Hydrodynamic blockage and reconfiguration of kirigami sheet under low-Re regime

Kirigami structures are thin elastic sheets that contain cutting lines. When interacting with fluid flow, these structures passively reconfigure through a balance between fluid force and bending force. In this study, we experimentally investigate the deformation and drag characteristics of circular kirigami sheets translating in a highly viscous fluid. When bending and viscous forces reach equilibrium, the initially planar cutting lines transform into three-dimensional pores. At high Reynolds numbers regime, flow penetrates these pores, resulting in a substantial reduction in drag compared to a flat sheet. In contrast, under low Reynolds number regime, thick shear layers develop and overlap within the pores, producing a hydrodynamic blockage effect. This effect causes the kirigami sheet to exhibit drag that is comparable to, or even greater than, that of a flat sheet. These hydrodynamic blockage effect was further elucidated through flow visualization. By systematically varying the sheet thickness and the size of the cutting lines, we examine how geometric and material parameters influence reconfiguration and hydrodynamic blockage effect. Additionally, we propose a governing parameter that captures the deformation and porosity of the kirigami sheet based on scaling analysis. The unique drag behavior of kirigami structures in viscous flows offers potential applications in the design of small-scale transport or propulsion systems that rely on drag-based mechanisms.