theory, it provides an accessible method to analyse
electrokinetic systems in general without the need of extensive numerical
methods. As an example, we analyse a Reverse Electrodialysis "blue energy"
system, and analyse how the many parameters that characterise such a system
affect the generated electrical power and efficiency
Fluids of platelike particles near a hard wall
Fluids consisting of hard platelike particles near a hard wall are
investigated using density functional theory. The density and orientational
profiles as well as the surface tension and the excess coverage are determined
and compared with those of a fluid of rodlike particles. Even for low densities
slight orientational packing effects are found for the platelet fluid due to
larger intermolecular interactions between platelets as compared with those
between rods. A net depletion of platelets near the wall is exhibited by the
excess coverage, whereas a change of sign of the excess coverage of hard-rod
fluids is found upon increasing the bulk density.Comment: 6 pages, 9 figure
Nonequilibrium steady states in fluids of platelike colloidal particles
Nonequilibrium steady states in an open system connecting two reservoirs of
platelike colloidal particles are investigated by means of a recently proposed
phenomenological dynamic density functional theory [M. Bier and R. van Roij,
Phys. Rev. E 76, 021405 (2007)]. The platelike colloidal particles are
approximated within the Zwanzig model of restricted orientations, which
exhibits an isotropic-nematic bulk phase transition. Inhomogeneities of the
local chemical potential generate a diffusion current which relaxes to a
nonvanishing value if the two reservoirs coupled to the system sustain
different chemical potentials. The relaxation process of initial states towards
the steady state turns out to comprise two regimes: a smoothening of initial
steplike structures followed by an ultimate relaxation of the slowest diffusive
mode. The position of a nonequilibrium interface and the particle current of
steady states depend nontrivially on the structure of the reservoirs due to the
coupling between translational and orientational degrees of freedom of the
fluid
Hard colloidal rods near a soft wall: wetting, drying, and symmetry breaking
Within an Onsager-like density functional theory we explore the thermodynamic
and structural properties of an isotropic and nematic fluid of hard needle-like
colloids in contact with a hard substrate coated with a soft short-ranged
attractive or repulsive layer. As a function of the range and the strength of
the soft interactions we find wetting and drying transitions, a pre-drying
line, and a symmetry-breaking transition from uniaxial to biaxial in the
wetting and drying film.Comment: 7 pages, 2 figure
Enhancement by polydispersity of the biaxial nematic phase in a mixture of hard rods and plates
The phase diagram of a polydisperse mixture of uniaxial rod-like and
plate-like hard parallelepipeds is determined for aspect ratios and
15. All particles have equal volume and polydispersity is introduced in a
highly symmetric way. The corresponding binary mixture is known to have a
biaxial phase for , but to be unstable against demixing into two
uniaxial nematics for . We find that the phase diagram for
is qualitatively similar to that of the binary mixture, regardless
the amount of polydispersity, while for a sufficient amount of
polydispersity stabilizes the biaxial phase. This provides some clues for the
design of an experiment in which this long searched biaxial phase could be
observed.Comment: 4 pages, 5 eps figure files, uses RevTeX 4 styl
Relaxation dynamics in fluids of platelike colloidal particles
The relaxation dynamics of a model fluid of platelike colloidal particles is
investigated by means of a phenomenological dynamic density functional theory.
The model fluid approximates the particles within the Zwanzig model of
restricted orientations. The driving force for time-dependence is expressed
completely by gradients of the local chemical potential which in turn is
derived from a density functional -- hydrodynamic interactions are not taken
into account. These approximations are expected to lead to qualitatively
reliable results for low densities as those within the isotropic-nematic
two-phase region. The formalism is applied to model an initially spatially
homogeneous stable or metastable isotropic fluid which is perturbed by
switching a two-dimensional array of Gaussian laser beams. Switching on the
laser beams leads to an accumulation of colloidal particles in the beam
centers. If the initial chemical potential and the laser power are large enough
a preferred orientation of particles occurs breaking the symmetry of the laser
potential. After switching off the laser beams again the system can follow
different relaxation paths: It either relaxes back to the homogeneous isotropic
state or it forms an approximately elliptical high-density core which is
elongated perpendicular to the dominating orientation in order to minimize the
surface free energy. For large supersaturations of the initial isotropic fluid
the high-density cores of neighboring laser beams of the two-dimensional array
merge into complex superstructures.Comment: low-resolution figures due to file size restrictions, revised versio
Social consequences of advanced cancer in patients and their informal caregivers:A qualitative study
Purpose Cancer threatens the social well-being of patients and their informal caregivers. Social life is even more profoundly affected in advanced diseases, but research on social consequences of advanced cancer is scarce. This study aims to explore social consequences of advanced cancer as experienced by patients and their informal caregivers. Methods Seven focus groups and seven in-depth semi-structured interviews with patients (n = 18) suffering from advanced cancer and their informal caregivers (n = 15) were conducted. Audiotapes were transcribed verbatim and open coded using a thematic analysis approach. Results Social consequences were categorized in three themes: "social engagement," "social identity," and "social network." Regarding social engagement, patients and informal caregivers said that they strive for normality by continuing their life as prior to the diagnosis, but experienced barriers in doing so. Regarding social identity, patients and informal caregivers reported feelings of social isolation. The social network became more transparent, and the value of social relations had increased since the diagnosis. Many experienced positive and negative shifts in the quantity and quality of their social relations. Conclusion Social consequences of advanced cancer are substantial. There appears to be a great risk of social isolation in which responses from social relations play an important role. Empowering patients and informal caregivers to discuss their experienced social consequences is beneficial. Creating awareness among healthcare professionals is essential as they provide social support and anticipate on social problems. Finally, educating social relations regarding the impact of advanced cancer and effective support methods may empower social support systems and reduce feelings of isolation
Many-body interactions and melting of colloidal crystals
We study the melting behavior of charged colloidal crystals, using a
simulation technique that combines a continuous mean-field Poisson-Boltzmann
description for the microscopic electrolyte ions with a Brownian-dynamics
simulation for the mesoscopic colloids. This technique ensures that many-body
interactions between the colloids are fully taken into account, and thus allows
us to investigate how many-body interactions affect the solid-liquid phase
behavior of charged colloids. Using the Lindemann criterion, we determine the
melting line in a phase-diagram spanned by the colloidal charge and the salt
concentration. We compare our results to predictions based on the established
description of colloidal suspensions in terms of pairwise additive Yukawa
potentials, and find good agreement at high-salt, but not at low-salt
concentration. Analyzing the effective pair-interaction between two colloids in
a crystalline environment, we demonstrate that the difference in the melting
behavior observed at low salt is due to many-body interactions
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