The objective of this study is to propose a method for precisely manipulating droplets on a superhydrophobic surface prepared using femtosecond laser technology with the aid of electrostatic tweezers. Highlighting the limitations of current droplet control methods, particularly passive approaches, and the demand for active, non-contact techniques, this research aims to address these challenges. Using femtosecond laser processing, the polydimethylsiloxane (PDMS) substrate surface becomes superhydrophobic, featuring unique micro/nanostructures and extreme contact angles. Subsequently, the design of electrostatic tweezers induces charges in neutral droplets within the electric field generated by the tweezers, thus exerting driving forces on the droplets to achieve controlled motion. Through electrostatic-tweezer-driven droplet manipulation, diverse functionalities in droplet motion control are demonstrated, encompassing the manipulation of droplets with various types and sizes, as well as precise control at certain tilt angles. Finally, a series of applications involving electrostatic tweezer-controlled droplet motion are realized, including chemical reactions through droplet mixing, flexible path adjustment, and surface self-cleaning.

The method begins with the use of a femtosecond laser processing system to fabricate a superhydrophobic surface on PDMS thin films, which are chosen for their inherent hydrophobic properties. By mirrors, the laser beam is directed and focused onto the PDMS surface, ablating the material and creating micro-nanostructures. The sample remains stationary while the laser focus moves line-by-line across the surface to ensure uniform coverage. Parameters such as scan speed, laser power, and spacing are adjusted to obtain the desired morphology. This process induces a uniform rough microstructure, which is further enhanced by nanoscale protrusions, contributing to the superhydrophobicity of the surface. For droplet manipulation, an electrostatic tweezer is formed by connecting a copper rod above the surface to a static electricity generator, which allows control of its surface potential. As the tweezers approach a neutral droplet on the superhydrophobic surface, the induced charges cause an electrostatic force that drives the motion of the droplet. This method combines femtosecond laser processing and electrostatic manipulation to achieve effective droplet control on superhydrophobic surfaces.

This method utilizes adjustable electrostatic interactions between droplets and electrostatic tweezers to manipulate droplet dynamics. By analyzing droplet behavior theoretically and observing it experimentally, three distinct scenarios are identified: in one, droplets remain stationary due to weak electrostatic forces; in another, increasing voltage and proximity enable the electrostatic tweezers to move droplets horizontally, which is the primary focus; in the third scenario, strong potential and close proximity cause droplets to be lifted uncontrollably. The electrostatic field induced by the electrostatic tweezers generates charges in nearby droplets, resulting in an electrostatic force that drives droplet motion. On a superhydrophobic surface, droplets experience various forces, including electrostatic and lateral adhesion forces, with the resultant force determining the droplet movement. The ability of electrostatic tweezers to attract droplets allows flexible manipulation, as demonstrated by handling droplets with different volumes and guiding them along complex paths. Additionally, electrostatic tweezers can manipulate corrosive droplets, showcasing their potential in practical applications such as droplet microreactions and precise cleaning of contaminated surfaces.

Electrostatic tweezers paired with superhydrophobic surfaces created via femtosecond laser technology allow for precise and contactless control of droplets. The electrostatic field induces charge redistribution in the neutral droplets, enabling their movement under electrostatic forces. This technique facilitates diverse droplet manipulation tasks such as guiding corrosive droplets on superhydrophobic surfaces with low surface tension. Its applications range from flexible droplet manipulation to the self-cleaning of contaminated surfaces and droplet mixing in microreactions. With further advancements, electrostatic tweezers hold promise in various fields, including biology, chemistry, microfluidics, and manufacturing.