48 research outputs found
Attraction Between Like-Charge Surfaces in Polar Mixtures
We examine the force between two charged surfaces immersed in aqueous
mixtures having a coexistence curve. For a homogeneous water-poor phase, as the
distance between the surfaces is decreased, a water-rich phase condenses at a
distance in the range 1-100nm. At this distance the osmotic pressure can
become negative leading to a long-range attraction between the surfaces. The
osmotic pressure vanishes at a distance , representing a very deep
metastable or globally stable energetic state. We give analytical and numerical
results for and on the Poisson-Boltzmann level.Comment: 6 pages, 6 figure
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
Dense ionic fluids confined in planar capacitors: in- and out-of-plane structure from classical density functional theory
The ongoing scientific interest in the properties and structure of electric
double layers (EDLs) stems from their pivotal role in (super)capacitive energy
storage, energy harvesting, and water treatment technologies. Classical density
functional theory (DFT) is a promising framework for the study of the in- and
out-of-plane structural properties of double layers. Supported by molecular
dynamics simulations, we demonstrate the adequate performance of DFT for
analyzing charge layering in the EDL perpendicular to the electrodes. We
discuss charge storage and capacitance of the EDL and the impact of screening
due to dielectric solvents. We further calculate, for the first time, the
in-plane structure of the EDL within the framework of DFT. While our
out-of-plane results already hint at structural in-plane transitions inside the
EDL, which have been observed recently in simulations and experiments, our DFT
approach performs poorly in predicting in-plane structure in comparison to
simulations. However, our findings isolate fundamental issues in the
theoretical description of the EDL within the primitive model and point towards
limitations in the performance of DFT in describing the out-of-plane structure
of the EDL at high concentrations and potentials
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
Microphase separation in oil-water mixtures containing hydrophilic and hydrophobic ions
We develop a lattice-based Monte Carlo simulation method for charged mixtures
capable of treating dielectric heterogeneities. Using this method, we study
oil-water mixtures containing an antagonistic salt, with hydrophilic cations
and hydrophobic anions. Our simulations reveal several phases with a spatially
modulated solvent composition, in which the ions partition between water-rich
and water-poor regions according to their affinity. In addition to the recently
observed lamellar phase, we find tubular, droplet, and even gyroid phases
reminiscent of those found in block copolymers and surfactant systems.
Interestingly, these structures stem from ion-mediated interactions, which
allows for tuning of the phase behavior via the concentrations, the ionic
properties, and the temperature.Comment: 5 pages, 4 figure
Phase-Separation Transition in Liquid Mixtures Near Charged Objects
We study the thermodynamic behavior of nonpolar liquid mixtures in the
vicinity of curved charged objects, such as electrodes or charged colloids. For
small enough charge on the object, or equivalently, small potential, the
dielectrophoretic force leads to enrichment of the more polar liquid close the
colloid. However, there is a critical value of charge (or potential), above
which a phase-separation transition occurs, and the interface between high- and
low-dielectric constant components becomes sharp. Analytical and numerical
composition profile are given, and the equilibrium front location as a function
of charge or voltage is found. We further employ a simple Cahn-Hilliard type
equation to study the dynamics of phase-separation in spatially nonuniform
electric fields. We find an exponential relaxation of the composition front
location, with a characteristic time depending on the charge, mixture
composition and ambient temperature.Comment: final version, includes small changes and typo correction