The impact dynamics of water droplets on the solid and liquid surfaces is an interesting phenomenon involving complex physical mechanisms, which has a long-term potential influence on the related industrial development and technologies [[1], [2], [3]]. In nature, raindrops hitting rocks, plant leaves and soil are found everywhere. Complex droplet-wall interactions also exist in industry such as paint spraying and coating, liquid atomization, trickled bed reactors, turbine blade and aircraft erosion, fire suppression, refrigeration cycles and fuel injection combustion. In general, water drops impacting the solid wall exhibit spreading, icing, boiling, bouncing, and splashing behaviours [[4], [5], [6], [7]]. In contrast, water drops impinging the liquid surface form crowns or splash to generate secondary droplets. Among them, the bouncing behaviour phenomenon of impacting droplets is relatively harsh because of various influencing factors such as droplet properties (size, viscosity, surface tension, etc.), surface properties (wettability, surface morphology, temperature, thermal conductivity, etc.) and surrounding environment (pressure, humidity, temperature, etc.). Since the dynamic behaviours of droplet impacting solid surfaces were proposed by Worthington [8], and large number of special phenomena were found to occur after droplet impacting, extensive theoretical and experimental investigations have been conducted in recent years. Among them, the bouncing behaviour of impacting droplets has attracted numerous interests with its uniqueness such as the shortest contact time and small energy dissipation and its great potential for applications.

However, several phenomena and related physical mechanisms of droplet impact dynamics cannot be verified theoretically or experimentally at present, such as the inability to solve accurately for the hydrodynamics of impacting droplets at free interfaces. This is because of the complex solid-liquid and liquid-liquid interactions involved, the expressions of surface tension closely related to the droplet morphology, the critical conditions for the appearance of boundary instabilities, and the problem of integrable stress singularities at the three-phase line. Taking the basic theoretical analysis as the origin, it is the investigating emphasis in recent years to modulate the droplet impacting process through the interfacial structure and heat transfer to achieve the process enhancement. Droplet impingement on the solid surface is a complex process in which physics and chemistry act jointly. The micro-structuring of the solid surface changes the wettability of the original surface, and the presence of these microstructures also affects the fluid flow. In addition, droplet impacting the liquid surface, including crown evolution and splashing behaviours, is also influenced by multiple factors. The coupling of these factors brings many challenges for in-depth understanding, as well as more options and opportunities for the design of advanced engineered surfaces.

Recently, investigations on the various properties of superhydrophobic/superhydrophilic surfaces and their technical potential during droplet impact have increased rapidly. Due to the rapid development of 3D nanoimaging equipment, high-speed camera measurement, surface treatment techniques and numerical modelling methods, the underlying physical mechanisms during the droplet impact have been revealed since the last decade [[9], [10], [11], [12]]. In addition, the surface morphology can be clearly observed using 3D micro and nano imaging techniques such as scanning electron microscopy, transmission electron microscopy and atomic force microscopy. Inspired by the high mobility of droplets on the lotus leaves, micro/nano-structured surfaces that mimic lotus leaves were designed to manipulate the bouncing of water droplets. Droplet deformation and dynamic behaviours are observed by high-speed imaging equipment to reveal the mechanism of droplet bouncing. Recent advances in droplet bouncing have been very attractive and meaningful, revealing the underlying physics of droplet-surface interactions and facilitating the development of numerous advanced applications, including surface self-cleaning [13], droplet transport [[14], [15], [16]], and spray cooling [[17], [18], [19]]. In addition, inspired by animal corneas, moss leaves and rice paper, superspreading surfaces are used in a wide range of applications from film fabrication to biofouling resistance and separation. Although significant progress and important applications have been made in the study of droplet impact dynamics, few comprehensive reviews have been conducted to highlight its significance, summarize its progress, and outline its direction of development, including complex droplet-surface interactions and coupled heat transfer between the droplet and surface.

This work provides a brief and systematic compilation of droplet impact dynamics as well as the heat and mass transfer laws involved herein. Firstly, a brief overview of the essential wetting mechanism and wettability transition of droplets on the solid surface is given. Secondly, the fundamental parameters regarding the droplet impact dynamics and the related dimensionless numbers are introduced. Thirdly, the dynamic behaviours and heat transfer characteristics of droplets impacting on the functional surfaces are reported from all aspects. The influence of impact velocity, surface curvature and morphology, droplet viscosity, substrate orientation and movement on droplet impact dynamics is summarized in detail. Fourthly, the influence of liquid film thickness, impact angle, initial velocity, physical property of liquid film and other factors on the crown evolution and splashing behaviours are analysed. The recent process of droplet impacting the SLIPS is summarized. Then, the limits of superhydrophobicity combined with various microstructures to reduce the contact time and the corresponding freezing/anti-icing properties are discussed. In addition, the regimes of droplet impingement on heated surfaces are discussed. As well, the cooling effect is evaluated. Droplet modulation strategies based on interfacial effects are summarized and analysed from the perspective of droplet dynamic behaviours and solid-liquid interfacial interaction mechanisms. An overview of potential applications of droplet impact dynamics is highlighted, involving self-cleaning, anti-corrosion, inkjet printing, anti-icing, heat transfer control and electricity generation. This paper will provide the readers with a comprehensive understanding of droplet impingement dynamics and the heat and mass transfer laws involved as well as the potential applications. Finally, conclusions and perspectives on droplet impact dynamics are presented.