Jiayao Wu

The controllable transport of droplets on solid surfaces is crucial for many applications, from water harvesting to bio-analysis. Herein, we propose a novel droplet transport controlling method, reconfigurable orbital electrowetting (ROEW) on inclined slippery liquid-infused porous surfaces (SLIPS), which enables controllable transport and dynamic handling of droplets by non-contact reconfiguration of orbital electrodes. The flexible reconfigurability is attributed to the non-contact wettability modulation and reversibly deformable flexible electrodes. ROEW graphically customizes stable wettability pathways by real-time and non-contact printing of charge-orbit patterns on SLIPS to support the continuous transport of droplets. Benefiting from the fast erase-writability of charges and the movability of non-contact electrodes, ROEW enables reconfiguration of the wetting pathways by designing electrode shapes and dynamically switching electrode configurations, achieving controllable transport of various pathways and dynamic handling of droplet sorting and mixing. ROEW provides a new approach for reconfigurable, electrode-free arrays and reusable microfluidics.

Droplet manipulation on open surfaces plays a key role in numerous applications. The enhancement in controllability, flexibility, and simplicity of droplet manipulation is sustainably demanded and developed. Herein, we create a unique droplet manipulation method by modulating wettability based on surface charges. With contactless charge deposition on tilting dielectric surfaces with the corona discharge generated by needle-plate electrodes, droplet sliding can be actuated in real time. Droplets show wettability enhancement after eliminating charge deposition, in which contact angle decreases reach up to ∼20°. The presented surface-charge-modulated wettability results in fast-speed (∼14 mm/s) and high-volume-range (10–80 μL) droplet manipulation. Experiments and theoretical analysis reveal the underlying mechanism of droplet actuation is surface charge gradients, which induce asymmetric electrostatic forces. Controlling charge distributions on dielectric surfaces with plate-electrode patterns, programmable, and reconfigurable droplet sliding along complex pathways can be achieved. Moreover, a droplet sliding value for controlling stepwise chemical reactions is demonstrated by moving plate electrodes.

Composite materials with precisely controlled particle assembly show exciting properties and enable numerous new applications. However, it is challenging to manipulate particle patterns in a programming mean during composite fabrication. In this study, we propose a novel electric-field-based method for particle assembly. In such a method, charges generated in air by corona discharge are injected into the liquid polymer and attached to particles, and charged particles are directionally driven by the electric field to form assembled patterns. Compared to the conventional electric-field-based particle assembly methods, the presented method eliminates the requirement of the particle’s electric properties, thus extending the potential applications. Moreover, enhancing particle charge amount can lead to high-efficiency particle assembly. By controlling the charge injection area with masks or tuning the electric field with the patterned electrodes, programmable particle assembly is achieved in a low-cost way. As a demonstration of the application, the proposed method is implemented in LED packaging, wherein phosphor particles are controllably deposited on a 1mm × 1mm LED chip in the silicone gel, which achieves directional sedimentation. The optical performances are enhanced significantly, especially in angle color uniformity (ACU) and luminous efficiency. Furthermore, this new charge-injection-based particle assembly method shows broad application prospects in large-scale composite manufacture and high-precision electronic device packaging.

The electric field is considered an effective stimulus to control droplet bounce or adhesion on demand on solid surfaces, which is important for various applications, including water harvesting and oil/water separation. However, it remains challenging to switch droplet bouncing/adhering on electrode surfaces smartly. Herein, we present a smart control method for droplet bouncing on dielectric surfaces by coupling charge transfer with contact electrification and a uniform electric field. Subject to electric fields, water droplets carrying the like charges in the insulating silicone oil present electric field-direction-dependent impact behaviors, bifurcating into bouncing and adhesion on hydrophobic and hydrophilic surfaces. Furthermore, oppositely charged water droplets with contact electrification led to contrary bouncing behaviors on hydrophobic and hydrophilic surfaces. The transfer charges and electric forces in the experiments are especially quantitively analyzed. By constructing dielectric pairs with hydrophobic and hydrophilic surfaces, reciprocating bouncing or selective adhesion can be modulated via switching electric field directions. This route of separately charging droplets and building electric fields facilitates droplet manipulation techniques and applications.