Hydrodynamic blockage and reconfiguration of a kirigami sheet in viscosity-dominant flow

Kirigami sheets, which contain cutting lines that introduce structural discontinuities, undergo significant deformation when they interact with the surrounding flow. We experimentally investigate the reconfiguration and drag characteristics of a kirigami sheet translating at a constant speed in the low-Reynolds-number regime. When initially planar cutting lines deform into three-dimensional holes due to fluid loading, thick shear layers that develop within the holes overlap, forming virtual fluid barriers. Despite the presence of holes, the flow bypasses the reconfigured sheet because of hydrodynamic blockage. By varying translation speed, sheet thickness and cutting ratio, we examine how the hydrodynamic blockage determines flow distribution, deformation and drag, in comparison with the high-Reynolds-number regime without blockage. A governing dimensionless parameter based on the balance between viscous fluid force and effective bending force, accounting for cutting lines, is proposed to characterise the reconfiguration-dependent porosity of the sheet. Despite high porosity, the reconfigured kirigami sheet exhibits drag comparable to or even greater than that of a flat rigid disk. The unique drag behaviour of kirigami sheets in response to highly viscous flow suggests potential applications to small-scale drag-based transport and propulsion systems.