Titre de la dissertation: « Emergence of fluid capture strategies through the coupling between viscous and elastic forces ».

Promoteur: Pascal DAMMAN

Résumé de la dissertation: Through out evolution animals have developed multiple ways to collect liquid either to stay hydrated or to get fed. Those drinking mechanisms depend, mainly, on the size of the animal and the viscosity of the liquid captured.In this thesis we concentrate our studies on mechanisms used by bees. Indeed they have, to our knowledge, at least two different ways to collect nectar. They can either use a lapping mechanism or a sucking mechanism. In the first case, bees dip their tongue into nectar and retract it to collect liquid trapped between the « hairs », called papillae, constituting the tongue. They repeat this movement around five times per second, independently of the viscosity on several magnitude of order. In the second case, bees simply dip their tongue and leave it in the nectar. Then, using a small pump inside their head, they can suck the liquid and use their tongue as a straw. They will choose one or the other depending on the viscosity, featuring a critical viscosity. Furthermore, the can deliberately switch between the two. They preferentially suck diluted nectar whereas they are prone to lap concentrated nectar. The latter induced higher viscous nectar. If this viscosity is too high we observe a drop of the quantity of nectar captured. This leads us to get a closer look at the tongue of the animal and study the relaxation of papillae specifically. We show that the drop is characterized by a second critical viscosity at which the papillae don’t have enough time to fully relax and capture efficiently the nectar.In the second part of this thesis we are interested in elastoviscous instabilities created by the compression of thin flexible sheet floating on top of a viscous liquid. Compressed sheet are well known when they rest on liquid and elastomer with a fixed compression rate. The latter will induced the wrinkling of the sheet characterized by what we call Static Wavelength. This wavelength depends on the substrate onto which the sheet is resting. Now imagine that the sheet is floating on a viscous fluid and that its ends are free. We can, again, induce a wrinkling of the sheet by compression but this compression need to be quick and/or need to occur on a highly viscous fluid. We show here the dependencies of this new Dynamic Wavelength which is smaller than the static one. For this study we built from scratch a custom cuvette made of elastomer that can be stretched and compressed with precise controlled speed. We show that increasing compression speed or viscosity or decreasing the bending modulus of the sheet will