In this work, we address the nucleation behavior of a supercooled monatomic cylindrical water droplet on nanoscale textured surfaces using molecular dynamics simulations. The ice nucleation rate at 203 K on graphite based textured surfaces with nanoscale roughness is evaluated using the mean fast-passage time method. The simulation results show that the nucleation rate depends on the surface fraction as well as the wetting states. The nucleation rate enhances with increasing surface fraction for water in the Cassie-Baxter state, while contrary behavior is observed for the case of Wenzel state. Based on the spatial histogram distribution of ice formation, we observed two pathways for ice nucleation. Heterogeneous nucleation is observed at a high surface fraction. However, the probability of homogeneous ice nucleation events increases with decreasing surface fraction. We further investigate the role of the nanopillar height in ice nucleation. The nucleation rate is enhanced with increasing nanopillar height. This is attributed to the enhanced contact area with increasing nanopillar height and the shift in nucleation events towards the three-phase contact line associated with the nanotextured surface. The ice-surface work of adhesion for the Wenzel state is found to be 1-2 times higher than that in the Cassie-Baxter state. Furthermore, the work of adhesion of ice in the Wenzel state is found to be linearly dependent on the contour length of the droplet, which is in line with that reported for liquid droplets.

The wetting behavior of an ethanol−water droplet is investigated on graphitic smooth and rough surfaces using molecular dynamics simulations. On a smooth surface, ethanol molecules prefer to stay at the vapor−liquid and solid− liquid interfaces. The contact angle of a droplet on a smooth surface decreases with an increase in the ethanol concentration from 0 to 30 wt %. The corresponding line tension increases from 3 × 10 −11 to 9.4 × 10 −11 N at 300 K. The critical weight percentage for complete wetting is found to be approximately 50%. In the case of a textured graphite surface, with the addition of ethanol molecules, the Cassie− Baxter state of a drop is transformed into the Wenzel state via the partial Wenzel state, with ethanol molecules filling the rough region, leading to an increase in its wettability. A linear relation of 1 + cos θ with the roughness parameter associated with the Cassie−Baxter and Wenzel states is observed, indicating that the solid− liquid interfacial tension is directly proportional to the roughness parameter. This behavior is akin to that seen for the case of pure liquid. The hydrogen bonding and density profile are analyzed to understand the wetting states of the blended drop.

Potent ice nucleating organic crystals display an increase in nucleation efficiency with pressure and memory effect after pressurization that set them apart from inorganic nucleants. These characteristics were proposed to arise from an ordered water monolayer at the organic−water interface. It was interpreted that ordering of the monolayer is the limiting step for ice nucleation on organic crystals, rendering their mechanism of nucleation nonclassical. Despite the importance of organics in atmospheric ice nucleation, that explanation has never been investigated. Here we elucidate the structure of interfacial water and its role in ice nucleation at ambient pressure on phloroglucinol dihydrate, the paradigmatic example of outstanding ice nucleating organic crystal, using molecular simulations. The simulations confirm the existence of an interfacial monolayer that orders on cooling and becomes fully ordered upon ice formation. The monolayer does not resemble any ice face but seamlessly connects the distinct hydrogen-bonding orders of ice and the organic surface. Although large ordered patches develop in the monolayer before ice nucleates, we find that the critical step is the formation of the ice crystallite, indicating that the mechanism is classical. We predict that the fully ordered, crystalline monolayer nucleates ice above −2 °C and could be responsible for the exceptional ice nucleation by the organic crystal at high pressures. The lifetime of the fully ordered monolayer around 0 °C, however, is too short to account for the memory effect reported in the experiments. The latter could arise from an increase in the melting temperature of ice confined by strongly icebinding surfaces.

In this work, we have considered the crystallisation behaviour of supercooled water in the presence of surface defects of varying size (surface fraction, α from 1 to 0.5). Ice nucleation on Ag exposed β-AgI (0001 plane) surface is investigated by molecular dynamics simulation at a temperature of 240 K. For systems with α > 0.67, the surface layers crystallise within 150 ns. In the system with defects, we observe two distinct stacking patterns in the layers near the surface and find that systems with AA stacking cause a monotonic decrease in the early nucleation dynamics with an increase in defect size.Where AB stacking (α = 0.833) is observed, the effect of the defect is diminished and the dynamics are similar to the plain AgI surface. This is supported by the variation in the orientational dynamics, hydrogen bond network stability, and tetrahedrality with respect to the defects. We quantify results in terms of the network topology using double-diamond cages (DDCs) and hexagonal cages (HCs). The configurations of the initially formed layers of ice strongly affect the subsequent growth even at long timescales. We assert that the retarded ice growth due to defects can be explained by the relative increase in DDCs with respect to HCs.

The structure and dynamics of water droplets on a bilayer graphene surface are investigated using molecular dynamics simulations. The effects of solid/water and air/water interfaces on the local structure of water droplets are analyzed in terms of the hydrogen bond distribution and tetrahedral order parameter. It is found that the local structure in the core region of a water droplet is similar to that in liquid water. On the other hand, the local structure of water molecules at the solid/water and air/water interfaces, referred to as the interface and surface regions, respectively, consists mainly of three-coordinated molecules that are greatly distorted from a tetrahedral structure. This study reveals that the dynamics in different regions of the water droplets affects the intermolecular vibrational density of states: It is found that in the surface and interface regions, the intensity of vibrational density of states at ∼50 cm−1 is enhanced, whereas those at ∼200 and ∼500 cm−1 are weakened and redshifted. These changes are attributed to the increase in the number of molecules having fewer hydrogen bonds in the interface and surface regions. Both single-molecule and collective orientation relaxations are also examined. Single-molecule orientation relaxation is found to be marginally slower than that in liquid water. On the other hand, the collective orientation relaxation of water droplets is found to be significantly faster than that of liquid water because of the destructive correlation of dipole moments in the droplets. The negative correlation between distinct dipole moments also yields a blueshifted libration peak in the absorption spectrum. It is also found that the water–graphene interaction affects the structure and dynamics of the water droplets, such as the local water structure, collective orientation relaxation, and the correlation between dipole moments. This study reveals that the water/solid and water/air interfaces strongly affect the structure and intermolecular dynamics of water droplets and suggests that the intermolecular dynamics, such as energy relaxation dynamics, in other systems with interfaces are different from those in liquid water.