Droplet manipulation on superhydrophobic surfaces based on external stimulation: A review

Superhydrophobic surfaces are smart surfaces with high water contact angles and low surface energy. When water droplets rest on superhydrophobic surfaces, they often stay at high contact angles, and the lower contact angle hysteresis of the surface makes the water droplets roll extremely easily. In recent years, materials based on response to external stimuli have received extensive attention and research, which has brought a positive impact on the research of superhydrophobic surfaces in droplet manipulation. With more and more attention, the research on droplet manipulation on superhydrophobic surfaces in response to external stimuli has achieved many promising results, which will have great application potential in the fields of liquid transport, biochemical separation, in situ detection, and microfluidics. In this paper, we mainly review the droplet manipulation on superhydrophobic surfaces in response to electrical, magnetic, photothermal, acoustic and other external stimuli in recent years. First, the basic concept of superhydrophobic surface is introduced, three wettability theories are discussed, and the connection between superhydrophobic surface and droplet manipulation is discussed, comparing the advantages and disadvantages of droplet manipulation on superhydrophobic surfaces and other surfaces. In addition, some special designs of superhydrophobic surfaces are also introduced. Secondly, the research results and progress of droplet manipulation on superhydrophobic surfaces induced by electrical, magnetic, optical, acoustic, mechanical stress and other external fields in recent years are mainly introduced and discussed. Finally, the advantages and disadvantages of droplet manipulation research are summarized, and the possible future development directions are prospected in order to promote the development of this field.