Droplet Rolling Transport on Hydrophobic Surfaces Under Rotating Electric Fields: A Molecular Dynamics Study

Abstract

Driving droplets by electric fields is usually achieved by controlling their wettability, and realizing a flexible operation requires complex electrode designs. Here, we show by molecular dynamics methods the droplet transport on hydrophobic surfaces in a rolling manner under a rotating electric field, which provides a simpler and promising way to manipulate droplets. The droplet internal velocity field shows the rolling mode. When the contact angle on the solid surface is 144.4°, the droplet can be transported steadily at a high velocity under the rotating electric field (E = 0.5 V nm −1 , ω = π/20 ps −1 ). The droplet center-of-mass velocities and trajectories, deformation degrees, dynamic contact angles, and surface energies were analyzed regarding the electric field strength and rotational angular frequency. Droplet transport with a complex trajectory on a two-dimensional surface is achieved by setting the electric field, which reflects the programmability of the driving method. Nonuniform wettability stripes can assist in controlling droplet trajectories. The droplet transport on the three-dimensional surface is studied, and the critical conditions for the droplet passing through the surface corners and the motion law on the curved surface are obtained. Droplet coalescence has been achieved by surface designs.

Dynamics of Droplets Moving on Lubricated Polymer Brushes

“…The receding contact angle decreases roughly linearly as a function of v , while the advancing angle remains roughly constant at a value close to the equilibrium angle. In previous molecular dynamics simulations of rolling droplets on hydrophobic surfaces or patterned superhydrophobic surfaces, and also of oil droplets that slide on polymer brush surfaces driven by shear flow, both the advancing and the receding angle were found to deviate strongly from the equilibrium value. In these examples, the surfaces were either hard or soft but with negative spreading coefficient S , i.e., a Neumann construction is possible.…”