Recent Progress in Technologies for Negatively Charging Droplets.

In recent years, droplet charging technology has become an interdisciplinary research hotspot due to its great potential for application in the frontier fields of microfluidics, materials science, and biomedicine. This technology can regulate droplet charge through physical (e.g., electric field, mechanical vibration, ultrasonic wave) or chemical (e.g., addition of surfactant, electrolyte) methods, which can achieve high-efficiency energy conversion and self-powering of microdevices in the field of energy, promote targeted drug delivery and precise cell manipulation in biomedicine, and enhance the efficiency of pollutant purification in the field of environmental management. However, relevant studies indicate that the charge polarity and magnitude of droplets are influenced by material properties, environmental conditions, and motion states. In both the design and application research of liquid-solid triboelectric nanogenerators (L-S TENG) employing fluoropolymers as solid friction layer materials, and in studies of droplet-atmosphere friction, a general tendency for droplets to become positively charged has been observed. In contrast, research on the generation mechanisms of negatively charged droplets and their efficient control strategies remains relatively scarce. This limitation constrains the comprehensive application and further advancement of droplet charging technologies. In this paper, we systematically review a series of methods to prepare negatively charged droplets and look forward to their future development, aiming to help scholars in the related fields to have a more comprehensive understanding of the current research status and trends in this field.