A team of researchers recently published a paper in the journal ACS Nano that investigated the evaporative behavior of water nanodroplets.
Study: Evaporation of Water Nanodroplets on Heated Surfaces: Does Nano Matter? Image Credit: LedyX/Shutterstock.com
Understanding the evaporation kinetics and wetting behavior of droplets on solid substrates is essential for several technological and engineering applications, such as evaporative cooling of electronics and inkjet printing.
Although the evaporative behavior of macroscopic droplets has been studied and understood extensively, such understanding of evaporative flux and kinetics and evaporation mode is absent in the case of nanodroplets.
At the nanoscale, several assumptions of the continuum models, based on which the evaporative behavior of the macroscopic droplets is predicted, break down. This increases the challenges of experimentally probing nanodroplets.
Thus, molecular dynamics (MD) simulations that do not depend on the assumptions of the continuum models were used to obtain insights into the wetting and evaporation behavior of water at the molecular level.
In this study, researchers investigated the evaporative behavior of water nanodroplets using MD simulations of water sessile nanodroplets on heated substrates that are chemically non-specific and compared the data from the study to the predictions of macroscopic models.
A large-scale atomic/molecular massively parallel simulator (LAMMPS) program was employed to perform all MD simulations in the present study.
The transferable intermolecular potential four-point (TIP4P-Ew) force field was used to model the water-water interactions, while the intramolecular bonded interactions in water were constrained using the SHAKE algorithm.
Two major simulations on water nanodroplets were performed. The first one was the evaporation of same size nanodroplets on substrates with different hydrophobicities heated at the same temperatures.
In the initial system, a semispherical nanodroplet with 2749 water molecules having a 3 nm radius was placed on the top of a non-chemically specific 1 nm thick substrate made of atoms organized in a face-centered cubic (FCC) structure.
The simulation box dimensions were 14 × 14 × 15 nm3 and the system consisted of an overall 21,501 atoms.
The second simulation was the equilibration of water nanodroplets with various sizes of 4985, 2763, and 1657 water molecules on substrates with dissimilar hydrophobicities heated at different temperatures of 400, 350, and 300 K.
The long-range Coulombic interactions were measured using the particle−particle−particle−mesh scheme, while the substrate-water and substrate-substrate interactions were modeled using a Lennard-Jones potential.
A time step of 2 fs was utilized in all simulations, while velocities and coordinates of the hydrogen and oxygen atoms were outputted every 2 ps for analysis.
The nanodroplet evaporation was stimulated on the substrate from very hydrophobic to very hydrophilic. The program MATrix LABoratory (MATLAB) was employed to perform analysis of the nanodroplets.
The evaporation time scale of the nanodroplets was of the same order for both the thermalization time between the nanodroplets and hot substrate and the diffusive relaxation of the vapor, indicating that the evaporating nanodroplets were not in thermal equilibrium with the substrate at every moment and the vapor concentration never attained a steady state.
These nanodroplets demonstrated a robust dynamic wetting behavior primarily caused by the thermal effects conforming with the traditional evaporation modes such as constant contact radius (CCR) or constant contact angle (CCA).
With the progression of evaporation and the increasing nanodroplet temperature, hydrophilic and hydrophobic nanodroplets became more hydrophilic and hydrophobic, respectively, and the nanodroplets that were most hydrophobic desorbed from the heated substrate before undergoing complete evaporation.
Evaporating nanodroplets displayed substantial non-equilibrium fluctuations in the contact radius and contact angle that were isovolumetric and concerted.
Such fluctuations are often negligible in macroscopic droplets.
No similarity was observed in the evaporation kinetics of macroscopic droplets and nanodroplets as nanodroplets did not follow the d2 law.
In the case of hydrophilic nanodroplets, the evaporation was primarily influenced by the instantaneous vapor concentration, which resulted in an exponential decay in the rate of evaporation over time.
In the case of hydrophobic nanodroplets, both evaporation and slow thermalization with the substrate was observed in the same time scale, indicating stretched exponential kinetics where the maximum rate of evaporation was observed at intermediate times.
In contrast to the predictions for macroscopic droplets, the evaporative lifetime of the studied nanodroplets was directly proportional to the thermalization time scale that increases monotonically with hydrophobicity owing to the reduced contact area between the substrate and the nanodroplet.
For hydrophilic nanodroplets, the evaporative flux was highest at the contact line, similar to the predictions for macroscopic droplets.
The flux was null for hydrophobic nanodroplets at the contact line and attained the highest value at the great circle, which was also qualitatively similar to the macroscopic predictions.
Significance of the Study
To summarize, the findings of this study demonstrated that the evaporative behavior of the water nanodroplets is considerably different from the macroscopic droplets.
Additionally, the mechanistic understanding of sessile nanodroplet evaporation gained in this study will be crucial for interpreting future studies on nanodroplet evaporation, and to guide interventions for modulating the evaporative behavior of nanodroplets in nanotechnological applications that involve evaporation of water nanodroplets.
Pestana, L.R., Head-Gordon, T. (2022) Evaporation of Water Nanodroplets on Heated Surfaces: Does Nano Matter?. ACS Nano. Avialable at: https://pubs.acs.org/doi/10.1021/acsnano.1c10244