Stratification in binary colloidal mixtures was investigated using implicit-solvent molecular dynamics simulations. For large particle size ratios and film Péclet numbers greater than unity, smaller colloids migrated to the top of the film, while big colloids were pushed to the bottom, creating an "inverted" stratification. This peculiar behavior was observed in recent simulations and experiments conducted by Fortini et al. [ Phys. Rev. Lett. 2016 , 116 , 118301 ]. To rationalize this behavior, particle size ratios and drying rates spanning qualitatively different Péclet number regimes were systematically studied, and the dynamics of the inverted stratification were quantified in detail. The stratified layer of small colloids was found to grow faster and to larger thicknesses for larger size ratios. Interestingly, inverted stratification was observed even at moderate drying rates where the film Péclet numbers were comparable to unity, but the thickness of the stratified layer decreased. A model based on dynamical density functional theory is proposed to explain the observed phenomena.

We studied the directed assembly of soft nanoparticles through rapid micromixing of polymers in solution with a nonsolvent. Both experiments and computer simulations were performed to elucidate the underlying physics and to investigate the role of various process parameters. In particular, we discovered that no external stabilizing agents or charged end groups are required to keep the colloids separated from each other when water is used as the nonsolvent. Furthermore, the size of the nanoparticles can be reliably tuned through the mixing rate and the ratio between polymer solution and nonsolvent. Our results demonstrate that this mechanism is highly promising for the mass fabrication of uniformly sized colloidal particles, using a wide variety of polymeric feed materials.

The structure of neat melts of polymer-grafted nanoparticles (GNPs) is studied via coarse-grained molecular dynamics simulations. We systematically vary the degree of polymerization and grafting density at fixed nanoparticle (NP) radius and study in detail the shape and size of the GNP coronas. For sufficiently high grafting density, chain sections close to the NP core are extended and form a dry layer. Further away from the NP, there is an interpenetration layer, where the polymer coronas of neighboring GNPs overlap and the chain sections have almost unperturbed conformations. To better understand this partitioning, we develop a two-layer model, representing the grafted polymer around an NP by spherical dry and interpenetration layers. This model quantitatively predicts that the thicknesses of the two layers depend on one universal parameter, x, the degree of overcrowding of grafted chains relative to chains in the melt. Both simulations and theory show that the chain extension free energy is nonmonotonic with increasing chain length at a fixed grafting density, with a well-defined maximum. This maximum is indicative of the crossover from the dry layer-dominated to interpenetration layer-dominated regime, and it could have profound consequences on our understanding of a variety of anomalous transport properties of these GNPs. Our theoretical approach therefore provides a facile means for understanding and designing solvent-free GNP-based materials.

When a colloidal suspension droplet evaporates from a solid surface, it leaves a characteristic deposit in the contact region. These deposits are common and important for many applications in printing, coating, or washing. By the use of superamphiphobic surfaces as a substrate, the contact area can be reduced so that evaporation is almost radially symmetric. While drying, the droplets maintain a nearly perfect spherical shape. Here, we exploit this phenomenon to fabricate supraparticles from bidisperse colloidal aqueous suspensions. The supraparticles have a core–shell morphology. The outer region is predominantly occupied by small colloids, forming a close-packed crystalline structure. Toward the center, the number of large colloids increases and they are packed amorphously. The extent of this stratification decreases with decreasing the evaporation rate. Complementary simulations indicate that evaporation leads to a local increase in density, which, in turn, exerts stronger inward forces on the larger colloids. A comparison between experiments and simulations suggest that hydrodynamic interactions between the suspended colloids reduce the extent of stratification. Our findings are relevant for the fabrication of supraparticles for applications in the fields of chromatography, catalysis, drug delivery, photonics, and a better understanding of spray-drying.

Drying polymer-polymer and colloid-polymer mixtures were studied using Langevin dynamics computer simulations. Polymer-polymer mixtures vertically stratified into layers, with the shorter polymers enriched near the drying interface and the longer polymers pushed down toward the substrate. Colloid-polymer mixtures stratified into a polymer-on-top structure when the polymer radius of gyration was comparable to or smaller than the colloid diameter, and a colloid-on-top structure otherwise. We also developed a theoretical model for the drying mixtures based on dynamical density functional theory, which gave excellent quantitative agreement with the simulations for the polymer-polymer mixtures and qualitatively predicted the observed polymer-on-top or colloid-on-top structures for the colloid-polymer mixtures.

We performed fluorescence correlation spectroscopy measurements to assess the long-time self-diffusion of a variety of spherical tracer particles in periodic porous nanostructures. Inverse opal structures with variable cavity sizes and openings in the nanometer domain were employed as the model system. We obtained both the exponent of the scaling relation between mean-square displacement and time and the slow-down factors due to the periodic confinement for a number of particle sizes and confining characteristics. In addition, we carried out Brownian dynamics simulations to model the experimental conditions. Good agreement between experimental and simulation results has been obtained regarding the slow-down factor. Fickian diffusion is predicted and seen in almost all experimental systems, while apparent non-Fickian exponents that show up for two strongly confined systems are attributed to polydispersity of the cavity openings. The utility of confining periodic porous nanostructures holds promise toward understanding of constrained diffusion with a wide range of applications ranging from water purification and drug delivery to tissue engineering.

In this contribution, we develop a coarse-graining methodology for mapping specific block copolymer systems to bead-spring particle-based models. We map the constituent Kuhn segments to Lennard-Jones particles, and establish a semi-empirical correlation between the experimentally determined Flory-Huggins parameter χ and the interaction of the model potential. For these purposes, we have performed an extensive set of isobaric-isothermal Monte Carlo simulations of binary mixtures of Lennard-Jones particles with the same size but with asymmetric energetic parameters. The phase behavior of these monomeric mixtures is then extended to chains with finite sizes through theoretical considerations. Such a top-down coarse-graining approach is important from a computational point of view, since many characteristic features of block copolymer systems are on time and length scales which are still inaccessible through fully atomistic simulations. We demonstrate the applicability of our method for generating parameters by reproducing the morphology diagram of a specific diblock copolymer, namely, poly(styrene-b-methyl methacrylate), which has been extensively studied in experiments.

The interplay of nematic order and phase separation in solutions of semiflexible polymers in solvents of variable quality is investigated by density functional theory (DFT) and molecular dynamics (MD) simulations. We studied coarse-grained models, with a bond-angle potential to control chain stiffness, for chain lengths comparable to the persistence length of the chains. We varied both the density of the monomeric units and the effective temperature that controls the quality of the implicit solvent.For very stiff chains only a single transition from an isotropic fluid to a nematic is found, with a phase diagram of "swan-neck" topology. For less stiff chains, however, also unmixing between isotropic fluids of different concentration, ending in a critical point, occurs for temperatures above a triple point. The associated critical behavior is examined in the MD simulations and found compatible with Ising universality.Apart from this critical behavior, DFT calculations agree qualitatively with the MD simulations.