50 research outputs found
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
Phase Diagrams of Binary Mixtures of Oppositely Charged Colloids
Phase diagrams of binary mixtures of oppositely charged colloids are
calculated theoretically. The proposed mean-field-like formalism interpolates
between the limits of a hard-sphere system at high temperatures and the
colloidal crystals which minimize Madelung-like energy sums at low
temperatures. Comparison with computer simulations of an equimolar mixture of
oppositely charged, equally sized spheres indicate semi-quantitative accuracy
of the proposed formalism. We calculate global phase diagrams of binary
mixtures of equally sized spheres with opposite charges and equal charge
magnitude in terms of temperature, pressure, and composition. The influence of
the screening of the Coulomb interaction upon the topology of the phase diagram
is discussed. Insight into the topology of the global phase diagram as a
function of the system parameters leads to predictions on the preparation
conditions for specific binary colloidal crystals.Comment: Submitte
Glassy dynamics, spinodal fluctuations, and the kinetic limit of hard-rod nucleation
Using simulations we identify three dynamic regimes in supersaturated
isotropic fluid states of short hard rods: (i) for moderate supersaturations we
observe nucleation of multi-layered crystalline clusters; (ii) at higher
supersaturation, we find nucleation of small crystallites which arrange into
long-lived locally favored structures that get kinetically arrested, while
(iii) at even higher supersaturation the dynamic arrest is due to the
conventional cage-trapping glass transition. For longer rods we find that the
formation of the (stable) smectic phase out of a supersaturated isotropic state
is strongly suppressed by an isotropic-nematic spinodal instability that causes
huge spinodal-like orientation fluctuations with nematic clusters diverging in
size. Our results show that glassy dynamics and spinodal instabilities set
kinetic limits to nucleation in a highly supersaturated hard-rod fluids.Comment: Accepted by Physical Review Letter
The electric double layer at high surface potentials : the influence of excess ion polarizability
By including the excess ion polarizability into the Poisson-Boltzmann theory, we show that the decrease in differential capacitance with voltage, observed for metal electrodes above a threshold potential, can be understood in terms of thickening of the double layer due to ion-induced polarizability holes in water. We identify a new length which controls the role of excess ion polarizability in the double layer, and show that when this is comparable to the size of the effective Debye layer, ion polarizability can significantly influence the properties of the double layer
Critical Casimir Forces and Colloidal Phase Transitions in a Near-Critical Solvent : A Simple Model Reveals a Rich Phase Diagram
From experimental studies it is well-known that colloidal particles suspended
in a near-critical binary solvent exhibit interesting aggregation phenomena,
often associated with colloidal phase transitions, and assumed to be driven by
long-ranged solvent mediated (SM) interactions (critical Casimir forces), set
by the (diverging) correlation length of the solvent. We present the first
simulation and theoretical study of an explicit model of a ternary mixture that
mimics this situation. Both the effective SM pair interactions and the full
ternary phase diagram are determined for Brownian discs suspended in an
explicit two-dimensional supercritical binary liquid mixture. Gas-liquid and
fluid-solid transitions are observed in a region that extends well-away from
criticality of the solvent reservoir. We discuss to what extent an effective
pair-potential description can account for the phase behavior we observe. Our
study provides a fresh perspective on how critical fluctuations of the solvent
might influence colloidal self-assembly.Comment: 4 pages, 4 figure
Effective charges and virial pressure of concentrated macroion solutions
The stability of colloidal suspensions is crucial in a wide variety of
processes including the fabrication of photonic materials and scaffolds for
biological assemblies. The ionic strength of the electrolyte that suspends
charged colloids is widely used to control the physical properties of colloidal
suspensions. The extensively used two-body Derjaguin-Landau-Verwey-Overbeek
(DLVO) approach allows for a quantitative analysis of the effective
electrostatic forces between colloidal particles. DLVO relates the ionic
double-layers, which enclose the particles, to their effective electrostatic
repulsion. Nevertheless, the double layer is distorted at high macroion volume
fractions. Therefore, DLVO cannot describe the many-body effects that arise in
concentrated suspensions. We show that this problem can be largely resolved by
identifying effective point charges for the macroions using cell theory. This
extrapolated point charge (EPC) method assigns effective point charges in a
consistent way, taking into account the excluded volume of highly charged
macroions at any concentration, and thereby naturally accounting for high
volume fractions in both salt-free and added-salt conditions. We provide an
analytical expression for the effective pair potential and validate the EPC
method by comparing molecular dynamics simulations of macroions and monovalent
microions that interact via Coulombic potentials to simulations of macroions
interacting via the derived EPC effective potential. The simulations reproduce
the macroion-macroion spatial correlation and the virial pressure obtained with
the EPC model. Our findings provide a route to relate the physical properties
such as pressure in systems of screened-Coulomb particles to experimental
measurements.Comment: 3 figure
Universal motion of mirror-symmetric microparticles in confined Stokes flow
Comprehensive understanding of particle motion in microfluidic devices is
essential to unlock novel technologies for shape-based separation and sorting
of microparticles like microplastics, cells and crystal polymorphs. Such
particles interact hydrodynamically with confining surfaces, thus altering
their trajectories. These hydrodynamic interactions are shape-dependent and can
be tuned to guide a particle along a specific path. We produce strongly
confined particles with various shapes in a shallow microfluidic channel via
stop flow lithography. Regardless of their exact shape, particles with a single
mirror plane have identical modes of motion: in-plane rotation and cross-stream
translation along a bell-shaped path. Each mode has a characteristic time,
determined by particle geometry. Furthermore, each particle trajectory can be
scaled by its respective characteristic times onto two master curves. We
propose minimalistic relations linking these timescales to particle shape.
Together these master curves yield a trajectory universal to particles with a
single mirror plane.Comment: 10 pages, 4 figures, 1 table, 1 PDF file containing Supplementary
Text, Figures and Tabl
The interplay between chemo-phoretic interactions and crowding in active colloids
Many motile microorganisms communicate with each other and their environments via chemical signaling which leads to long-range interactions mediated by self-generated chemical gradients. However, consequences of the interplay between crowding and chemotactic interactions on their collective behavior remain poorly understood. In this work, we use Brownian dynamics simulations to investigate the effect of packing fraction on the formation of non-equilibrium structures in a monolayer of diffusiophoretic self-propelled colloids as a model for chemically active particles. Focusing on the case when a chemical field induces attractive positional and repulsive orientational interactions, we explore dynamical steady-states of active colloids of varying packing fractions and degrees of motility. In addition to collapsed, active gas, and dynamical clustering steady-states reported earlier for low packing fractions, a new phase-separated state emerges. The phase separation results from a competition between long-range diffusiophoretic interactions and motility and is observed at moderate activities and a wide range of packing fractions. Our analysis suggests that the fraction of particles in the largest cluster is a suitable order parameter for capturing the transition from an active gas and dynamical clustering states to a phase-separated state
Absence of anomalous underscreening in highly concentrated aqueous electrolytes confined between smooth silica surfaces
Recent surface forces apparatus experiments that measured the forces between two mica surfaces and a series of subsequent theoretical studies suggest the occurrence of universal underscreening in highly concentrated electrolyte solutions. We performed a set of systematic Atomic Force Spectroscopy measurements for aqueous salt solutions in a concentration range from 1 mM to 5 M using chloride salts of various alkali metals as well as mixed concentrated salt solutions (involving both mono- and divalent cations and anions), that mimic concentrated brines typically encountered in geological formations. Experiments were carried out using flat substrates and submicrometer-sized colloidal probes made of smooth oxidized silicon immersed in salt solutions at pH values of 6 and 9 and temperatures of 25 °C and 45 °C. While strong repulsive forces were observed for the smallest tip-sample separations, none of the conditions explored displayed any indication of anomalous long range electrostatic forces as reported for mica surfaces. Instead, forces are universally dominated by attractive van der Waals interactions at tip-sample separations of ≈2 nm and beyond for salt concentrations of 1 M and higher. Complementary calculations based on classical density functional theory for the primitive model support these experimental observations and display a consistent decrease in screening length with increasing ion concentration
Crystallization And Reentrant Melting Of Charged Colloids In Nonpolar Solvents
We explore the crystallization of charged colloidal particles in a nonpolar solvent mixture. We simultaneously charge the particles and add counterions to the solution with aerosol-OT (AOT) reverse micelles. At low AOT concentrations, the charged particles crystallize into body-centered-cubic (bcc) or face-centered-cubic (fcc) Wigner crystals; at high AOT concentrations, the increased screening drives a thus far unobserved reentrant melting transition. We observe an unexpected scaling of the data with particle size, and account for all behavior with a model that quantitatively predicts both the reentrant melting and the data collapse