Non-Traditional Fabrication Technique for Promoting Industrial Applications of Biomimetic Surfaces
Motion control is a critical feature for both natural and artificial systems as it facilitates a range of applications, including improved heat transfer and high-throughput cell screening. Generally, motion control emanates from natural concepts. For example, lotus leaf-inspired superhydrophobic surfaces with contact angle greater than 150° are promising materials for controlling droplet motion when combined with the effects of hydrophilic or hydrophobic patterns or external stimuli. Unfortunately, efficient motion control has remained a challenge due to the defects induced by either mass loss on hydrophilic patterns or limited droplet motion on hydrophobic patterns. The external elements used together with stimulus-responsive surfaces are not suitable for some applications making it difficult to manipulate in some scenarios.
Generally, exciting functions result when nature matches different textures and solids. For instance, Nepenthes alata pitcher plant produces an intermediary lubricant by using microtextures. The lubricant automatically fills the vacancies created by the textures to produce a stable and smooth film that is slippery enough to push the insects into the rim. The magnificent concept of Nepenthes pitcher has inspired the creation of slippery liquid-infused porous surfaces (SLIPS) with remarkable features such as stable pressure and self-healing. Subsequent research has led to the identification of physiochemical properties and surface topology of the infused lubricant and as the main factors influencing the droplet motion control on slippery surfaces. Nevertheless, despite the remarkable progress on droplet motion control, there are limited uniformity, directionality, and magnitude of the motion control on the entire surface. This hinders programming and conducting more efficient and sophisticated droplet motion control in different scenarios.
To overcome these challenges, researchers at the Nanjing University of Aeronautics and Astronautics Professor Xiaolong Yang, Mr. Kai Zhuang and Professor Xiaolei Wang in collaboration with Professor Yao Lu from the Queen Mary University of London developed topological ultra-slippery surfaces for effective and efficient droplet motion control. In their approach, SLIPS were integrated with different topologies with specific functions inspired by nature, such as grooved structures of rice leaves. A three-dimensional (3D) topological SLIPS inspired by the nanotextured slippery property of the Nepenthes pitcher and the leaf-like grooved structures of rice was fabricated on 3D metal substrate structures via ultraviolet laser milling followed by alkaline oxidation of nanotextures. Their research work is currently published in the journal, ACS Nano.
Results showed the resulting grooved nanostructured SLIPS could effectively shape footprint droplets to realize a sliding resistance anisotropy of 109.8 µN, which is significantly larger than that of natural rice leaf and sufficient to aid droplet transportation. Consequently, continuous self-driven droplet transportation could be achieved when the wedge-shaped nanostructured SLIPS effectively squeezed the confined droplet footprint to produce a sufficient Laplace pressure gradient. The authors also noted that the fabricated surfaces could manipulate different droplets solutions, including those of salt, alkali and acid. Furthermore, the proposed concept could be extended for different applications involving condensing heat transfer, lab-on-chip devices, and energy fields.
In summary, a 3D topological SLIP configuration fabricated via laser milling of metal substrates and alkaline oxidation was reported to effectively control slippery droplet motion. The droplet footprint, similar to the superhydrophobic Wenzel state, made contact with the structure due to the high adhering force between the lubricant and water droplet, allowing for droplet transportation. This, combined with sufficient Laplace pressure gradient, allowed self-driven droplet transport. The concept could be modified to realize more sophisticated topologies for autonomous droplet motion control via external stimuli and specific materials. In a statement to Advances in Engineering, the authors said the new fabrication method and combinative surface have great potential in advancing precise microdroplet motion control in different fields.
Yang, X., Zhuang, K., Lu, Y., & Wang, X. (2021). Creation of Topological Ultraslippery Surfaces for Droplet Motion Control. ACS Nano, 15(2), 2589-2599.