Mechanism for Asymmetric Nanoscale Electrowetting of an Ionic Liquid on Graphene

Abstract

The electrowetting behavior of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) confined between two oppositely charged graphene layers is investigated using molecular dynamics simulations. By introducing charges on the surface, counterions are attracted to the surface and co-ions are repelled from it, leading to the reduction of the solid-liquid interfacial free energy and consequently the contact angle. Recently, we have shown that changes in the contact angle upon charging the surface are asymmetric with respect to surface polarity and opposite to the changes in the solid-liquid interfacial free energy. In this work, the asymmetry of the solid-liquid interfacial free energy is shown to originate from differences in structural organization of the ions at the interface, with positively polarized surfaces inducing a more favorable electrostatic arrangement of the ions. Analysis of the liquid structure in the vicinity of the three phase contact line, however, shows that the ion size asymmetry, together with differences in orientational ordering of the cations on oppositely polarized surfaces, instead leads to enhanced spreading on the negatively polarized surfaces, resulting in a corresponding contact angle asymmetry.

Capillary evaporation of the ionic liquid [EMIM][BF4] in nanoscale solvophobic confinement

“…(2)]. 63,[80][81][82] We conclude by noting that small changes in the flexibility of confining plates, not included here, can significantly impact the thermodynamics and kinetics of capillary evaporation 60 and can be used to further tune solvophobic effects in IL-based systems.…”

Molecular Dynamics Simulations of the Influence of Drop Size and Surface Potential on the Contact Angle of Ionic-Liquid Droplets

“…Herrara et al performed recent molecular dynamics (MD) simulations of the amino-acid IL 1-ethyl-3-methylimidazolium glycine (EMIM-GLY) and reported 40 degree variation in contact angle with the size of the nanodrops (from 100 to 500 IL pairs) on highly attractive graphene sheets. Taherian et al used MD simulations for an electrowetting study of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF 4 ) ions bridging between graphene surfaces of opposing and fixed Coulombic charges. In their simulations, ions were bonding with each other in one of the two dimensions parallel to the sheets, such that the IL was continuous in this direction and formed a pillar-like bridge between electrodes.…”

Molecular insights into the electrowetting behavior of aqueous ionic liquids

“…Taherian et al [24] used MD simulation to calculate 2D contact angles of [BMIM][BF4] on positive and negative charged graphene surfaces. They found an asymmetry in the electrowetting aspect of the IL.…”