A fishbone-inspired liquid splitter enables directional droplet transportation and spontaneous separation

Abstract

Despite extensive efforts made in directional manipulation of water and oil microdroplets, challenges still remain in the ultrafast self-transportation and the rapid separation of droplets with different surface tensions due to the lack of driving force. The natural inspiration from fish medullary spines provides a potential way to realize directional oil movement by using an anisotropic surface with micropits. Herein, a fish-spine-like liquid splitter (FSLLS) is developed based on a precise 3D printing and subsequent femtosecond laser structuring. The liquid splitter enables directional transportation of various microdroplets, with the maximum velocity reaching 265.3 mm s⁻¹. More interestingly, it can spontaneously transport liquids with the surface tension ranging from 15 to 48 mN m⁻¹, achieving an outstanding selectivity of specific liquids. Moreover, the lower the surface tension for the liquid in the surface tension range, the faster the transport speed on the FSLLS. This feature can be attributed to the unique micro-pit structure on the FSLLS surface and its oleophilic properties. The combination of ultrafast directional transportation and peculiar selective ability makes the FSLLS achieve the pumping of microdroplets with lower surface tension. As a proof-of-concept, the separation of the mixed droplets with different surface tensions is demonstrated using the FSLLS and the scalable device. This study offers a new bionic design concept for selective microdroplet pumping devices and opens up an avenue for the separation and extraction of oil droplets in the field of multiphase separation, biomedical analysis, microfluidics, etc.