Transient evolution of electrowetting induced oscillating droplets on hydrophobic substrates

Axisymmetrical resonance modes in an electrowetting optical lens

Electrowetting-based adaptive optics are of great interest for applications ranging from confocal microscopy to LIDAR, but the impact of low-frequency mechanical vibration on these devices remains to be studied. We present a simple theoretical model for predicting the resonance modes induced on the liquid interface in conjunction with a numerical simulation. We experimentally confirm the resonance frequencies by contact angle modulation. They are found to be in excellent agreement with the roots of the zero-order Bessel functions of the first kind. Next, we experimentally verify that external axial vibration of an electrowetting lens filled with density mismatched liquids (Δρ = 250 kg/m³) will exhibit observable Bessel modes on the liquid-liquid interface. An electrowetting lens filled with density matched liquids (Δρ = 4 kg/m³) is robust to external axial vibration and is shown to be useful in mitigating the effect of vibrations in an optical system.

Dynamical behaviors of nanodroplets impinging on solid surfaces in the presence of electric fields

The collision of droplets with solid surfaces is a common phenomenon in nature. However, droplets exhibit interesting motion states when captured by surfaces. This work investigates the dynamical behavior and the wetting condition of droplets captured by different surfaces in electric fields via molecular dynamics (MD) simulations. By adjusting the initial velocity of droplets (V₀), electric field intensity (E) and directions, the spreading and wetting properties of droplets are analyzed systematically. The results indicate that the electric stretching effect occurs when a droplet strikes the solid surface in electric fields and the stretch length (h_t) of droplets continuously increases with the enhancement of E. In the low field strength regime, the direction of electric fields has an effect on h_t: the value of h_t is larger in the case of positive electric fields as compared to negative electric fields. In the high field strength regime, the direction of electric fields makes no difference to h_t: the droplet is stretched observably, and the breakdown voltage U is calculated to be 0.57 V nm^-1 under both positive and negative electric fields. Droplets impacting with surfaces at initial velocities display various states. The droplet bounces off the surface regardless of the direction of electric field at V₀ ≥ 1.4 nm ps^-1. The maximum spreading factor β_max and h_t both increase with V₀ and are not affected by field directions. The simulation results are consistent with experiments, and the relationships between E, β_max, h_t and V₀ are proposed, which provide the theoretical basis for large-scale numerical calculations such as computational fluid dynamics.