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A striking and highly versatile feature of magnetic (nano)particles is their ability to be manipulated at will at a distance by external fields. In this paper, the influence of gravity on the self-assembly of dipolar particles near a surface in the presence of a strong vertical magnetic field is investigated theoretically. A rich ground-state phase diagram stems from the effects of the number of particles N and gravity. Two distinct regimes are discovered for the gravity-mediated breakup of a standing chain. When N is small, there is a chain fragmentation (with two widely separated repulsive chain fragments) above a critical value for the gravity, whereas for higher chains, ribbonization (with two cohesive chain fragments) sets in. In both scenarios, simple algebraic decays for the transition gravity as a function of N are analytically predicted and accurately corroborate the exact numerical results. Further intricate chain fragmentations and internal ribbon transformations operate upon further increasing the gravity until all N constitutive particles lie on the surface. Our findings shed additional light on various recent experiments and computer simulations on magnetic colloids and granular media.

The interaction of two dipolar hard spheres near a surface and under the influence of gravity and external perpendicular magnetic fields is investigated theoretically. The full ground-state phase diagram as a function of gravity and magnetic field strengths is established. A dimer (i.e., two touching beads) can only exist when the gravity and magnetic field strengths are simultaneously not too large. Thereby, upon increasing the magnetic field strength, three dimeric states emerge: a lying state (dimer axis parallel to the substrate), an inclined state (intermediate state between the lying and standing ones) and a standing state (dimer axis normal to the substrate). It is found that the orientation angles of the dimer axis and the dipole moment in the newly discovered inclined phase are related by a strikingly simple Snell-Descartes-like law. We argue that our findings can be experimentally verified in colloidal and granular systems.

This paper examines the effect of an external magnetic on the crystallization and magnetization of layered dipolar particles. Exact results are provided for monolayers and bilayers where it is shown that increasing the layer thickness provides enhanced cohesion and weaker susceptibility.

International audience; Sedimentation enables self-assembly of colloidal particles into crystalline structures, as needed for catalysis or photonics applications. Here we combine experiments, theory, and simulations to investigate the equilibrium structure of a colloidal monolayer with tunable interparticle repulsion via an applied external magnetic field. Experimental observations of the equilibrium structure are in excellent agreement with density functional theory. Within a (zero-temperature) local density approximation, we derive a simple analytical expression that quantitatively captures the inhomogeneous ordering ranging from solid to liquidlike states. Monte Carlo simulations corroborate these findings and explore an even wider range of sedimentation conditions, thus providing a global view of the sedimentation-mediated ordering in colloidal monolayers with tunable long-ranged interparticle repulsions. Our findings shed further light on the classical sedimentation problem in colloidal science and related areas.

Intermediate filaments are the least explored among the large cytoskeletal elements. We show here that they display conformational anomalies in narrow microfluidic channels. Their unusual behavior can be understood as the consequence of a previously undetected, large-scale helically curved superstructure. Confinement in a channel orders the otherwise soft, strongly fluctuating helical filaments and enhances their structural correlations, giving rise to experimentally detectable, strongly oscillating tangent correlation functions. We propose an explanation for the detected intrinsic curving phenomenon-an elastic shape instability that we call autocoiling. The mechanism involves self-induced filament buckling via a surface stress located at the outside of the cross section. The results agree with ultrastructural findings and rationalize for the commonly observed looped intermediate filament shapes. Beyond curvature, explaining the molecular origin of the detected helical torsion remains an interesting challenge.

We report a new phase called clinohexagonal prism (CHP) that accounts for all the ground states of dipolar hard spheres prepared at any density. This phase merely consists of an oblique prismatic lattice with a hexagonal base. Our calculations show that at intermediate densities, a special close packed body-centered orthorhombic phase coincides with the CHP phase in the ground state for a wide density window. In the high packing regime, i.e., in the vicinity of the density of the hexagonal close packed phase, it is a limiting case of the CHP phase with vanishing obliquity that emerges. These findings provide a unified and clarified view of the solid-solid transitions occurring at zero temperature in dipolar systems and should be relevant in other related molecular or soft matter systems governed by anisotropic (and possibly isotropic) soft potentials.

The interactions of dipolar filaments such as magnetic needles and chains in strong homogeneous magnetic/electric field are investigated theoretically. Revisiting the case of uniformly magnetized/polarized parallel needles of finite size L and separated by a distance R , all the relevant regimes of attraction and/or repulsion are properly addressed and discussed. At short inter-needle separation ( R/L $ \lesssim$ 0.2, the repuive pair potential of two facing needles is governed by R-1 in strong contrast with R-3 at long separations (R/L ≳ 2.5). This softening is attributed to an efficient long-range screening owing to the relatively long needle extension in this regime. This whole understanding of dipolar needles effective interaction is then used to grasp that of dipolar chains made up of spherical dipolar beads. When excluded-volume correlations are weak (i.e., the chains are a few beads apart), chains and needles possess virtually the same effective interaction. However, at short separation there is a remarkable hardening upon approaching two chains in registry in qualitative contrast to the needles case.

International audience; Crystallization near structured substrates is explored by extensive Brownian dynamics computer simulations of two-dimensional equimolar binary colloidal suspensions. The particles are interacting via repulsive dipole-dipole forces. The two species have different (big and small) dipole moments which are tunable by an external magnetic field. A quench is realized by a sudden drastic increase of the magnetic field which formally corresponds to a quench to zero temperature. After the quench the equilibrium state is an alternating square lattice which coincides exactly with that imposed by the external wall. The relaxation behavior following the quench is explored. The amount of local crystallites with triangular and square symmetry is monitored as a function of time for different initial temperatures. It found that the number and structure of crystallites near the walls strongly depend on the wall pattern. Even though local square structures are favored energetically and the equilibrium state is an alternating square lattice, the number of triangular crystallites close to the wall which has outermost fixed small particles is significantly higher than in the unconfined case. The actual number of triangular clusters depends on the depth of the quench. This effect is not contained in classical nucleation theory.

This paper deals with the investigation of cohesive energy in dipolar helices made up of hard spheres. Such tubular helical structures are ubiquitous objects in biological systems. We observe a complex dependence of cohesive energy on surface packing fraction and dipole moment distribution. As far as single helices are concerned, the lowest cohesive energy is achieved at the highest surface packing fraction. Besides, a striking non-monotonic behavior is reported for the cohesive energy as a function of the surface packing fraction. For multiple helices, we discover a new phase, exhibiting markedly higher cohesive energy. This phase is referred to as ZZ tube consisting of stacked crown rings (reminiscent of a pile of zig-zag rings), resulting in a local triangular arrangement with densely packed filaments parallel to the tube axis.

International audience; Athermal two-dimensional granular systems are exposed to external mechanical noise leading to Brownian-like motion. Using tunable repulsive interparticle interaction, it is shown that the same microstructure as that observed in colloidal suspensions can be quantitatively recovered at a macroscopic scale. To that end, experiments on granular and colloidal systems made up of magnetized particles as well as computer simulations are performed and compared. Excellent agreement throughout the range of the magnetic coupling parameter Gamma is found for the pair distribution as well as the bond-orientational correlation functions. This finding opens new ways to efficiently and very conveniently explore phase transitions, crystallization, nucleation, etc in confined geometries.

International audience; The self-assembly in two dimensions of spherical magnets is addressed theoretically. Minimal energy structures are obtained by optimization procedures as well as Monte Carlo computer simulations. For a small number of constitutive magnets N = 18, the magnets form touching concentric rings that are reminiscent of the onion-like structures. At sufficiently large N, the (edgy) shells are hexagonal where dipole moments tend to align to the edge direction. All these relevant predicted shapes are experimentally reproduced by manipulating millimetric magnets.

Employing computer simulations and Poisson-Boltzmann theory, we show that a symmetric electrolyte in an electric condenser undergoes a localisation-delocalisation tran- sition as a function of the external voltage U. The transition occurs at the critical voltage Uc corresponding to the situation where the surface charge density of the plates equals the area charge density of completely charge-separated electrolytes. The average distance of the electrolyte ions to the plates diverges logarithmically for UUc. This transition is expected to be observable in micro-electrodes.

We studied the structures formed in aqueous dispersions of charged colloidal spheres under a constant low salt concentration of c = 6 × 10−6 mol l−1. Particles of diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation 0

A review is given on recent studies of charged colloidal suspensions and polyelectrolytes both in static and non-equilibrium situations. As far as static equilibrium situations are concerned, we discuss three different problems: 1) Sedimentation density profiles in charged suspensions are shown to exhibit a stretched non-bariometric wing at large heights and binary suspensions under gravity can exhibit an analog of the brazil-nut effect known from granular matter, i.e. the heavier particles settle on top of the lighter ones. 2) Soft polyelectrolyte systems like polyelectrolyte stars and microgels show an ultra-soft effective interaction and this results into an unusual equilibrium phase diagram including reentrant melting transitions and stable open crystalline lattices. 3) The freezing transition in bilayers of confined charged suspensions is discussed and a reentrant behaviour is obtained. As far as nonequilibrium problems are concerned, we discuss an interface instability in oppositely driven colloidal mixtures and discuss possible approaches to simulate electrokinetic effects in charged suspensions.

In this article we review the complexation behavior of charged polymers adsorbed onto charged substrates. In the first part we summarize the results obtained on the adsorption properties of a single polyelectrolyte chain with one or several oppositely charged spherical colloids. The results of various simulational studies are compared with theoretical predictions. After that, we show that under strong electrostatic coupling conditions one can also obtain complexations, where chain and colloid carry the same charge (like-charge complexation). This effect is due to counterion correlation effects. Finally, we investigate the case of polyelectrolyte multilayers structures, which are composed of alternating polycations and polyanions on various substrate geometries. This structure, normally obtained by a so-called layer-by-lay deposition, is very stable experimentally, but only poorly understood in terms of theoretical approaches. Our simulation results suggest that short range attractive interactions (e.g., van der Waals) are important for obtaining a stable sequence of multilayers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3557–3570, 2004

The adsorption of highly \textit{oppositely} charged flexible polyelectrolytes (PEs) on a charged planar substrate is investigated by means of Monte Carlo (MC) simulations. We study in detail the equilibrium structure of the first few PE layers. The influence of the chain length and of a (extra) non-electrostatic short range attraction between the polycations and the negatively charged substrate is considered. We show that the stability as well as the microstructure of the PE layers are especially sensitive to the strength of this latter interaction. Qualitative agreement is reached with some recent experiments., Comment: 28 pages; 11 (main) Figs - Revtex4 - Higher resolution Figs can be obtained upon request. To appear in Macromolecules

The adsorption of highly charged flexible polycations and polyanions on a charged cylindrical substrate is investigated by means of Monte Carlo (MC) simulations. A detailed structural study, including monomer and fluid charge distributions, is provided. The influence of a short range attraction between the polycations and the negatively charged substrate is also considered. We demonstrate that the building up of multilayer structures is highly prohibited mainly due to the high entropy penalty stemming from the low dimensionality of the substrate at strong curvature., 18 pages - 7 (main) EPS figs - revtex4 - to appear in J. Chem. Phys

The effects of image charges (i.e., induced surface charges of polarization) in spherical geometry and their implication for charged colloidal systems are investigated. We study analytically and exactly a single microion interacting with a dielectric sphere and discuss the similarities and discrepancies with the case of a planar interface. By means of extensive Monte Carlo (MC) simulations, we study within the framework of the primitive model the effects of image charges on the structure of the electrical double layer. Salt-free environment as well as salty solutions are considered. A remarkable finding of this study is that the position of the maximum in the counterion density (appearing at moderately surface charge density) remains quasi-identical, regardless of the counterion valence and the salt content, to that obtained within the \textit{single}-counterion system., 14 pages, 12 figures, RevTeX4, to appear in J. Chem. Phys

In this Letter we investigate the mechanism for overcharging of a single spherical colloid in the presence of aqueous salts within the framework of the primitive model by molecular dynamics (MD) simulations as well as integral-equation theory. We find that the occurrence and strength of overcharging strongly depends on the salt-ion size, and the available volume in the fluid. To understand the role of the excluded volume of the microions, we first consider an uncharged system. For a fixed bulk concentration we find that upon increasing the fluid particle size one strongly increases the local concentration nearby the colloidal surface and that the particles become laterally ordered. For a charged system the first surface layer is built up predominantly by strongly correlated counterions. We argue that this a key mechanism to produce overcharging with a low electrostatic coupling, and as a more practical consequence, to account for charge inversion with monovalent aqueous salt ions., Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letters

We report the coupled effects of macroion charge discretization and counterion valence in the primitive model for spherical colloids. Instead of considering a uniformly charged surface, as it is traditionally done, we consider a more realistic situation where \textit{discrete monovalent microscopic charges} are randomly distributed over the sphere. Monovalent or multivalent counterions ensure global electroneutrality. We use molecular dynamics simulations to study these effects at the ground state and for finite temperature. The ground state analysis concerns the counterion structure and \textit{charge inversion}. Results are discussed in terms of simple analytical models. For finite temperature, strong and weak Coulomb couplings are treated. In this situation of finite temperature, we considered and discussed the phenomena of ionic pairing (pinning) and unpairing (unpinning)., 24 pages, 12 (main) figures (28 EPS files). To appear in Physica A

We investigate the columnar aggregation of dipolar chains theoretically. At very small number of constitutive chains , ribbons are found to be stable. For a larger number of chains , chain bundles in zipper configuration with narrow sections emerge. The underlying lattice is always a body-centered tetragonal lattice which corresponds to the bulk ground-state limit. Remarkable low-energy ground states are obtained when the section of the chain bundle corresponds to a square or a rectangle with aspect ratio close to the former (i.e. , unity). Our findings should rationalize recent experimental observations of the columnar assembly of dipolar chains.

Abnormal grain growth in the presence of second-phase particles is investigated with the help of a two-dimensional Monte Carlo simulation. An aggregate of equiaxed grains is considered with constant grain boundary energy and mobility. The only driving force accounted for stems from the grain boundary curvature. The process of abnormal grain growth is investigated as a function of two governing parameters, the initial degree of pinning of the matrix grains by the particles and the initial size advantage of the anomalous grain. In such conditions, moderate growth is obtained whose specific features are discussed with respect to the available models. It is shown that it is possible to obtain drastic grain growth by introducing the thermally activated unpinning of grain boundaries from particles. For this purpose, a simplified but effective procedure is proposed and discussed that includes the influence of the capillary force on the height of the local energy barrier for grain unpinning.

The effect of fixed discrete colloidal charges in the primitive model is investigated for spherical macroions. Instead of considering a central bare charge, as it is traditionally done, we distribute \textit{discrete} charges randomly on the sphere. We use molecular dynamics simulations to study this effect on various properties such as overcharging, counterion distribution and diffusion. In the vicinity of the colloid surface the electrostatic potential may considerably differ from the one obtained with a central charge. In the strong Coulomb coupling, we showed that the colloidal charge discretization qualitatively influences the counterion distribution and leads to a strong colloidal charge-counterion pair association. However, we found that \textit{charge inversion} still persists even if strong pair association is observed., 16 pages, 16 ps figures, REVTEX, accepted for publication in EPJE

In this letter, we study the ground state of two spherical macroions of identical radius, but asymmetric bare charge ((Q_{A}>Q_{B})). Electroneutrality of the system is insured by the presence of the surrounding divalent counterions. Using Molecular Dynamics simulations within the framework of the primitive model, we show that the ground state of such a system consists of an overcharged and an undercharged colloid. For a given macroion separation the stability of these ionized-like states is a function of the difference ((\sqrt{N_{A}}-\sqrt{N_{B}})) of neutralizing counterions (N_{A}) and (N_{B}). Furthermore the degree of ionization, or equivalently, the degree of overcharging, is also governed by the distance separation of the macroions. The natural analogy with ionic bonding is briefly discussed., published version

The effect of second-phase particles on grain growth was simulated in two dimensions with the help of a Monte Carlo method. A wide range of particle surface fractions was considered, from 10 −4 to 10 −1 . Particle unpinning from grain boundaries was only observed for small volume fractions ( f ≤2×10 −2 ). The average size of the grains in the pinned microstructure was investigated as a function of f and the fraction φ of particles interacting with the grain boundaries. The normalised stagnant grain size was found to scale with ( φf ) −1/2 . These results are discussed in terms of the available non-random approaches of grain boundary pinning by particles.

Anomalous grain growth has been studied in the solid state in an N18 superalloy. An unexpected observation was the presence of small grains, either isolated or clustered, embedded in the large grains. The misorientations of the grains and clusters with respect to the large grain have been measured by electron backscattering diffraction. It is found that a majority of the isolated grains exhibit boundaries of low interfacial energy, while in the clusters the misorientations mainly correspond to high energy random boundaries. A simple wetting model in terms of interfacial energies is proposed and discussed, which explains the occurrence of the low energy insular grains. A specific mechanism for the occurrence of the large energy grains is proposed, which qualitatively explains the experimental distributions of misorientations.

International audience; The self-assembly of spherical magnets (magnetic balls) is addressed theoretically. Minimal energy structures are obtained by optimization procedures as well as Monte Carlo computer simulations. Three typical shapes are obtained depending on the number of constitutive magnets N. In the regime of small N, chains are stable as dimers or trimers (i.e., N = 14). The number of stacked rings is found to increase as N-2/3, leading to a tubular structure at large N. All the relevant predicted shapes are experimentally reproduced by manipulating millimetric magnets.

Recent studies on confined crystals of charged colloidal particles are reviewed, both in equilibrium and out of equilibrium. We focus in particular on direct comparisons of experiments (light scattering and microscopy) with lattice sum calculations and computer simulations. In equilibrium we address buckling and crystalline multilayering of charged systems in hard and soft slit confinement. We discuss also recent crystalline structures obtained for charged mixtures. Moreover, we put forward possibilities to apply external perturbations, in order to drive the system out of equilibrium. These include electrolyte gradients as well as the application of shear and electric fields., Comment: Review article, 18 pages, 5 figures

Two-dimensional binary mixtures made up of particles carrying similar parallel dipole moments are investigated. Using Monte Carlo simulations a detailed structural analysis based on partial pair distribution functions and microstructure snapshots is presented for high dipolar coupling. At equimolar composition, the relevance of the coexistence of triangular superlattices with stoichiometry AB 2 and A 2 B is revealed, with A(B) standing for the large(small) dipole moments. This key finding is in excellent qualitative agreement with the zero-temperature theoretical prediction of Assoud L. et al., EPL, 80 (2007) 48001. These structures are detectable in experiments on colloidal and granular matter.

[EN] We show that hard spheres confined between two parallel hard plates pack denser with periodic adaptive prismatic structures which are composed of alternating prisms of spheres. The internal structure of the prisms adapts to the slit height which results in close packings for a range of plate separations, just above the distance where three intersecting square layers fit exactly between the plates. The adaptive prism phases are also observed in real-space experiments on confined sterically stabilized colloids and in Monte Carlo simulations at finite pressure. © 2012 American Physical Society., We thank Elvira Bonet, Moises Garin, and Kevin Mutch for helpful discussions. This work was partially supported by the DFG within the SFB TR6 (Project D1), and by the Spanish CICyT Projects FIS2009-07812 and PROMETEO/2010/043. F. R.-M. acknowledges the support from the EU Marie Curie Project APPCOPTOR-275150 (FP7-PEOPLE-2010-IEF).

We combine real-space experiments and lattice sum calculations to investigate the phase diagram of charged colloidal particles under soft confinement. In the experiments we explore the equilibrium phase diagram of charged colloidal spheres in aqueous suspensions confined between two parallel charged walls at low background salt concentrations. Motivated by the experiments, we perform lattice sum minimizations to predict the crystalline ground state of point-like Yukawa particles which are exposed to a soft confining wall potential. In the multilayered crystalline regime, we obtain good agreement between the experimental and numerical findings: upon increasing the density we recover the sequence 2 ! 24! 2hcp?! 3 ! 34! 3hcp?! 4 . (Some figures may appear in colour only in the online journal)

We investigated driven crystal formation events in thin layers of sedimented colloidal particles under low salt conditions. Using optical microscopy, we observe particles in a thermodynamically stable colloidal fluid to move radially converging towards cation exchange resin fragments acting as seed particles. When the local particle concentration has become sufficiently large, subsequently crystallization occurs. Brownian dynamics simulations of a 2D system of purely repulsive point-like particles exposed to an attractive potential, yield strikingly similar scenarios, and kinetics of accumulation and micro-structure formation. This offers the possibility of flexibly designing and manufacturing thin colloidal crystals at controlled positions and thus to obtain specific micro-structures not accessible by conventional approaches. We further demonstrate that particle motion is correlated with the existence of a gradient in electrolyte concentration due to the release of electrolyte by the seeds.

The crystalline ground state of macroions confined between two neutral parallel plates in the presence of their homogeneously spread counterions is calculated by lattice sum minimization of candidate phases involving up to six layers. For increasing macroion density, a cascade of solid-solid transitions is found involving various multilayered crystals. The cascade includes triangular monolayer and buckled bilayer as well as rhombic, squared and triangular phase structures.

A penalty method is developed for predicting crystal structures in the presence of hard-core interactions. Applications to binary mixtures of neutral as well as charged disks are in particular investigated. For the case of neutral hard disks, a new stable A₃B₃ phase is reported. Concerning the charged case (i.e., two-dimensional ionic crystals), the full phase diagram as a function of the size ratio of the constitutive ions is calculated.

Using non-equilibrium Brownian dynamics computer simulations, we have investigated the steady state statistics of a polymer chain under three different shear environments: i) linear shear flow in the bulk (no walls), ii) shear vorticity normal to the adsorbing wall, iii) shear gradient normal to the adsorbing wall. The statistical distribution of the chain end-to-end distance and its orientational angles are calculated within our monomer-resolved computer simulations. Over a wide range of shear rates, this distribution can be mapped onto a simple theoretical finite-extensible-nonlinear-elastic dumbbell model with fitted anisotropic effective spring constants. The tails of the angular distribution functions are consistent with scaling predictions borrowed from the bulk dumbbell model. Finally, the frequency of the characteristic periodic tumbling motion has been investigated by simulation as well and was found to be sublinear with the shear rate for the three set-ups, which extends earlier results done in experiments and simulations for free and tethered polymer molecules without adsorption., 10 figures

An ultra-fast quench is applied to binary mixtures of superparamagnetic colloidal particles confined at a two-dimensional water-air interface by a sudden increase of an external magnetic field. This quench realizes a virtually instantaneous cooling which is impossible in molecular systems. Using real-space experiments, the relaxation behavior after the quench is explored. Local crystallites with triangular and square symmetry are formed on different time scales and the correlation peak amplitude of the small particles evolves nonmonotonically in time in agreement with Brownian dynamics computer simulations., Comment: 4 pages, 4 figures

The phase diagram of Yukawa particles confined between two parallel hard walls is calculated at zero-temperature beyond the bilayer regime by lattice-sum-minimization. Tuning the screening, a rich phase behavior is found in the regime bounded by stable two-triangular layers and 3-square layers. In this regime, alternating prism phases with square and triangular basis, structures derived from a hcp bulk lattice, and a structure with two outer layers and two inner staggered rectangular layers, reminiscent of a Belgian waffle iron, are stable. These structures are verifiable in experiments on charged colloidal suspensions and dusty plasma sheets., Comment: 11 pages, 6 figures

The adsorption of charged colloids (macroions) onto an oppositely charged planar substrate is investigated theoretically. Taking properly into account the finite size of the macroions, unusual behaviors are reported. It is found that the role of the coions (the little salt-ions carrying the same sign of charge as that of the substrate) is crucial to understand the mechanisms involved in the process of macroion adsorption. In particular, the coions can accumulate near the substrate's surface and lead to a counter-intuitive {\it surface charge amplification}., Comment: 11 pages - 4 figures. To appear in JCP

The phase diagram of binary mixtures of particles interacting via a pair potential of parallel dipoles is computed at zero temperature as a function of composition and the ratio of their magnetic susceptibilities. Using lattice sums, a rich variety of different stable crystalline structures is identified including $A_mB_n$ structures. [$A$ $(B)$ particles correspond to large (small) dipolar moments.] Their elementary cells consist of triangular, square, rectangular or rhombic lattices of the $A$ particles with a basis comprising various structures of $A$ and $B$ particles. For small (dipolar) asymmetry there are intermediate $AB_2$ and $A_2B$ crystals besides the pure $A$ and $B$ triangular crystals. These structures are detectable in experiments on granular and colloidal matter., Comment: 6 pages - 2 figs - phase diagram updated

The behavior of polyampholytes near a charged planar surface is studied by means of Monte Carlo simulations. The investigated polyampholytes are overall electrically neutral and made up of oppositely charged units (called blocks) that are highly charged and of the the same length. The influence of block length and substrate's surface-charge-density on the adsorption behavior is addressed. A detailed structural study, including local monomer concentration, monomer mean height, transversal chain size, interface-bond orientation correlation, is provided. It is demonstrated that adsorption is favored for long enough blocks and/or high enough Coulomb interface-ion couplings. By explicitly measuring the chain size in the bulk, it is shown that the charged interface induces either a swelling or a shrinkage of the transversal dimension of the chain depending, in a non trivial manner, on the block length., 23 pages - 10 eps figs. Submitted for publication

Crystalline bilayers of charged colloidal suspensions which are confined between two parallel plates and sheared via a relative motion of the two plates are studied by extensive Brownian dynamics computer simulations. The charge-stabilized suspension is modeled by a Yukawa pair potential. The unsheared equilibrium configuration is two crystalline layers with a nested quadratic in-plane structure. For increasing shear rates (.)gamma, we find the following steady states: First, up to a threshold of the shear rate, there is a static solid which is elastically sheared. Above the threshold, there are two crystalline layers sliding on top of each other with a registration procedure. Higher shear rates melt the crystalline bilayers and even higher shear rates lead to a reentrant solid stratified in the shear direction. This qualitative scenario is similar to that found in previous bulk simulations. We have then studied the relaxation of the sheared steady state back to equilibrium after an instantaneous cessation of shear and found a nonmonotonic behavior of the typical relaxation time as a function of the shear rate (.)gamma. In particular, application of high shear rates accelerates the relaxation back to equilibrium since shear-ordering facilitates the growth of the equilibrium crystal. This mechanism can be used to grow defect-free colloidal crystals from strongly sheared suspensions. Our theoretical predictions can be verified in real-space experiments of strongly confined charged suspensions.

The behaviour of colloidal mono- and bilayers in external magnetic fields and under shear is discussed and recent progress is summarized. Superparamagnetic colloidal particles form monolayers when they are confined to a air–water interface in a hanging water droplet. An external magnetic field allows us to tune the strength of the mutual dipole–dipole interaction between the colloids and the anisotropy of the interaction can be controlled by the tilt angle of the magnetic field relative to the surface normal of the air–water interface. For sufficiently large magnetic field strength crystalline monolayers are found. The role of fluctuations in these two-dimensional crystals is discussed. Furthermore, clustering phenomena in binary mixtures of superparamagnetic particles forming fluid monolayers are predicted. Finally, we address sheared colloidal bilayers and find that the orientation of confined colloidal crystals can be tailored by a previously applied shear direction.

The behavior of highly charged short rod-like polyelectrolytes near oppositely charged planar surfaces is investigated by means of Monte Carlo simulations. A detailed microstructural study, including monomer and fluid charge distribution, and chain orientation, is provided. The influence of chain length, substrate's surface-charge-density and image forces is considered. Due to the lower chain-entropy (compared to flexible chains), our simulation data show that rod-like polyelectrolytes can, in general, better adsorb than flexible ones do. Nonetheless, at low substrate-dielectric-constant, it is found that repulsive image forces tend to significantly reduce this discrepancy., Comment: Updated results - 19 pages - 10 figs - RevTex 4

The adsorption of flexible and highly charged polyelectrolytes onto oppositely charged planar surfaces is investigated by means of Monte Carlo simulations. The effect of image forces stemming from the dielectric discontinuity at the substrate interface is considered. The influence, at fixed polyelectrolyte volume fraction, of chain length and surface-charge density is also considered. A detailed structural study, including monomer and fluid charge distributions, is provided. It is demonstrated that image forces can considerably reduce the degree of polyelectrolyte adsorption and concomitantly inhibit the charge inversion of the substrate by polyelectrolytes., 19 pages - 7 eps figs - RevTex 4 - submitted for publication

The restricted primitive model of an electrical double layer around a spherical macroparticle is studied by using integral equation theories and Monte Carlo simulations. The resulting theoretical curves for the Helmholtz and surface potentials versus the macroparticle charge show an unexpected positive curvature when the ionic size of uni- and divalent electrolyte species is increased. This is a novel effect that is confirmed here by computer experiments. An explanation of this phenomenon is advanced in terms of the adsorption and layering of the electrolytic species and of the compactness of the diffuse double layer. It is claimed that the interplay between electrostatic and ionic size correlation effects, absent in the classical Poisson-Boltzmann view, is responsible for this singularity.

We study the counterion distribution around a spherical macroion and its osmotic pressure in the framework of the recently developed Debye-H"uckel-Hole-Cavity (DHHC) theory. This is a local density functional approach which incorporates correlations into Poisson-Boltzmann theory by adding a free energy correction based on the One Component Plasma. We compare the predictions for ion distribution and osmotic pressure obtained by the full theory and by its zero temperature limit with Monte Carlo simulations. They agree excellently for weakly developed correlations and give the correct trend for stronger ones. In all investigated cases the DHHC theory and its computationally simpler zero temperature limit yield better results than the Poisson-Boltzmann theory., Comment: 10 pages, 4 figures, 2 tables, RevTeX4-style

The phase diagram of crystalline bilayers of particles interacting via a Yukawa potential is calculated for arbitrary screening lengths and particle densities. Staggered rectangular, square, rhombic and triangular structures are found to be stable including a first-order transition between two different rhombic structures. For varied screening length at fixed density, one of these rhombic phases exhibits both a single and even a double reentrant transition. Our predictions can be verified experimentally in strongly confined charged colloidal suspensions or dusty plasma bilayers., Comment: 4 pages, 3 eps figs - revtex4. PRL - in press

We report results from a molecular dynamics (MD) simulation on the conformations of a long flexible polyelectrolyte complexed to a charged sphere, \textit{both negatively charged}, in the presence of neutralizing counterions in the strong Coulomb coupling regime. The structure of this complex is very sensitive to the charge density of the polyelectrolyte. For a fully charged polyelectrolyte the polymer forms a dense two-dimensional "disk", whereas for a partially charged polyelectrolyte the monomers are spread over the colloidal surface. A mechanism involving the \textit{overcharging} of the polyelectrolyte by counterions is proposed to explain the observed conformations., Comment: 4 pages, 4 figures (6 EPS files)