Re-Entrant Microstructures for Robust Liquid Repellent Surfaces

Superhydrophobic surfaces have many interesting applications because of their self-cleaning, waterproof, anti-biofouling, anti-corrosion, and low-adhesion properties. Accordingly, numerous surfaces with hierarchical micro/nanostructures are designed and engineered to achieve superhydrophobicity. However, these surfaces have two major problems. First, they lose superhydrophobic properties over time, primarily because of environmental conditions such as vibration, external pressure, evaporation, and pollution. Second, most superhydrophobic surfaces fail to repel all types of liquids, especially those with low surface tensions. To address this bottleneck, microstructures with re-entrant curvature have emerged, demonstrating excellent liquid-repellent abilities and robustness. Additionally, microstructures with re-entrant curvature have significant applications in designing surfaces with unidirectional wetting properties for passive liquid handling. Accordingly, this review systematically summarizes the design and fabrication strategies of these re-entrant microstructures. The emphasis is given to wettability studies and other surface properties of re-entrant microstructures and their applications, especially for liquid self-transporting. This paper also highlights the potential applications and remaining technical challenges of fabricating these structures. Finally, the study is concluded by providing the future directions in this promising field.