How does the electric field make a droplet exhibit the ejection and rebound behaviour on a superhydrophobic surface?

Abstract

A droplet impinging on a superhydrophobic substrate in an electric field is an important process in droplet manipulation and electrostatic spraying. Here, the entire impinging dynamic of the droplet in a vertical electric field is studied by a visualization experiment and numerical simulation with OpenFOAM. We investigate the effect of an electrostatic force on droplet impact in depth, where four ejection modes and three rebound modes are found experimentally. In particular, the filamentous ejecting phenomenon occurs after a droplet impinging on a superhydrophobic substrate is first discovered. In the numerical simulation, the strong coupling between the dynamic distribution of the interface electric charge and the evolution of the droplet profile can lead to different ejection modes, and the different ejection behaviours are caused by the combined effects of electrostatic pressure, capillary pressure, dynamic pressure and static pressure on the droplet apex. A charge scaling law for the ejection droplets is proposed. Furthermore, a set of theoretical models is established, which can successfully predict the threshold electric capillary number for different droplet ejection modes. The results reveal some important characteristics for a droplet impinging on a superhydrophobic surface in an electric field, which could facilitate the design of electrically operated droplet equipment and guide the safe and stable operation of the device.

Efficient fabrication of bioinspired soft, ridged-slippery surfaces with large-range anisotropic wettability for droplet manipulation

“…20 In addition, the vulnerability of microstructures and nanostructures and the instability of separation air cushions in them on these surfaces can lead to an undesired conversion of droplets from the Cassie state to the Wenzel state, especially when exposed to liquid impact, abrasion, and moisture environments. 21,22 Unlike superhydrophobic surfaces, Nepenthes pitcher plantinspired slippery surfaces have a firmly attached insoluble lubricant between the droplet and the solid surface, offering a more adaptable platform for manipulating droplets. [23][24][25] The lubricant injection ensures excellent repellence to various liquids with low contact angle hysteresis, while the substantial interfacial tension between the droplet and the lubricant prevents droplets from hard-controlled detaching.…”

“…20 In addition, the vulnerability of microstructures and nanostructures and the instability of separation air cushions in them on these surfaces can lead to an undesired conversion of droplets from the Cassie state to the Wenzel state, especially when exposed to liquid impact, abrasion, and moisture environments. 21,22…”

Mosaic Patterned Surfaces toward Generating Hardly‐Volatile Capsular Droplet Arrays for High‐Precision Droplet‐Based Storage and Detection

“…[1][2][3][4][5][6][7][8][9][10][11][12] Nowadays, inspired by the "dorsum of the Namib desert beetle," creating droplet arrays with a specific geometry and volume on heterogeneous patterned surfaces has been developed as an efficient method to facilitate high-throughput and programmable on-surface droplets processing. [13][14][15][16][17] The droplet repellency from the superhydrophobic (SHB) or superamphiphobic (SAB) backgrounds and the droplet anchoring property from the hydrophilic or amphiphilic regions can be integrated into a surface. The heterogeneous wettability surface-based method has remarkable extendibility, addressability, and accessibility, showing more controllable interactions with droplet arrays than homogeneous surfaces.…”

Electrohydrodynamic-induced partial coalescence between a droplet and a liquid–air interface

“…Previous studies usually converted the surface charge to the volume charge as a simplification (Das, Dalal & Tomar 2021; Tian et al. 2022), since it is too difficult to directly solve the jumps of stresses at the interface by the two methods. The electric volumetric forces are then included in the momentum conservation equation as a numerical treatment.…”

“…For a typical leaky-dielectric fluid (Saville 1997), the density of charge is zero in the bulk but both tangential and normal electric stresses appear at the two-phase interface to balance with the viscous stress. Previous studies usually converted the surface charge to the volume charge as a simplification (Das, Dalal & Tomar 2021;Tian et al 2022), since it is too difficult to directly solve the jumps of stresses at the interface by the two methods. The electric volumetric forces are then included in the momentum conservation equation as a numerical treatment.…”