  Decapod crustaceans have appendages with an array of rigid hairs covered in
chemo-receptors, used to sense and track food. Crustaceans directly influence
the flow behavior by changing the speed of flow past the hairy surface, thereby
manipulating the Reynolds number (Re). Hairs act either as a rake - diverting
flow around the hair array, or as a sieve - filtering flow through the hairs.
In our experiments, we uncover a third transitional phase: deflection - where
the flow partially penetrates the hair array and is deflected laterally. We
develop a reduced order model that predicts the flow phase based on the depth
of the boundary layer on a single hair. This model with no fitting parameters
agrees very well with our experimental data. Additionally, our model agrees
well with measurements of both chemo-sensing and suspension-feeding
crustaceans, and can be generalized for many different geometries.
