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Complex fluids dripped into a liquid bath : impact, relaxation and gelation dynamics

Abstract : Dripping alginate solutions into a bath of calcium ions is a robust process to obtain gelled beads which can be used in various applications. This process relies on the ability of calcium ions to bridge the alginate chains of the droplets to form a rigid gel. The Reynolds and Weber numbers, defining the competition between inertia, viscous dissipation and surface tension forces are known to influence the final shape of the beads. Indeed inertia tends to drive the spreading of droplets upon impact with the bath while high droplet viscosity and surface tension tend to limit their extension upon impact.However, very little is known about the dynamics of the droplets shape during impact with a liquid bath. In this thesis, using a high speed camera, we study the shape evolution of alginate-ceramic suspension droplets as they impact and penetrate a bath of calcium ions. We observe that the drops first extend upon impact with the liquid surface, and then relax to a more spherical shape inside the bath, while gelling at the same time.As expected, the drops maximum elongation increases with increasing impact velocity and decreasing droplet viscosity. The effect of the calcium concentration in the bath is less intuitive. As it increases, the maximum extension of the droplets tends to decrease and the droplets tend to relax faster to a spherical shape. This relaxation is related to a syneresis phenomenon which instantaneously occurs as the droplet hits the bath: the gelled membrane in formation tends to shrink by expelling water. This phenomenon puts the outer membrane under tensile stress and therefore is the driving force of the capsule relaxation. We evidence the main obstacles to relaxation, resulting in capsules frozen in an ellipsoidal shape before reaching a perfect sphere and before being fully gelled.
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Submitted on : Monday, October 19, 2020 - 3:50:25 PM
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  • HAL Id : tel-02971590, version 1

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Julie Godefroid. Complex fluids dripped into a liquid bath : impact, relaxation and gelation dynamics. Soft Condensed Matter [cond-mat.soft]. Université Paris sciences et lettres, 2019. English. ⟨NNT : 2019PSLET039⟩. ⟨tel-02971590⟩

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