Wave Boundary Layer
An innovative methodology to measure bottom shear stresses based on the use of ferro-magnetic nanomaterials, named ferrofluids, is presented. Subject to an external magnetic field, these materials show hexagonal patterns by remaining at a fluidic state. A flow additionally deforms such a pattern. Here, a small drop of ferrofluid is used as a sensor of bottom shear stress. Flow-induced deformations of the ferrofluid are measured by an optical readout strategy, in the presence of a mobile beds, to investigate the critical bed shear stress of both uniform sand and sediment mixtures.