Liquid droplets, formed due to the surface tension of a liquid, can be seen everywhere in daily life and nature, such as dewdrops on leaves, raindrops on lotus leaves, and wine tears hanging on the surface of wine glasses [[1], [2], [3]]. Because of their unique properties and size compatibility, liquid droplets are also crucially important in many tranditional industrial applications and chemical engineering processes, such as heat transfer, separation, and microfluidics [[4], [5], [6], [7], [8]]. Of late, they have become an integral part of a plethora of modern applications due to their potential to replace a great deal of conventional processes [9]. For example, droplets offer high interfacial area per unit mass, limited usage of materials for the reduction in wastage, incompressible system, and open thermodynamic system for mass, momentum, and energy transfer [10]. So far, droplets have emerged as cutting-edge engineering applications in the areas of electronics [11], energy harvesting [12], chemical reactions [13], and healthcare [14]. Therefore, many droplet manipulation methods have been constantly explored for meeting increased on-demand applications [[15], [16], [17]]. In particular, with the development of functional surface fabrication technology and droplet dynamics theory, droplet manipulation on surfaces has been realized by introducing surface wettability gradient or external stimuli for various applications. Inspired by natural creatures, many bionic surface structures including cactus, spider spindle and pine needles have been fabricated to create specific microstructures and wetting gradient surfaces for collecting and directed transport of droplets [18,19]. Subsequently, responsive surfaces that can adjust their microstructure according to changes of external stimuli such as temperature, light and magnetism for causing different wetting properties have been used to control the movement of droplets [20,21]. By making use of responsive surfaces and the effect of external stimuli, numerous methods have been developed for realizing the manipulation of complex behaviors of droplets, consequently, droplets have achieved a wide applications in various fields [22,23].

With the exacerbation of the energy crisis and environmental pollution, it is imperative to explore and fully utilize green, renewable resources. Water, covering approximately 71 % of the Earth's surface, absorbs around 70 % of the solar radiation that reaches the ground and gains up to 60 trillion kilowatts (1015 W) of energy [24]. Conventional water energy scavenging methods are mostly based on electromagnetic generators, which require large areas, and rigorous requirements of velocity, level, and flow [25]. As a common energy-rich substance, droplets are regarded as one of the most potential candidates for sustainable power sources due to their enormous energy [26]. In 1867, Lord Kelvin developed the Kelvin water dropper based on electrostatic induction, regarded as an earlier classic droplet energy harvesting technology [27]. As a result, the theories and models of raindrop energy harvesting based on the piezoelectric, reverse electrowetting, and hydrovoltaic technology are sequentially proposed for practical applications in diverse fields. Significantly, the appearance of triboelectric nanogenerator (TENG) accelerates the development of droplet power generation, which is applied to scavenge the raindrop energy [28]. Since 2012, TENG relying on the coupling effect of contact electrification and electrostatic induction, has made great progress and achievements on mechanical energy harvesting and self-powered sensor systems [29]. Recently, the progress working based on the bulk effect of water droplets can achieve high instantaneous electrical output, which presents an effective strategy for pushing raindrop energy investigations [30]. At the same time, different two-phase interfaces of droplet energy harvesting are also extensively studied for realizing a high instantaneous power density [31]. The development of droplet based energy harvesting not only demonstrates the capability of transforming droplet energy into electricity but also confirms their promising applications in green energy. For instance, droplet based TENGs are able to convert ambient mechanical energy sources into electricity and easily generate an open-circuit voltage of kV level [32]. On the other hand, droplets also have a wide range of applications in the biomedical field, including cell culturing [33], drug synthesis and delivery [34], biochemical analysis [35], biosensor [36], and clinical diagnosis [37]. Particularly, droplet based TENGs could be used as droplet sensors for realizing the remote monitoring of blood through a self-power supply, which display great promising applications of biosensors [38].

As we known, bulk water can be served as an inert solvent for many chemical and biological reactions. In 1892, the Nobel Laureate Philipp Lenard reported that when water transformed into tiny droplets, the negative and positive charges will be separated, which is the earliest indication of water microdroplets formed with a net charge [39]. Then, many studies have revealed that chemical reactions take place in water microdroplets can be accelerated than in bulk solution [40,41]. Further investigations indicate that the ability to form high electric fields at the interface of water microdroplets and the inability to form a three-dimensional solvation shell around ions at the interface cause the acceleration of reactions [39]. Zhang's group demonstrates the strong spontaneous redox power of water microdroplets, which are critical for chemical synthesis [42]. The charge transfer between water microdroplets is size-dependent, it has been found that interfacial electrons transfer from large microdroplets to small microdroplets during ultrasonic atomization [43]. Deep studies by Zare's group have shown that water microdroplets can generate a certain amount of free electrons and form H2O2 at their surfaces [44]. More and more studies confirm the importance of interface/surface of water droplets, as they enable electron transfer to induce chemical reactions, such as in the uncatalyzed oxidation of SO2 to sulfate observed in atmospheric chemistry, and the observation of water-derived H2 formation from water–oil emulsions due to the electrons pulled from the water microdroplet interface [45,46]. Recent investigation discloses that the air–water interface of microdroplets can break the strong chemical bond of nitrogen (N2), producing nitrogen oxides in the environment. The spontaneous formation of nitrogen oxides at the air–water interface of microdroplets has important implications in chemical synthesis and atmospheric chemistry. In addition to participating in chemical reactions as a reducing agent, droplets also can act as microreactors. For example, droplet-based reactors can be applied to synthesis of nanocrystals at the liquid/solid interface and unravel a nucleation mechanism at atomic scale [47]. Droplet microreactors also can be used to conduct many multistep reactions by repeatedly adding controlled quantities of reagents to droplets [48]. Recent results reveal that microdroplets can spontaneously degrade perfluorooctanoic acid at room temperature and atmospheric pressure conditions [49]. This study promotes the understanding of the environmental fate and chemistry of polyfluoroalkyl substances and facilitates the development of effective methods for their elimination. Furthermore, droplet microreactors also can be used as transport vehicles for directionally delivering products after chemical reactions completed [50]. So, droplets have been recognized as reducing agent, microreactor and transport vehicle, which can play multiple and continuous critical functions in many scientific processes.

In this review, we present a comprehensive introduction of the recent progress of droplet applications in the fields of energy, biomedical, microreactor and chemical synthesis. It should be pointed out that droplets in this review are ranged from microscale to nanoscale, including volume and size. As the applications of droplets are highly relied on the manipulation of droplets by various methods [51], we provide a brief introduction of the methods of droplet manipulation at first. With an emphasis on the comparison of various methods, different strategies of manipulation of droplets have been categorized as passive strategies (Laplace pressure and wettability gradients) and active strategies (electric, magnetic field, light and temperature) [52]. At the same time, design and usages of different droplet robots also have been presented. Then, besides the main introduction of applications of droplets within the fields of energy, biomedical, microreactor and chemical synthesis, droplet studies of water harvesting, directional transportation, and environmental protection are also introduced. Finally, we provide conclusions and outlooks for droplet manipulation and applications, as well as challenges for their further developments. We hope this review not only provides comprehensive understandings of the manipulation and cutting-edge applications of droplets, but also inspires more researches about droplets for solving critical problems originated from strategic fields like energy, healthcare, and environment.