Melting–Freezing Transition of Monolayer Water Confined by Phosphorene Plates

Adjustable high-speed and directional diffusion of water nanodroplets confined by graphene sheets

Diffusion of confined water is important in nanofluidic and other water transport systems. In this study, the diffusion of water nanodroplets confined by graphene sheets is investigated based on molecular dynamics simulations. We find that the confined water nanodroplets can achieve a high-speed and directional motion. The impact of the size of water nanodroplets and distance of graphene sheets on diffusion is studied. The results show that the diffusion of confined water nanodroplets is adjustable and the speed is about 3 orders of magnitude faster than that of the self-diffusing water molecules in liquid water. Subsequently, the most suitable morphology of confined nanodroplets for rapid movement is found. We also find that the direction of diffusion of confined water nanodroplets is affected by the thermal vibrations of carbon atoms. Finally, the interaction energy and friction coefficient between confined nanodroplets and graphene sheets are analyzed to give an insight into the fast and directional diffusion behaviors of water nanodroplets. Our results reveal that a variation in the structure of interfacial water molecules with the distance of graphene sheets is the key to the rapid movement of confined water nanodroplets. The phenomena reported here can enrich the knowledge of molecular mechanisms for nanoconfined water systems, and may stimulate more ideas for the rapid removal of confined water.

Building and Breaking Bonds by Homogenous Nucleation in Glass-Forming Melts Leading to Transitions in Three Liquid States

The thermal history of melts leads to three liquid states above the melting temperatures T_m containing clusters-bound colloids with two opposite values of enthalpy +Δε_lg × ΔH_m and -Δε_lg × ΔH_m and zero. All colloid bonds disconnect at T_n+ > T_m and give rise in congruent materials, through a first-order transition at T_LL = T_n+, forming a homogeneous liquid, containing tiny superatoms, built by short-range order. In non-congruent materials, (T_n+) and (T_LL) are separated, T_n+ being the temperature of a second order and T_LL the temperature of a first-order phase transition. (T_n+) and (T_LL) are predicted from the knowledge of solidus and liquidus temperatures using non-classical homogenous nucleation. The first-order transition at T_LL gives rise by cooling to a new liquid state containing colloids. Each colloid is a superatom, melted by homogeneous disintegration of nuclei instead of surface melting, and with a Gibbs free energy equal to that of a liquid droplet containing the same magic atom number. Internal and external bond number of colloids increases at T_n+ or from T_n+ to T_g. These liquid enthalpies reveal the natural presence of colloid-colloid bonding and antibonding in glass-forming melts. The Mpemba effect and its inverse exist in all melts and is due to the presence of these three liquid states.

Adjustable diffusion enhancement of water molecules in a nanoscale water bridge

The emergence of nanofluidics in the last few decades has led to the development of various applications such as water desalination, ultrafiltration, osmotic energy conversion, etc. In particular, understanding water molecule transport in nanotubes is of importance for designing novel ultrafiltration and filtering devices. In this paper, we use an electric field to form a nanoscale water bridge as an artificial water channel to connect two separate disjoint nanotubes by molecular dynamics simulations. The extended length of the water bridge under different electric field strengths could adjust the diffusion process of the water molecules crossing the two disjoint nanotubes and the diffusion coefficients could be remarkably enhanced up to 4 times larger than the value in bulk water. By analyzing the structure of the water bridge, it is found that the diffusion enhancement originates from the strengthened interactions and the increase of hydrogen bonds between the water molecules due to the restrained reorientation from the external electric field. Our result provides a promising insight for realizing an efficient mass transport between various disjoint nanochannels.

Disentangling the liquid phase exfoliation of two-dimensional materials: an "in silico" perspective

Liquid Phase Exfoliation (LPE) is one of the most successful synthetic roots for the preparation of two-dimensional (2D) materials from their bulk counterparts. In recent years, significant progress has been accomplished for the development and modification of LPE techniques. However, precise identification of the hierarchical steps of the molecular mechanism of LPE remains to some extent elusive. Additionally, the a priori choice of suitable solvents for successful exfoliation and dispersion of various layered materials poses a challenge for both academia and industry. Computational methods, particularly Molecular Dynamics (MD) simulations with classical force-fields have contributed a great deal towards the understanding of the underlying mechanism of LPE, providing remarkable insights into the molecular-level details of the solvent-material interactions at the nanoscale and predicting "good" and "bad" solvents for exfoliation as well as stabilization of the dispersed state. With an intention to build up a unified understanding, in this perspective article, we summarize the recent advancements of molecular simulation techniques employed to decipher the mechanism of LPE, pointing out the key features of molecular interactions and identifying several thermodynamic parameters governing the phenomena. In addition, we outline the necessary characteristics of solvent molecules, essential for their use as "good" solvents towards LPE. Also, we highlight the limitations of simulation methods for the modelling of LPE. We believe that this article will be beneficial for the selection of solvents for the synthesis of novel 2D materials via LPE and will also provide a comprehensive view to computational material scientists towards the development of novel simulation protocols for investigating and analysing such complex molecular events.

Improved Dreiding force field for single layer black phosphorus

We present an improved Dreiding force field for single layer black phosphorus (SLBP) obtained by first-principle calculations in conjunction with the particle swarm optimization algorithm and molecular dynamics (MD) simulations.

Phase transition-like behavior of the water monolayer close to the polarized surface of a nanotube

By molecular dynamics simulations, we have investigated effects of temperature on the dynamical behavior of water layers at the charged surface of a nanotube. The behavior of the first water monolayer at the charged surface is very different from that of bulk water. There are three different temperature regions for the axial diffusion coefficient and they increase in different ways (linearly or exponentially) with temperature. The dipole distribution of water molecules was chosen as the order parameter to analyze the phase transition-like behavior. The simulation results indicate that the transition from ordered water to disordered water is continuous, which has not been found in the bulk counterpart. The mechanism behind the unexpected phenomenon was also investigated.