Viscous dipping, application to the capture of fluids in living organisms

Various animals (among insects, birds and mammals) use flower nectar as their energy supply. They then developed specific skills to ingest highly viscous fluids. Depending on the sugar content in the nectar, different strategies are observed: hummingbirds have a tongue made from two thin flexible sheets that bend to form a tube when immersed in the nectar; other animals exhibit in contrast a specific papillary structure. Bees and some bats possess a tongue decorated with complex structures that, according biologists, are optimized for fluid capture. In this talk, we first make an extensive investigation of the viscous drag/drainage with smooth rods. For high capillary numbers, a switch from the classical LLD model to a visco-gravitational regime is observed. In a second stage, the influence of micro-structures that mimic biological morphologies is investigated. The micro-structures has several purposes: to trap fluid in the controlled roughness, and to alter the viscous drag process. Interestingly, it appears that increasing the size of protuberances increases the amount of trapped fluid but to the detriment of the dragged fluid. There should thus be an optimum of the structure determined by the viscosity of nectar and the characteristics of the tongue.