A comprehensive understanding on droplets.

Droplets are ubiquitous and necessary in natural phenomena, daily life, and industrial processes, which play a crucial role in many fields. So, the manipulation of droplets has been extensively investigated for meeting widespread applications, consequently, a great deal of progresses have been achieved across multiple disciplines ranging from chemistry to physics, material, biological, and energy science. For example, microdroplets have been utilized as reactors, colorimetric or electrochemical sensors, drug-delivery carriers, and energy harvesters. Moreover, droplet manipulation is the basis in both fundamental researches and practical applications, especially the combination of smart materials and external fields for achieving multifunctional applications of droplets. In view of this background, this review initiates discussion of the manipulation strategies of droplets including Laplace pressure, wettability gradients, electric field, magnetic force, light and temperature. Thereafter, based on their manipulation strategies, this review mainly summarizes the applications of droplets in the fields of robot, green energy, sensors, biomedical treatments, microreactors and chemical reactions. Application related basic concepts, theories, principles and progresses also have been introduced. Finally, this review addresses the challenges of manipulation and applications of droplets and provides the potential directions for their future development. By presenting these results, we aim to provide a comprehensive overview of water droplets and establish a unified framework that guides the development of droplets in various fields.