Synergetic optimization strategy of asymmetric wettability and structure for directional droplet transport

Controllable manipulating droplet transportation is a fundamental and applied topic in the fields of chemistry, physics, and materials science. Despite scientific developments achieved in recent years, controllable and accurate droplet manipulation still remains challenging. To adapt to the complex environment, some creatures in nature evolved asymmetric wettability and structures for manipulating droplets for survival. The bio-inspirations could promote the fabrication of excellent materials, which combine with asymmetric wettability and structure for directional droplet transport. Herein, the state-of-the-art advances in directional droplet transport devices (DDTDs) are reviewed. Firstly, the basic concepts of droplet in contacting with solid surfaces and the driving forces of asymmetric wettability and structure for directional droplet transport are illustrated. Secondly, the pioneering design strategies and fabrication techniques of DDTDs are presented, especially highlighting the synergetic optimization of asymmetric wettability and structures of DDTDs. The superiority and inferiority of each DDTD are discussed. Thirdly, the most emerging applications, such as unidirectional water transport fabrics, water harvesting, digital microfluidics, liquid-liquid separation, and other applications are also presented. Finally, a critical summarization of present challenges and further development is prospected, aiming to stimulate the next generation of DDTDs from lab research to practical implementation.