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

Colloid Transport in Bicontinuous Nanoporous Media

Colloid transport and retention in porous media are critical processes influencing various Earth science applications, from groundwater remediation to enhanced oil recovery. These phenomena become particularly complex in the confined spaces of nanoporous media, where strong boundary layer effects and nanoconfinement significantly alter colloid behavior. In this work, we use particle dynamics models to simulate colloid transport and retention processes in bicontinuous nanoporous (BNP) media under pressure gradients. By utilizing particle-based models, we track the movement of each colloid and elucidate the underlying colloid retention mechanisms. Under unfavorable attachment conditions, the results reveal two colloid retention mechanisms: physical straining and trapping in low-flow zone. Furthermore, we investigate the effects of critical factors including colloid volume fraction, d, pressure difference, ΔP, interaction between colloids and BNP media, Ec-p, and among colloids, Ec-c, on colloid transport. Analysis of breakthrough curves and colloid displacements demonstrates that higher values of d, lower values of ΔP, and strong Ec-p attractions significantly increase colloid retention, which further lead to colloid clogging and jamming. In contrast, Ec-c has minimal impact on colloid transport due to the limited colloid-colloid interaction in nanoporous channels. This work provides critical insights into the fundamental factors governing colloid transport and retention within stochastic nanoporous materials.

Low porosity, high areal-capacity Prussian blue analogue electrodes enhance salt removal and thermodynamic efficiency in symmetric Faradaic deionization with automated fluid control

Prussian blue analogues (PBAs) show great potential for low-energy Faradaic deionization (FDI) with reversible Na-ion capacity approaching 5 M in the solid-state. However, past continuous-flow demonstrations using PBAs in FDI were unable to desalinate brackish water to potable levels using single-pass architectures. Here, we show that recirculation of effluent from a symmetric cation intercalation desalination cell into brine/diluate reservoirs enables salt removal exceeding 80% at thermodynamic efficiency as high as 80% when cycled with 100 mM NaCl influent and when controlled by a low-volume, automated fluid circuit. This exceptional performance is achieved using a novel heated, alkaline wet phase inversion process that modulates colloidal forces to increase carbon black aggregation within electrode slurries to solidify crack-free, high areal-capacity PBA electrodes that are calendered to minimize cell impedance and electrode porosity. The results obtained demonstrate the need for co-design of auxiliary fluid-control systems together with electrode materials to advance FDI beyond brackish salinity.

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.

Thermal transport phenomena in nanoparticle suspensions

Nanoparticle suspensions in liquids have received great attention, as they may offer an approach to enhance thermophysical properties of base fluids. A good variety of applications in engineering and biomedicine has been investigated with the aim of exploiting the above potential. However, the multiscale nature of nanosuspensions raises several issues in defining a comprehensive modelling framework, incorporating relevant molecular details and much larger scale phenomena, such as particle aggregation and their dynamics. The objectives of the present topical review is to report and discuss the main heat and mass transport phenomena ruling macroscopic behaviour of nanosuspensions, arising from molecular details. Relevant experimental results are included and properly put in the context of recent observations and theoretical studies, which solved long-standing debates about thermophysical properties enhancement. Major transport phenomena are discussed and in-depth analysis is carried out for highlighting the role of geometrical (nanoparticle shape, size, aggregation, concentration), chemical (pH, surfactants, functionalization) and physical parameters (temperature, density). We finally overview several computational techniques available at different scales with the aim of drawing the attention on the need for truly multiscale predictive models. This may help the development of next-generation nanoparticle suspensions and their rational use in thermal applications.

Hydrodynamic interactions enhance gelation in dispersions of colloids with short-ranged attraction and long-ranged repulsion

We show that discrete element simulations of colloidal gelation must account for hydrodynamic interactions between suspended particles through investigation of gelation in a dispersion of colloids interacting pair-wise via short-ranged attraction and long-ranged repulsion (SALR). These dynamic simulations juxtapose self-assembly with and without hydrodynamic interactions between the particles. The long-ranged repulsion impacts the relative rates of coagulation and compaction of colloidal aggregates pre-gel, and introduces a surprising sensitivity to the nature of hydrodynamic interactions between the suspended colloids. For such SALR dispersions, we observe a significant disparity between the percolation boundaries predicted by simulations including and neglecting long-ranged hydrodynamic interactions. Additionally, we find that the percolation boundaries predicted by simulations including hydrodynamic interactions agree well with those measured experimentally. Long-ranged repulsion promotes gelation via growth of anisotropic clusters regardless of the hydrodynamic model employed. However, differences between the models, which persist far from the percolation boundary, are apparent via measurements of the fractal dimension, local bond order parameters, and the collective relaxation dynamics. Notably, the growth of elongated clusters is augmented in simulations that incorporate long-ranged hydrodynamic interactions due to the anisotropic diffusion of elongated bodies at low Reynolds numbers, which favors percolation over a transition of anisotropic clusters to their more isotropic ground states. It is only in relatively dense suspensions that a combination of hydrodynamic screening and significantly faster aggregation combine to bring the two simulation methods into agreement. These results demonstrate the necessity of long-ranged hydrodynamic forces in discrete element simulations of heterogeneous gelation at the colloidal scale.

Comparison of different coupling schemes between counterions and charged nanoparticles in multiparticle collision dynamics

We applied the multiparticle collision dynamics (MPC) simulation technique to highly asymmetric electrolytes in solution, i.e., charged nanoparticles and their counterions in a solvent. These systems belong to a domain of solute size which ranges between the electrolyte and the colloidal domains, where most analytical theories are expected to fail, and efficient simulation techniques are still missing. MPC is a mesoscopic simulation method which mimics hydrodynamics properties of a fluid, includes thermal fluctuations, and can be coupled to a molecular dynamics of solutes. We took advantage of the size asymmetry between nanoparticles and counterions to treat the coupling between solutes and the solvent bath within the MPC method. Counterions were coupled to the solvent bath during the collision step and nanoparticles either through a direct interaction force or with stochastic rotation rules which mimic stick boundary conditions. Moreover, we adapted the simulation procedure to address the issue of the strong electrostatic interactions between solutes of opposite charges. We show that the short-ranged repulsion between counterions and nanoparticles can be modeled by stochastic reflection rules. This simulation scheme is very efficient from a computational point of view. We have also computed the transport coefficients for various densities. The diffusion of counterions was found in one case to increase slightly with the volume fraction of nanoparticles. The deviation of the electric conductivity from the ideal behavior (solutes at infinite dilution without any direct interactions) is found to be strong.