Despite the industrial implications and worldwide abundance of gas hydrates, the formation mechanism of these compounds remains poorly understood. We report direct molecular dynamics simulations of the spontaneous nucleation and growth of methane hydrate. The multiple-microsecond trajectories offer detailed insight into the process of hydrate nucleation. Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages. The early cages are mostly face-sharing partial small cages, favoring structure II; however, larger cages subsequently appear as a result of steric constraints and thermodynamic preference for the structure I phase. The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are linked by uncommon 5(12)6(3) cages that facilitate structure coexistence without an energetically unfavorable interface.

[1] We propose that the recently conceptualized suction stress characteristic curve represents the effective stress for the shear strength behavior of unsaturated soil. Mechanically, suction stress is the interparticle stress called tensile stress. The working hypothesis is that the change in the energy of soil water from its free water state is mostly consumed in suction stress. We demonstrate that the suction stress lies well within the framework of continuum mechanics where free energy is the basis for any thermodynamic formulation. Available experimental data on soil water characteristic curves and suction stress characteristic curves are used to test the hypothesis, thus validating a closed-form equation for effective stress in unsaturated soil. The proposed closed-form equation is intrinsically related to the soil water characteristic curve by two pore parameters: the air entry pressure and pore size spectrum number. Both semiquantitative and quantitative validations show that the proposed closed-form equation well represents effective stress for a variety of earth materials ranging from sands to clays. Of important practical implications are (1) the elimination of the need for any new shear strength criterion for unsaturated soil, (2) the elimination of the need for determining the Bishop's effective stress parameter c because the new form of effective stress is solely a function of soil suction, and (3) the ready extension of all classical soil mechanics work on limit equilibrium analysis to unsaturated soil conditions.

Fundamental understanding of gas hydrate formation and decomposition processes is critical in many energy and environmental areas and has special importance in flow assurance for the oil and gas industry. These areas represent the core of gas hydrate applications, which, albeit widely studied, are still developing as growing fields of research. Discovering the molecular pathways and chemical and physical concepts underlying gas hydrate formation potentially can lead us beyond flowline blockage prevention strategies toward advancing new technological solutions for fuel storage and transportation, safely producing a new energy resource from natural deposits of gas hydrates in oceanic and arctic sediments, and potentially facilitating effective desalination of seawater. The state of the art in gas hydrate research is leading us to new understanding of formation and dissociation phenomena that focuses on measurement and modeling of time-dependent properties of gas hydrates on the basis of their well-established thermodynamic properties.

Methane hydrate nucleation rates are reported from over 200 μs of molecular dynamics simulations across a range of thermodynamic conditions and varying degrees of methaneÀwater interfacial curvature. Calculated nucleation rates increase with aqueous phase methane concentration (X CH 4 ), consistent with experimental results. The effect of interfacial curvature on X CH 4 is quantified, with dissolved methane concentration increasing with the degree of curvature (i.e., the number of dimensions in which curvature exists). Nucleation rates are reported for system sizes of 3456 and 13 824 molecules (H 2 O + CH 4 ). Among the smaller simulation systems (which comprise the majority of the data), the calculated hydrate nucleation rates follow the same trend when plotted against X CH 4 regardless of whether the predominant contribution to the effective system pressure is the simulation barostat or the methaneÀwater interfacial curvature (YoungÀLaplace pressure). The incipient hydrate nuclei are destabilized in the immediate vicinity (∼ 1 nm) of the methaneÀwater interface, and the calculated nucleation rates for the larger simulation systems (in which the incipient hydrate solids are less affected by interfacial destabilization) fall above the trend observed in the smaller systems.

Chiral molecules with opposite handedness exhibit distinct physical, chemical, or biological properties. They pose challenges as well as opportunities in understanding the phase behavior of soft matter, designing enantioselective catalysts, and manufacturing single-handed pharmaceuticals. Microscopic particles, arranged in a chiral configuration, could also exhibit unusual optical, electric, or magnetic responses. Here we report a simple method to assemble achiral building blocks, i.e., the asymmetric colloidal dimers, into a family of chiral clusters. Under alternating current electric fields, two to four lying dimers associate closely with a central standing dimer and form both right-and left-handed clusters on a conducting substrate. The cluster configuration is primarily determined by the induced dipolar interactions between constituent dimers. Our theoretical model reveals that in-plane dipolar repulsion between petals in the cluster favors the achiral configuration, whereas outof-plane attraction between the central dimer and surrounding petals favors a chiral arrangement. It is the competition between these two interactions that dictates the final configuration. The theoretical chirality phase diagram is found to be in excellent agreement with experimental observations. We further demonstrate that the broken symmetry in chiral clusters induces an unbalanced electrohydrodynamic flow surrounding them. As a result, they rotate in opposite directions according to their handedness. Both the assembly and propulsion mechanisms revealed here can be potentially applied to other types of asymmetric particles. Such kinds of chiral colloids will be useful for fabricating metamaterials, making model systems for both chiral molecules and active matter, or building propellers for microscale transport.hirality is a fundamental concept presented ubiquitously in the molecular world. For example, small molecules such as the amino acids, phospholipids, and sugars with specific handedness build many biomacromolecules whose chirality is essential to living organisms. Although the right-and left-handed molecules are identical in chemical composition, the catalytic activity (1), pharmacological impact (2, 3), biological recognition (4), and optical response (5) can be strikingly different. Extending the chiral structure to microscopic objects such as colloids has become increasingly desirable for several reasons. First, the chiral arrangement of colloidal particles can exhibit unusually strong optical, electric, and magnetic responses (6-10) that are not manifested either at the single-particle level or in achiral forms. Therefore, chiral clusters can be potentially used to build metamaterials (11-14) with exotic properties or sensors for detection of molecules. Second, chiral particles can be conveniently characterized by real-time optical microscopy. As the macroscopic analog of chiral molecules, they can be used to study fundamental questions related to the crystallization (15) or enantiomeric separation (16, 17) of a racemic...