Two-Steps Versus One-Step Solidification Pathways of Binary Metallic Nanodroplets.

The solidification of AgCo, AgNi, and AgCu nanodroplets is studied by molecular dynamics simulations in the size range of 2-8 nm. All these systems tend to phase separate in the bulk solid with surface segregation of Ag. Despite these similarities, the simulations reveal clear differences in the solidification pathways. AgCo and AgNi already separate in the liquid phase, and they solidify in configurations close to equilibrium. They can show a two-step solidification process in which Co-/Ni-rich parts solidify at higher temperatures than the Ag-rich part. AgCu does not separate in the liquid and solidifies in one step, thereby remaining in a kinetically trapped state down to room temperature. The solidification mechanisms and the size dependence of the solidification temperatures are analyzed, finding qualitatively different behaviors in AgCo/AgNi compared to AgCu. These differences are rationalized by an analytical model.

Study of the aggregation behavior of Janus particles by coupling experiments and Brownian dynamics simulations.

HYPOTHESIS: New colloids such as inverse patchy particles or Janus particles are considered as smart building blocks in the development of innovative and performant materials. For example, the control of the self-assembly of oxide-based charged Janus particles is interesting for ceramic shaping. Thus, the synthesis of silica based Janus particles as well as a detailed study of their behavior in suspension are presented in this paper. EXPERIMENTS: Fluorescent silica particles are partially modified in surface by grafting amine groups using a Pickering emulsion route. Zeta potential measurements, sedimentation tests and confocal microscopy observations are carried out to analyze the aggregation of the obtained particles in aqueous suspension as a function of the patch size and of the pH. Brownian dynamics simulations are also performed to better understand the aggregate structures. FINDINGS: The aggregation of the synthesized silica-based Janus particles can be tuned by modifying the experimental parameters, and elongated or on the contrary more compact structures could be observed. This control of aggregation makes such particles promising to build new ceramic materials.

Self-Organization of Large Alumina Platelets and Silica Nanoparticles by Heteroaggregation and Sedimentation: Toward an Alternative Shaping of Nacre-Like Ceramic Composites.

Nacre-like ceramic composites are of importance in a wide range of applications, because of their mechanical properties, combining high mechanical strength and high fracture toughness. Those mechanical properties are the result of strongly aligned platelets glued in a matrix. Different methods exist to shape such a "brick-and-mortar" hierarchical structure. In this paper, we propose to use the phenomenon of heteroaggregation between silica nanoparticles and large alumina platelets. Experimental and numerical results show that silica nanoparticles can adsorb on alumina platelets with good distribution. This adsorption promotes the deagglomeration of alumina that can self-organize in layers by sedimentation. This phenomenon can be exploited to shape alumina-silica nacre-like composites.

Brownian dynamics simulations of one-patch inverse patchy particles.

Inverse patchy particles are promising colloids to develop new architectures in ceramic materials based on their self-assembly. Nonetheless, a good understanding of their aggregation is required. Several previous studies have shown that the behavior of ceramic colloids can be well described by the DLVO interaction potential. In the present paper, we develop new coarse-grained Brownian dynamics simulations, where particles are represented by an assembly of beads interacting via DLVO interactions, whose parameters can be directly linked to experimental characterization. First, the validity of the simulations is proved by studying the heteroaggregation of homogeneously charged particles. Then, simulations are applied to one-patch inverse patchy particles to study the effect of the patch size. They show that the smaller the patch, the more elongated the aggregates. Simulations are also performed to understand the role of the Debye screening length in the particular case of large patches and they show that aggregation leads always to compact aggregates.

Role of Electrostatic Interactions in Oil-in-Water Emulsions Stabilized by Heteroaggregation: An Experimental and Simulation Study.

Oil-in-water emulsion stabilization by heteroaggregation of hydrophilic particles without a surfactant is of importance in a wide range of applications; however, the stabilization mechanism is little described. To shed light on the early stage of the stabilization mechanism, a model system composed of an oil wax phase dispersed in water with oppositely charged colloidal particles is studied experimentally and numerically. Experiments show that the colloids do not penetrate deeply in the oil phase, suggesting that adsorption of the colloidal particles on the wax droplets is mainly due to electrostatic interactions. Experiments and Brownian dynamics simulations show also that when oppositely charged colloidal particles are present in the emulsion, a multilayer coating of heteroaggregated colloidal particles is formed around the wax droplets. This protective coating is expected to prevent from the oil droplet coalescence and therefore to stabilize the emulsion.

Influence of different surfactants on Pickering emulsions stabilized by submicronic silica particles.

HYPOTHESIS: Pickering emulsions were prepared using wax and silica submicronic particles (650 nm), as a first step towards the synthesis of Janus particles. Surfactants added to silica particles control the emulsion stability and particles arrangement, i.e. their penetration depth into the wax and their ability to form a monolayer. Thus, a systematic study of surfactants is proposed. EXPERIMENTS: Zeta potential measurements and sedimentation tests are conducted to evaluate interactions of two cationic (CTAB: hexadecyltrimethylammonium bromide and DDAB: didodecyldimethylammonium bromide) and two polymeric surfactants with silica surface. Surfactants affinity for the wax is estimated by contact angle measurements. Emulsions stability is compared to evaluate the ability of particles to stabilize wax droplets. Through microscopic analyses, the penetration depth into the wax is measured as well as the ability to form a monolayer or multilayers/aggregates. FINDINGS: All surfactants modify silica surface properties, but only CTAB and DDAB give stable Pickering emulsions. Because of a better affinity for the wax, DDAB presents the best characteristics for Janus particle synthesis, allowing a larger variation of particles penetration depth into the wax.

Computer simulations of heteroaggregation with large size asymmetric colloids.

HYPOTHESIS: Hetero-aggregation of inorganic colloids is influenced by numerous parameters, which dictate the suspension properties. When particles are different in size, the suspension can be either stable or unstable according to concentration of components, ionic strength, and pH. Experimentally, understanding the role of each parameter is sometimes difficult because parameters cannot easily be modified independently. Numerical simulations are thus very useful to discriminate between different effects. SIMULATIONS: Brownian dynamics simulations are used here to study the heteroaggregation of dilute suspensions composed of two populations of colloids with large size asymmetry. Special attention is paid to the effect of small-particle concentration, surface potentials, and ionic strength. FINDINGS: The simulation results show that hetero-aggregation can be tuned by modifying these different parameters, and that the resulting aggregate structures depend more on the surface properties of small particles than on those of large particles. The simulations shed light on a further parameter crucially influencing hetero-aggregation, i.e. the mobility of small particles when adsorbed on large ones. The present results rationalize numerous experimental observations reported in the literature and can be used as reference to explain future experimental observations.

Early Dynamics and Stabilization Mechanisms of Oil-in-Water Emulsions Containing Colloidal Particles Modified with Short Amphiphiles: A Numerical Study.

Emulsions stabilized by mixtures of particles and amphiphilic molecules are relevant for a wide range of applications, but their dynamics and stabilization mechanisms on the colloidal level are poorly understood. Given the challenges to experimentally probe the early dynamics and mechanisms of droplet stabilization, Brownian dynamics simulations are developed here to study the behavior of oil-in-water emulsions stabilized by colloidal particles modified with short amphiphiles. Simulation parameters are based on an experimental system that consists of emulsions obtained with octane as the oil phase and a suspension of alumina colloidal particles modified with short carboxylic acids as the continuous aqueous medium. The numerical results show that attractive forces between the colloidal particles favor the formation of closely packed clusters on the droplet surface or of a percolating network of particles throughout the continuous phase, depending on the amphiphile concentration. Simulations also reveal the importance of a strong adsorption of particles at the liquid interface to prevent their depletion from the droplet surface when another droplet approaches. Strongly adsorbed particles remain immobile on the droplet surface, generating an effective steric barrier against droplet coalescence. These findings provide new insights into the early dynamics and mechanisms of stabilization of emulsions using particles and amphiphilic molecules.

Interdiffusion and crystallization of oppositely charged colloids.

The aggregation of oppositely charged colloids, usually denoted as heteroaggregation, is often used in colloidal processing, for which a precise control of the basic mechanisms of aggregate formation is of crucial importance. A promising way to achieve a better degree of control is to guide heteroaggregation by imposing geometric constraints. Here, we consider this possibility by simulating the heteroaggregation of two oppositely charged suspensions which are initially separated and then put into contact through a planar interface. Our Brownian dynamics simulations show that this type of heteroaggregation allows the formation of mixed films whose thickness can be controlled by tuning the interactions between the particles or by changing the colloidal concentration in the initial suspensions. The dependence of the type of crystalline order in these films on these parameters is also analyzed.

Shear viscosity in hard-sphere and adhesive colloidal suspensions with reverse non-equilibrium molecular dynamics.

We employ the reverse non-equilibrium molecular dynamics method (RNEMD) of Müller-Plathe [Phys. Rev. E, 1999, 59, 4894] to calculate the shear viscosity of colloidal suspensions within the stochastic rotation dynamics-molecular dynamics (SRD-MD) simulation method. We examine the influence of different coupling schemes in SRD-MD on the colloidal volume fraction ϕc dependent viscosity from the dilute limit up to ϕc = 0.3. Our results demonstrate that the RNEMD method is a robust and reliable method for calculating rheological properties of colloidal suspensions. To obtain quantitatively accurate results beyond the dilute regime, the hydrodynamic interactions between the effective fluid particles in the SRD and the MD colloidal particles must be carefully considered in the coupling scheme. We benchmark the method by comparing with the hard sphere suspension case, and then calculate relative viscosities for colloids with mutually attractive interactions. We show that the viscosity displays a sharp increase at the onset of aggregation of the colloidal particles with increasing volume fraction and attraction.

Aggregation of binary colloidal suspensions on attractive walls.

The adsorption of colloidal particles from a suspension on a solid surface is of fundamental importance to many physical and biological systems. In this work, Brownian Dynamics simulations are performed to study the aggregation in a suspension of oppositely charged colloidal particles in the presence of an attractive wall. For sufficiently strong attractions, the wall alters the microstructure of the aggregates so that B2 (CsCl-type) structures are more likely obtained instead of B1 (NaCl-type) structures. The probability of forming either B1 or B2 crystallites depends also on the inverse interaction range κa. Suspensions with small κa are more likely to form B2 crystals than suspensions with larger κa, even if the energetic stability of the B2 phase decreases with decreasing κa. The mechanisms underlying this aggregation and crystallization behaviour are analyzed in detail.

How colloid-colloid interactions and hydrodynamic effects influence the percolation threshold: A simulation study in alumina suspensions.

The percolation behavior of alumina suspensions is studied by computer simulations. The percolation threshold ϕc is calculated, determining the key factors that affect its magnitude: the strength of colloid-colloid attraction and the presence of hydrodynamic interactions (HIs). To isolate the effects of HIs, we compare the results of Brownian Dynamics, which do not include hydrodynamics, with those of Stochastic Rotation Dynamics-Molecular Dynamics, which include hydrodynamics. Our results show that ϕc decreases with the increase of the attraction between the colloids. The inclusion of HIs always leads to more elongated structures during the aggregation process, producing a sizable decrease of ϕc when the colloid-colloid attraction is not too strong. On the other hand, the effects of HIs on ϕc tend to become negligible with increasing attraction strength. Our ϕc values are in good agreement with those estimated by the yield stress model by Flatt and Bowen.

Compact and ordered colloidal clusters from assembly-disassembly cycles: a numerical study.

Brownian dynamics simulations are used to investigate the assembly of attractive colloids whose interaction potential well is periodically changed over time. Our system is composed of spherical, mono-disperse, highly charged, alumina particles whose interactions are modeled by the DLVO theory. The depth of the potential well is periodically changed by varying the ionic strength of the liquid medium. The simulations show that, with a right choice of some key parameters, a potential well depth alternating between low and higher values allows a faster aggregation into more compact and also more ordered colloidal clusters. This result is quantified by the computation of two relevant coordination parameters during the aggregation. This finding may help elucidate the assembly of colloidal particles in complex biological processes (e.g. biomineralization) and could be useful for the development of photonic crystals from attractive colloidal particles.

Numerical and experimental study of suspensions containing carbon blacks used as conductive additives in composite electrodes for lithium batteries.

Suspensions of carbon blacks and spherical carbon particles are studied experimentally and numerically to understand the role of the particle shape on the tendency to percolation. Two commercial carbon blacks and one lab-synthesized spherical carbon are used. The percolation thresholds in suspensions are experimentally determined by two complementary methods: impedance spectroscopy and rheology. Brownian dynamics simulations are performed to explain the experimental results taking into account the fractal shape of the aggregates in the carbon blacks. The results of Brownian dynamics simulations are in good agreement with the experimental results and allow one to explain the experimental behavior of suspensions.

Brownian dynamics simulations of colloidal suspensions containing polymers as precursors of composite electrodes for lithium batteries.

Dilute aqueous suspensions of silicon nanoparticles and sodium carboxymethylcellulose salt (CMC) are studied experimentally and numerically by brownian dynamics simulations. The study focuses on the adsorption of CMC on silicon and on the aggregation state as a function of the suspension composition. To perform simulations, a coarse-grained model has first been developed for the CMC molecules. Then, this model has been applied to study numerically the behavior of suspensions of silicon and CMC. Simulation parameters have been fixed on the basis of experimental characterizations. Results of brownian dynamics simulations performed with our model are found in qualitative good agreement with experiments and allow a good description of the main features of the experimental behavior.

Granulating titania powder by colloidal route using polyelectrolytes.

A new, convenient, and inexpensive approach to process and granulate titania powders by a chemical route is proposed. It is based on the use of a formulation that includes a polyanion such as poly(sodium 4-styrenesulfonate) (PSS). Such a polyelectrolyte is most often considered to achieve dispersion of oxide powders in water. Basically, it adsorbs onto the surface of particles and induces electrical and/or steric interactions between particles in the suspension, which prevents agglomeration and rapid sedimentation. The advantages of polyelectrolytes in ceramic processing is well documented in the literature to produce low viscosity suspensions that are further used to form ceramic parts. In the case of TiO2 powders, such aqueous dispersions were obtained by adding small quantities of PSS. However, when exploring the behavior of mixtures containing lower contents of dispersant, we have discovered that, well below the optimum concentration required to get stable dispersions, the polyelectrolyte can act as a binder for titania particles. This can confer cohesion to the agglomerates, which can be processed to form large size (e.g., millimeter size) spheres. This phenomenon takes place when the oxide surface carries both positive and negative electrical charges and can be explained on a simple basis involving surface chemistry. For the optimum concentration of PSS that disperses titania, a polycation such as chitosan should be added to get spheres. This simple technique is expected to receive increasing attention due its potentialities and strong advantages with respect to other granulation techniques, such as spray-drying, which are energy consuming.