Surfactants are amphiphilic compounds that can be used to control the evaporation rate on a liquid-vapor interface. Previous work has shown that molecular monolayers can reduce the evaporation rate on a flat liquid-vapor interface, while surfactant-modified sessile droplets show an increased evaporation rate due to changes in their wetting properties. A surfactant-induced increase in evaporation rate on a flat interface has yet to be demonstrated.

In this project, we applied molecular dynamics (MD) simulations to study the evaporation of a Lennard-Jones (LJ) liquid with a variety of surfactants. A non-equilibrium MD (NEMD) simulation with a temperature difference across a solid/liquid/vapor/liquid/solid system was designed to directly extract the evaporation mass flux. The solid atoms have the same mass and length scale as the fluid but an energy scale ten times larger. Periodic boundary conditions are applied in the two directions parallel to the liquid-vapor interfaces. In the direction normal to the interface, two layers of atoms at each end of the system serve as fixed boundaries. 

Surfactants are added to the evaporating liquid-vapor interface. The mass and energy scales of the surfactant atoms are artificially adjusted so as to keep the surfactants on the liquid-vapor interface. Bonds and angles in the surfactants are set to be harmonic, and all angles have an equilibrium value of 180 degrees. Their strengths are carefully tuned based on the LJ potential. 

We found that an anchor-shaped surfactant with a flat hydrophobic top and attached hydrophilic chains enhances the evaporation rate by a factor of two compared to the pure liquid. Afterwards, additional equilibrium MD simulations of the pure liquid and liquid-surfactant systems were also performed. Thermal conductivities predicted using the Green-Kubo method show an increase in that for the liquid-surfactant mixture compared to the pure liquid. To identify the origin of this unexpected result, we calculated the diffusion coefficient of the evaporating bulk liquid layer for the case with and without the surfactants. The results show that the case with surfactants have a smaller diffusion coefficient than the case with pure fluid, indicating differences in liquid structures when the anchor-shaped surfactants are present. Vibrational spectra calculated from the velocity autocorrelation function is also obtained in the NEMD system. We found an increase in the liquid structure near the created surfactant, which may lead to a lower thermal resistance. Different surfactant sizes and structures are currently being investigated to further elucidate the origin of the enhanced evaporation rate.

 


Anchor-Shaped Surfactants Enhance Evaporation on a Flat Liquid Interface

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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