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Abstract The wetting effect and related applications of nanodroplets have attracted extensive research interest. In this work, we report molecular dynamics simulations of the confinement dynamics of nanodroplets at different droplet-surface interactions, surface separations, and ion concentrations under the electric field. The results show that in the case of a single droplet, there is a critical value of surface wettability for the shape transition of the confined droplet. Below this value, the droplet shrinks into a spherical shape and attaches to the surface. Above this value, the droplet splits into two smaller droplets which locate at the bottom and top surfaces, respectively. For the two-droplet cases, a liquid bridge forms and some ions migrate into the pure water droplet under the electric field. The results reveal that the ion concentration has a significant effect on the shape evolution of droplets. The stable morphology of droplets under the electric field depends on the droplet-surface interactions as well as the ion concentration. We explore the underlying mechanisms causing the morphological transition by analyzing the intermolecular interactions and interfacial characteristics. [ABSTRACT FROM AUTHOR]

• Transmission of molecular interactions through graphene can drive migration of droplets. • Enhancing electric field accelerates migration velocity of two droplets towards each other. • Dipole–dipole interactions of interfacial water enhance deflection of water dipoles. • Critical electric field to induce large deformation becomes lower for cases of two droplets. Understanding the physical mechanism of wetting transparency of monolayer graphene, or how atoms or molecules located on opposite sides of the atomically thin carbon monolayer transmit molecular interactions (such as van der Waals and electrostatic forces) is of significant importance for many extraordinary applications from scientific and practical perspectives. In the current work, we carried out molecular dynamics simulations to explore the interactions between two droplets (TD) separated by the monolayer graphene under applied electric field. The results indicate that in the absence of the electric field the transmission of molecular interactions can drive migration of two droplets towards each other, implying partial wetting transparency of the graphene. Compared to the cases of single droplet (SD), the critical electric field to induce large deformation is lower for the TD cases. However, the droplet–droplet interactions under strong electric fields suppress the reduction of the number of water molecules in the graphene/droplet interface and the elongation of droplets. To understand the enhancement of interactions between two droplets owing to the electric field, we analyze microscopic structures of interfacial water (dipole orientation distribution and hydrogen bonding). The dipole–dipole interaction through the graphene mainly contributes to the droplet–droplet interactions. It was found that significant deflection of water dipoles for the TD cases occurs as the electric field enhances, but the deflection for the SD cases is smaller and even the droplet is detached from the graphene surface. Our findings provide fundamental insights on the influence of the electric field on molecular interactions between two droplets through the monolayer graphene and clues for developing potential graphene-based nanofluidic applications controlled by external electric field. [ABSTRACT FROM AUTHOR]

The mechanisms governing the electrocoalescence between droplets are intricate owing to the presence of many influential parameters. In this work, we conduct molecular dynamics simulations to explore the effects of electric field strength and ionic concentration on the electrocoalescence behaviors of one pure water droplet and one salty droplet. The dynamic evolution is difierent from that for two identical droplets. The electric field strengths are chosen near the critical value. When the electric field strength is less than the critical value, the coalescence process is similar regardless of the ionic concentration of the salty droplet. The analysis on the mobility of ions demonstrates that the diffusion time of ions is much longer compared to the characteristic time of deformation of the coalesced droplet. At a stronger electric field than the critical field, the coalescence behavior exhibits dependence on the ionic concentration. For cases of high ionic concentrations, a rebounding behavior between two droplets is identified due to aggregation of ions close to the contact region of the droplets. Further, we also investigate molecular interaction energy, hydrogen bonding and ejection of daughter droplets during the electrocoalescence. Our study reveals that the coupling effect between the electric field and the ionic concentration plays an important role in controlling the electrocoalescence behavior of droplets. • Mechanisms of electrocoalescence between pure water and salty droplets are explored. • Coupling effect between electric field and ionic concentration plays a critical role. • Remarkable inconsistency between ion diffusion and droplet deformation is identified. [ABSTRACT FROM AUTHOR]