Helical pipes may act as hydrodynamic “diodes”: Anomalous fluid flow inspired by shark intestines

The intestines of sharks and rays form complex helical structures that are unique in the animal kingdom. In the literature, this peculiar shape is commonly attributed to maximizing nutrient absorption by greatly increasing the surface area. However, a recent study suggests that these structures also induce asymmetric flows, favoring flows down the digestion track (i.e., anterior to posterior). Such ability to control the direction of a flow without any moving parts retains significant potential in engineering applications ranging from industrial pipelines to microfluidic devices and medical implants. Motivated by this remarkable phenomenon, we aim to understand the underlying physics. We design and 3D print biomimetic simplified models of helical shark intestines. By measuring the flow of various fluids through these structures in both directions, we can extract their hydrodynamical properties. Our models are characterized by a small number of parameters, mainly the radius, pitch, and thickness of the inner helical structure, which allows us to recognize the role of each of them, generalizing our results to arbitrary scales. Finally, by printing structures from softer materials, we study the coupling between pipe deformation and flow rate. Together, these results can be used to identify relevant applications and help design the ideal structure for a given task.