External-field-induced directional droplet transport: A review

Directional transport of fluids is crucial for vital activities of organisms and numerous industrial applications. This process has garnered widespread research attention due to the wide breadth of flexible applications such as medical diagnostics, drug delivery, and digital microfluidics. The rational design of functional surfaces that can achieve the subtle control of liquid behavior. Previous studies were mainly dependent on the special asymmetric structures, which inevitably have the problem of slow transport speed and short distance. To improve controllability, researchers have attempted to use external fields, such as thermal, light, electric fields, and magnetic fields, to achieve controllable droplet transport. On the fundamental side, much of their widespread applicably is due to the degree of control over droplet transport. This review provides an overview of recent progress in the last three years toward the transport of droplets with different mechanisms induced by various external stimuli, including light, electric, thermal, and magnetic field. First, the relevant basic theory and typical induced gradient for directional liquid transport are illustrated. We will then review the latest advances in the external-field-induced directional transport. Moreover, the most emerging applications such as digital microfluidics, harvesting of energy and water, heat transfer, and oil/water separation are also presented. Finally, we will outline possible future perspectives to attract more researchers interest and promote the development of this field.