Directed droplet manipulation is paramount in various applications, including chemical micro-reaction and biomedical analysis. The existing strategies include some kinds of gradients (structure, inherent wettability, and charge density), whereas they suffer from several limitations, such as low velocity, limited volume range, poor durability, and inefficient environmental suitability. Moreover, active bi-directional reversal of omni-droplets remains challenging. In order to overcome these challenges, Professor Wu Sizhu's team has developed a smooth cross-scale structure that uses bi-directional (vertical and horizontal) vibration to realize active and directional steering of omni-droplets, and the results were published in the internationally renowned journal Applied Physics Letters [Applied Physics Letters, 2023, 122(17)]. 

Using femtosecond laser patterned oblique etching and lubricant infusion, the team designed and fabricated a cross-scale structure that includes macro millimeter-scale circular arc arrays and micro/nanometer-scale slant ratchet arrays. Using bidirectional (vertical and horizontal) vibrations, the structure enables active and directional steering of omni-droplets. Under vertical vibration, the droplet is propelled along the forward direction, while the droplet is actuated along the backward direction under horizontal vibration. The physical mechanism of active droplet steering lies in the relative competition between the forces under vertical and horizontal vibration, which mainly arise from the circular arc arrays and slant ratchet arrays, respectively. Specifically, forward transport is due to the asymmetry of the length of the contact line at the anterior and posterior edges of the droplet, while backward transport is due to the anisotropic distribution of capillary forces at the angle close to the three-phase contact line. The results of this study can be applied to circuit on/off and droplet-based chemical microreactions, especially in high-throughput whole microfluids quantification and directional manipulation.

Figure 1. (a) Schematic illustration of the directional droplet steering driven by mechanical vibration on the SCSA. (b) Optical images of circular arc arrays before and after infusing the lubricant at a macro scale. (c) The scanning electron microscopy (SEM) image of slant ratchet arrays. The side optical view indicates a thin film of the infused lubricant. (d) Steering transport of water droplets on the SCSA. (e) Transport displacement of various droplets vs time.

Hefei University of Technology is the first signatory of the paper, Professor Wu Sizhu is the first author of the paper, and Zhang Yiyuan, a postdoctoral fellow of the University of Hong Kong, and Professor Wang Liqiu of the Hong Kong Polytechnic University are the corresponding authors. This work was supported by the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, the National Key R&D Program of China, the Key Laboratory of Biomimetic Engineering of Jilin University (Ministry of Education), and the Research Grants Council of Hong Kong.

Link to original article: https://doi.org/10.1063/5.0146217