Spontaneous and unidirectional transportation of underwater bubbles on superhydrophobic dual rails

Superhydrophobic/superhydrophilic surfaces (SBS/SLS) with excellent water repellency/adhesion are important in both academic research and industrial settings owing to their intriguing functions in tiny droplet and gas bubble manipulation. However, most manipulation strategies involving SBS/SLS are limited to their large-area fabrication or sophisticated morphology designs, which distinctly hinders their practical uses. In this paper, we design and fabricate superhydrophobic polydimethylsiloxane narrowing dual rails (SNDRs) beneath a superhydrophilic stainless steel sheet by one-step femtosecond laser ablation. Our SNDR tracks are capable of transporting gas bubbles in various volumes from wide end to narrow end spontaneously and unidirectionally underwater, even when they are bent. The mechanical analysis for diverse geometrical dual-rail configurations in bubble transportation performance is further discussed. Finally, we experimentally demonstrate the intriguing capability of lossless mixing of gas bubbles at a designed volume ratio on a multiple SNDR combination. This approach is facile and flexible, and will find broad potential applications such as intelligent bubble transport, mixing, and controllable chemical reactions in interfacial science and microfluidics.