Wind Tweezer for Versatile Droplet Manipulation on the Femtosecond Laser-Structured Superhydrophobic Platform

Although various physical tweezers and external fields based on optical, magnetic, electrical, and acoustic interactions have been developed for noncontact droplet manipulation, existing techniques are limited by the use of responsive additives in droplets/substrates or changes in the droplet's physical and chemical properties during operation. Herein, we report a wind tweezer (WT) that uses a controlled airflow field (wind force) to remotely and programmatically manipulate droplets in a highly precise and flexible manner. A comprehensive equation is developed to characterize the wind force acting on a droplet within the WT system, taking into account variables such as wind speed, WT height, pipe diameter, droplet size, and droplet's deviation distance. The versatile WT can manipulate droplets with variable volumes (100 nL to 3 mL), chemical compositions, and high velocities (up to 160 mm/s), as well as droplet arrays on various substrates and Leidenfrost droplets on superheated smooth surfaces. Vertical droplet manipulation is also allowed. The WT has been successfully used in various exciting applications, such as circuit welding/repair, cell transport in bioactive liquids, and improving the detection performance of surface-enhanced Raman scattering (SERS).