807 research outputs found
Statistical thermodynamics of supercapacitors and blue engines
We study the thermodynamics of electrode-electrolyte systems, for instance
supercapacitors filled with an ionic liquid or blue-energy devices filled with
river- or sea water. By a suitable mapping of thermodynamic variables, we
identify a strong analogy with classical heat engines. We introduce several
Legendre transformations and Maxwell relations. We argue that one should
distinguish between the differential capacity at constant ion number and at
constant ion chemical potential, and derive a relation between them that
resembles the standard relation between heat capacity at constant volume and
constant pressure. Finally, we consider the probability distribution of the
electrode charge at a given electrode potential, the standard deviation of
which is given by the differential capacity.Comment: To be published in "New challenges in Electrostatics of Soft and
Disordered Matter", Eds. J. Dobnikar, A. Naji, D.Dean and R. Podgornik,
PanStanford Pub. Singapore (2012
Solvo-osmotic flow in electrolytic mixtures
We show that an electric field parallel to an electrically neutral surface
can generate flow of electrolytic mixtures in small channels. We term this
solvo-osmotic flow, since the flow is induced by the asymmetric preferential
solvation of ions at the liquid-solid interface. The generated flow is
comparable in magnitude to the ubiquitous electro-osmotic flow at charged
surfaces, but for a fixed surface charge density, it differs qualitatively in
its dependence on ionic strength. Solvo-osmotic flow can also be sensitively
controlled with temperature. We derive a modified Helmholtz-Smoluchowski
equation that accounts for these effects.Comment: 11 pages, 4 figure
Self-propulsion mechanism of active Janus particles in near-critical binary mixtures
Gold-capped Janus particles immersed in a near-critical binary mixture can be
propelled using illumination. We employ a non-isothermal diffuse interface
approach to investigate the self-propulsion mechanism of a single colloid. We
attribute the motion to body forces at the edges of a micronsized droplet that
nucleates around the particle. Thus, the often-used concept of a surface
velocity cannot account for the self-propulsion. The particle's swimming
velocity is related to the droplet shape and size, which is determined by a
so-called critical isotherm. Two distinct swimming regimes exist, depending on
whether the droplet partially or completely covers the particle. Interestingly,
the dependence of the swimming velocity on temperature is non-monotonic in both
regimes.Comment: 5 pages, 3 figure
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
The effect of flexibility and bend angle on the phase diagram of hard colloidal boomerangs
We study the effect of flexibility and bend angle on systems of hard
semiflexible boomerangs. These are modelled as two rodlike segments joined at
one end with an angle that can fluctuate about a preferred angle. We use a
second-virial theory for semiflexible chains with two segments, and numerically
solve for the full orientation distribution function as a function of the four
angles that determine the boomerang's orientation. We plot the single segment
distributions as a function of two angles as well as the interarm angle
distribution. For stiff boomerangs, we find prolate, oblate, and biaxial
nematic phases depending on the bend angle and density, in partial agreement
with previous results on rigid boomerangs. For the case that the preferred
interarm angle is , however, we find that the biaxial nematic phase
has four-fold rather than two-fold rotational symmetry, and thus requires
fourth-rank order parameters to describe it. In addition, we find that
flexibility drastically reduces the region of stability for the biaxial nematic
phase, with the prolate nematic becoming more favourable.Comment: 14 pages, 7 figure
Poisson-Boltzmann cell model for heterogeneously charged colloids
We introduce the Poisson-Boltzmann cell model for spherical colloidal
particles with a heterogeneous surface charge distribution. This model is
obtained by generalizing existing cell models for mixtures of homogeneously
charged colloidal spheres. Our new model has similar features as Onsager's
second-virial theory for liquid crystals, but it predicts no orientational
ordering if there is no positional ordering. This implies that all phases of
heterogeneously charged colloids that are liquid-like with respect to
translational degrees of freedom are also isotropic with respect to particle
orientation.Comment: 9 pages, 3 figure
Boosting capacitive blue-energy and desalination devices with waste heat
We show that sustainably harvesting 'blue' energy from the spontaneous mixing
process of fresh and salty water can be boosted by varying the water
temperature during a capacitive mixing process. Our modified Poisson-Boltzmann
calculations predict a strong temperature dependence of the electrostatic
potential of a charged electrode in contact with an adjacent aqueous 1:1
electrolyte. We propose to exploit this dependence to boost the efficiency of
capacitive blue engines, which are based on cyclically charging and discharging
nanoporous supercapacitors immersed in salty and fresh water, respectively [D.
Brogioli, Phys. Rev. Lett. 103, 058501 (2009)]. We show that the energy output
of blue engines can be increased by a factor of order two if warm
(waste-heated) fresh water is mixed with cold sea water. Moreover, the
underlying physics can also be used to optimize the reverse process of
capacitive desalination of water
Tuning colloid-interface interactions by salt partitioning
We show that the interaction of an oil-dispersed colloidal particle with an
oil-water interface is highly tunable from attractive to repulsive, either by
varying the sign of the colloidal charge via charge regulation, or by varying
the difference in hydrophilicity between the dissolved cations and anions. In
addition, we investigate the yet unexplored interplay between the
self-regulated colloidal surface charge distribution with the planar double
layer across the oil-water interface and the spherical one around the colloid.
Our findings explain recent experiments and have direct relevance for tunable
Pickering emulsions.Comment: 5+4 pages, 3+4 figures, V2: improved text and figures, more detailed
supplementar
Harvesting vibrational energy with liquid-bridged electrodes: thermodynamics in mechanically and electrically driven RC-circuits
We theoretically study a vibrating pair of parallel electrodes bridged by a
(deformed) liquid droplet, which is a recently developed microfluidic device to
harvest vibrational energy. The device can operate with various liquids,
including liquid metals, electrolytes, as well as ionic liquids. We numerically
solve the Young-Laplace equation for all droplet shapes during a vibration
period, from which the time-dependent capacitance follows that serves as input
for an equivalent circuit model. We first investigate two existing energy
harvesters (with a constant and a vanishing bias potential), for which we
explain an open issue related to their optimal electrode separations, which is
as small as possible or as large as possible in the two cases, respectively.
Then we propose a new engine with a time-dependent bias voltage, with which the
harvested work and the power can be increased by orders of magnitude at low
vibration frequencies and by factors 2-5 at high frequencies, where frequencies
are to be compared to the inverse RC-time of the circuit.Comment: 9 pages, 6 figure
Free Minimization of the Fundamental Measure Theory Functional: Freezing of Parallel Hard Squares and Cubes
Due to remarkable advances in colloid synthesis techniques, systems of
squares and cubes, once an academic abstraction for theorists and simulators,
are nowadays an experimental reality. By means of a free minimization of the
free-energy functional, we apply Fundamental Measure Theory to analyze the
phase behavior of parallel hard squares and hard cubes. We compare our results
with those obtained by the traditional approach based on the Gaussian
parameterization, finding small deviations and good overall agreement between
the two methods. For hard squares our predictions feature at intermediate
packing fraction a smectic phase, which is however expected to be unstable due
to thermal fluctuations. This implies that for hard squares the theory predicts
either a vacancy-rich second-order transition or a vacancy-poor weakly
first-order phase transition at higher density. In accordance with previous
studies, a second-order transition with a high vacancy concentration is
predicted for hard cubes
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