Portable Triboelectric Electrostatic Tweezer for Precise Non-Contact Droplet Manipulation in Enclosed Systems

The precise and controllable manipulation of liquid droplets is essential in development of microfluidics, biomedical analysis, and combinatorial chemistry. Techniques that allow for the non-contact manipulation of droplets are particularly valued due to their ability to minimize contamination and enhance the precision of liquid handling processes. However, current methods are limited when it comes to manipulating droplets within confined or closed spaces which is a significant challenge. Traditional strategies for droplet manipulation can be classified into those relying on gradient-based surfaces involve designing surfaces with geometric, chemical, wetting, or charge gradients that induce asymmetric forces on droplets,  driving their movement and those using external stimuli such as magnetic, light, or electric fields.  While these methods can achieve spontaneous droplet motion, they are constrained by fixed transport directions, limited transport distances, and irreversible movement. In contrast, stimulus-based strategies apply external forces directly to the droplets or modify the substrate’s properties to induce droplet motion. These methods, although more flexible and capable of longer transport distances, require complex surface or droplet pretreatments and are predominantly effective on open surfaces and it is still a challenge to develop a technique that can externally manipulate droplets within closed systems without the need for direct contact or surface pretreatment. To this account, new study published in Nano Letters and led by Professor Jiale Yong, Xinlei Li, Youdi Hu, Yiming Wang, Yubin Peng, Zhenrui Chen, Yachao Zhang, Suwan Zhu, Professor Chaowei Wang, and Professor Dong Wu from the University of Science and Technology of China, developed a novel portable triboelectric electrostatic tweezer (TET) which integrates electrostatic forces with a superhydrophobic surface and enables the precise manipulation of droplets even in enclosed environments. The TET uses the electrostatic induction of droplets to exert a direct force and allows for highly controlled movement on a femtosecond laser-treated superhydrophobic platform.

The team began by fabricating the superhydrophobic platform, an essential component of the TET system, using a femtosecond laser to ablate the surface of a hydrophobic polytetrafluoroethylene (PTFE) sheet which ensured extremely low adhesion to droplets suitable for efficient droplet manipulation. They found the superhydrophobic surface allowed the TET to move droplets with minimal resistance, as demonstrated by the smooth movement of water droplets during the initial tests. The authors’ core of the TET system is a glass rod charged by rubbing with silk, which creates an electrostatic field that induces an electrostatic force on the droplets placed on the superhydrophobic platform. They fixed the charged rod vertically above the droplet at a suitable distance and when the rod moved horizontally, the droplets followed it due to the induced electrostatic forces, effectively demonstrated the system’s ability to manipulate droplets with high precision and flexibility.

The researchers investigated the electrostatic potential, the height of the TET, and droplet volume to understand the factors influencing droplet manipulation and found that increasing the electrostatic potential or decreasing the height of the TET enhanced the electrostatic force acting on the droplets, thus improving manipulation precision. The droplet volume had a relatively minor effect on the manipulation process compared to the electrostatic potential and height. These findings were crucial in optimizing the TET system for different droplet sizes and manipulation tasks. The TET demonstrated exceptional control over droplets, including the ability to pull off-center droplets back to the center position under the TET. In one experiment, the TET moved a droplet at an average velocity of 32 mm/s with minimal lag, maintaining precise control even at higher speeds up to 101.8 mm/s. The researchers successfully guided droplets through a complex maze, highlighting the TET’s high precision and flexibility. Moreover, the authors tested the TET’s robustness where they manipulated droplets of different chemical compositions, including acidic, alkaline, and saline solutions. The superhydrophobic platform’s stability allowed for the transportation of these corrosive liquids without degradation of performance.

The researchers demonstrated their system to move droplets inside a closed polystyrene   plastic tube from the outside, which showcase the TET’s potential for applications where direct contact is not feasible. They extended to biological applications, where the TET performed cell labeling experiments inside a sealed Petri dish and managed to manipulate droplets containing cells and staining agents, the TET successfully labeled cell nuclei and membranes without opening the dish, preventing contamination and maintaining the culture environment’s integrity. Furthermore, the researchers compared the TET with other droplet manipulation methods, and they evaluated motion behavior, manipulation conditions, and droplet characteristics and found the TET outperformed other techniques in precision, flexibility, and applicability to various droplet types and environmental conditions.

In conclusion, the non-contact TET system developed by Professor Jiale Yong and colleagues reduces the risk of contamination, making it highly suitable for sensitive applications such as biomedical research, pharmaceutical production, and chemical analysis. Maintaining a sterile environment, the TET enables more accurate and reliable experimental outcomes, particularly in applications involving cell cultures and biological assays. Moreover, the TET can be employed in a wide range of chemical and industrial processes, from precise chemical synthesis to material processing and it be resilient to high temperatures and its effective on different substrates, including superhydrophobic and slippery surfaces, which even further enhance its practical applicability. Furthermore, the TET can operate within sealed reactors or processing chambers in industrial processes which enhance safety and efficiency. Indeed, the reported TET system’s capabilities align well with the needs of microfluidics and lab-on-a-chip technologies, where precise and flexible control of small liquid volumes is essential and will contribute to the advancements in diagnostics, drug development, and environmental monitoring.