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Applications and rheological characterisation of polymer solutions
Polymer solutions, such as saliva or egg white, belong to an important category of non-Newtonian liquids called viscoelastic liquids. Such liquids are known to exhibit a strong “strain-hardening” behaviour in extensional flows, meaning that strong stresses can arise due to the stretching and progressive unravelling of polymer chains. This is why filaments of saliva are surprisingly resistant in spite of their low shear viscosity. Over the years, polymers have been found to be useful additives in many flow applications such as satellite droplets suppression in inkjet printing and turbulent drag reduction in pipe flow.
In this talk, I will start by presenting our experimental results on the effect of viscoelasticity in two flow situations where, in spite of many attempts, no fully convincing theory has yet been proposed. These flow situations, which are particularly relevant in agriculture, are (i) the fragmentation of a liquid sheet into droplets (spray) and (ii) the dynamics of a droplet impacting on a hydrophobic substrate. We show that polymer addition to a Newtonian solvent leads to the formations of larger droplets in sprays, which helps minimising pollution in pesticide spraying where small aerosol droplets could otherwise be carried away by the wind instead of reaching the target plant, and slows down the retraction dynamics of impacting droplets, hence increasing the efficiency of droplet deposition on plants by preventing them to bounce off the leaves.
In order to rationalise such experimental results and to compare them with predictions of viscoelasticity models, measurements of the mechanical (rheological) properties of the liquid are required. However, our recent experimental results show that the most standard extensional rheology technique, which relies on the thinning dynamics of a liquid bridge during droplet pinch-off, can surprisingly give different outcomes for a given liquid depending on the initial size of the bridge, hence questioning the claim of universality of this method.