Droplet formation and growth inside a polymer network: A molecular dynamics simulation study.

We present a molecular dynamics simulation study that focuses on the formation and growth of nanoscale droplets inside polymer networks. Droplet formation and growth are investigated by the liquid-vapor phase separation of a dilute Lennard-Jones (LJ) fluid inside regularly crosslinked, polymer networks with varying mesh sizes. In a polymer network with small mesh sizes, droplet formation can be suppressed, the extent of which is dependent on the attraction strength between the LJ particles. When droplets form in a polymer network with intermediate mesh sizes, subsequent growth is significantly slower when compared with that in bulk without a polymer network. Interestingly, droplet growth beyond the initial nucleation stage occurs by different mechanisms depending on the mesh size: droplets grow mainly by diffusion and coalescence inside polymer networks with large mesh sizes (as observed in bulk), whereas Ostwald ripening becomes a more dominant mechanism for droplet growth for small mesh sizes. The analysis of droplet trajectories clearly reveals the obstruction effect of the polymer network on the movement of growing droplets, which leads to Ostwald ripening of droplets. This study suggests how polymer networks can be used to control the growth of nanoscale droplets.

In-layer stacking competition during ice growth.

When ice grows, the growth rates are unequal [corrected] along different growth directions and some layers contain planar defective regions. With the aim of helping to understand these phenomena, we report the molecular dynamics simulations of ice growth on the basal and prismatic faces of initial hexagonal ice, using the TIP5P-E water model. By presenting the time evolution of the two-dimensional density profiles of water molecules in each layer and the kinetics of layer formation during ice growth at the temperature of 11 K supercooling, we show that two forms of ice arrangements, hexagonal and cubic, develop competitively within the same ice layer on the basal face, whereas such in-layer stacking-competition is insignificant on the prismatic face. It is shown that, on the basal face, the occurrence of significant in-layer stacking competition in one of the layers significantly delays the layer formation in several overlying layers and explains the overall delay in ice growth on the basal face compared to that on the prismatic face. In addition, it is observed that large planar defects form on the basal face, as a consequence of the long-lasting in-layer stacking competition when the overlying layer grows rapidly.

Understanding anisotropic growth behavior of hexagonal ice on a molecular scale: a molecular dynamics simulation study.

Although distinct growth behaviors on different faces of hexagonal ice have long been suggested, their understanding on a molecular scale has been hampered due to experimental difficulties near interfaces. We present a molecular dynamics simulation study to unravel the molecular origin of anisotropy in the growth kinetics of hexagonal ice by visualizing the formation of transient water structures in the growing ice interface. During ice growth, the formation of transient structures and their rearrangement to the final ice configuration are observed irrespective of growth direction. However, we find that their structure and duration differ significantly depending on growth direction. In the direction perpendicular to the basal face of hexagonal ice along which growth occurs most slowly, a two-dimensional transient structure, which is formed by competing hexagonal and cubic arrangements within the same layer, persists for a significant period of time, contrasted with short-lived transient structures in other directions. This observation of such transient water structures and their rearrangement during ice growth provides a clear explanation of different growth rates on each face of hexagonal ice on a molecular scale.

Vesicle-like assemblies of ligand-stabilized nanoparticles with controllable membrane composition and properties.

Here, we report that nanoparticles modified with simple end-functionalized alkyl thiol ligands show interesting directional self-assembly behavior and can act as an effective surfactant to encapsulate other functional molecules and nanoparticles. Gold nanoparticles modified with the mixture of alkyl thiols and hydroxyl-terminated alkyl thiols organize into unique vesicle-like structures with controllable membrane thicknesses. Molecular dynamics simulations showed that the ligand segregation and the edge-to-edge ligand binding are responsible for the two-dimensional assembly formation. Furthermore, the nanoparticle assemblies can encapsulate other functional nanoparticles into the membrane or inside the cavity, generating multicomponent inorganic vesicles with various morphologies. The light-induced release profiles of encapsulated dye molecules showed that the membrane properties can be controlled by varying the membrane thickness and ligand composition.