Dynamic capillarity-driven texture morphing in flexible fins

Elastocapillarity, the capillary-induced deformation of slender materials, is a common phenomenon observed in our everyday activities like painting, washing hair, and seeing wet grass or leaves. In this study, we develop an unusual actuation system influenced by hydrodynamics, capillarity, and elasticity. We create an elastomeric fin placed between two parallel walls with different spacing gaps to break the symmetry. The submerged fins of the soft cell exhibit bimorphic textures upon emerging, dictated by the drainage rates governed by a syringe pump. Slow drainage results in a flexible fin to deform towards the narrow side due to a strong capillary pressure within the limited space (N-mode). Conversely, during rapid drainage (W-mode), the fin inclines towards the wider side, largely due to the suction pressure generated within the soft cell by drainage dynamics. Accounting for the viscous, capillary, and dynamic factors, we establish a theoretical model to offer a comprehensive regime map for the texture patterns of these flapping fins. Additionally, we provide a more complex polymorphism within the flapping fin array. We predict that this novel switchable texture system can lead to several practical uses, including reflective displays and multimodal signaling systems.