Evaporation of Water Nanodroplets on Heated Surfaces: Does Nano Matter?

Abstract

While experiments and continuum models have provided a relatively good understanding of the evaporation of macroscopic water droplets, elucidating how sessile nanodroplets evaporate is an open question critical for advancing nanotechnological applications where nanodroplets can play an essential role. Here, using molecular dynamics simulations, we find that evaporating nanodroplets, in contrast to their macroscopic counterparts, are not always in thermal equilibrium with the substrate and that the vapor concentration on the nanodroplet surface does not reach a steady state. As a result, the evaporative behavior of nanodroplets is significantly different. Regardless of hydrophobicity, nanodroplets do not follow conventional evaporation modes but instead exhibit dynamic wetting behavior characterized by huge, non-equilibrium, isovolumetric fluctuations in the contact angle and contact radius. For hydrophilic nanodroplets, the evaporation rate, controlled by the vapor concentration, decays exponentially over time. Hydrophobic nanodroplets follow stretched exponential kinetics arising from the slower thermalization with the substrate. The evaporative half-lifetime of the nanodroplets is directly related to the thermalization time scale and therefore increases monotonically with the hydrophobicity of the substrate. Finally, the evaporative flux profile along the nanodroplet surface is highly nonuniform but does not diverge at the contact line as the macroscopic continuum models predict.

Chemical transformations and transport phenomena at interfaces

“…Therefore, to study evaporation, one solution has been to heat up the system up to temperatures typically in the range of 400-500 K, where the process proceeds fast enough to be observed during the time scale of the simulation. 8,274 Because thermostats introduce artifacts in the dynamics of the atoms during the simulation, care must be exercised when heating up the liquid. For sessile droplets (droplets on solid substrates), for example, it can be done by thermosetting a substrate while still integrating the dynamics of the liquid and vapor molecules in the microcanonical (NVE) ensemble.…”

“…As already mentioned, the evaporation of water, particularly at room temperature, is a slow process in relation to the duration of an MD simulation. Therefore, to study evaporation, one solution has been to heat up the system up to temperatures typically in the range of 400–500 K, where the process proceeds fast enough to be observed during the time scale of the simulation 8,274 . Because thermostats introduce artifacts in the dynamics of the atoms during the simulation, care must be exercised when heating up the liquid.…”

“…For that reason, some previous computational studies of evaporation defined an interphase region of finite thickness that contains molecules that are neither liquid nor vapor. 8 The benefit of such an approach is that the calculation of the number of molecules in the vapor as a function of time is less prone to the fast fluctuations that arise in the interphase region, thus allowing to characterize of the evaporation kinetics much more precisely. Other density-based criteria have also been used to define the interface of evaporating nanodroplets, such as, for example, using the mean between the maximum and minimum densities of the system.…”

“…While bulk water freezes at low temperatures, and depending on pressure, into different ice phases, it has been shown that water confined between graphene sheets can exhibit different phases and phase‐coexistence depending on the degree of flexibility of the 2D interface 6 . Finally, empirical studies indicate that the key to mass and heat transfer enhancement lies in controlling nanoscale features near the solid–liquid–vapor contact line via the surface chemistry and wettability 7,8 …”

“…6 Finally, empirical studies indicate that the key to mass and heat transfer enhancement lies in controlling nanoscale features near the solid-liquid-vapor contact line via the surface chemistry and wettability. 7,8 However, the underlying relationship between the molecular properties of the interface and the resulting emergent behavior and/or chemical functional outcome still needs further development. In particular, we seek to identify and classify the chemical features and physical properties of interfaces that focus on the specifics (charge, hydrophobicity, roughness, etc.…”