Tank-treading of microcapsules in shear flow

Clément de Loubens (1, 2) , Julien Deschamps (1) , Florence Edwards-Lévy (3) , Marc Leonetti (1)

1 IRPHE - Institut de Recherche sur les Phénomènes Hors Equilibre
2 LIPhy - Laboratoire Interdisciplinaire de Physique [Saint Martin d’Hères]
3 ICMR - Institut de Chimie Moléculaire de Reims - UMR 7312

We investigated experimentally the deformation of soft microcapsules and the dynamics of their membrane in simple shear flows. Firstly, the tank-treading motion, i.e. the rotation of the membrane, was visualized and quantified by tracking particles included in the membrane by a new protocol. The period of membrane rotation increased quadratically with the extension of the large axis. The tracking of the distance between two close micro-particles showed membrane contraction at the tips and stretching on the sides, a specific property of soft particles such as capsules. Present experimental results are discussed in regard to previous numerical simulations. This analysis showed that the variation of the tank-treading period with the Taylor parameter (deformation) cannot be explained by purely elastic membrane models. It suggests a strong effect of membrane viscosity whose order of magnitude is determined. Secondly, two distinct shapes of sheared microcapsules were observed. For moderate deformations, the shape was a steady ellipsoid in the shear plane. For larger deformations, the capsule became asymmetric and presented a S-like shape. When the viscous shear stress increased by three orders of magnitude, the small axis decreased by 70 % whereas the large axis increased by 100% before any break-up. The inclination angle decreased from 40 • to 8 • , almost aligned with the flow direction as expected by theory/numerics on capsules and experiments/theory/numerics on drops and vesicles. Whatever the microcapsule size and the concentration of proteins, the characteristic lengths of the shape, the Taylor parameter and the inclination angle satisfy master curves versus the longest axis or the normalized shear stress or the capillary number in agreement with theory for non negligible membrane viscosity in the regime of moderate deformations. Finally, we observed that very small deviation from sphericity gave rise to swinging motion, i.e. shape oscillations, in the small deformation regime. In conclusion, this study of tank-treading motion supports the role of membrane viscosity on the dynamics of microcapsules in shear flow by both independent methods which compare experimental data with numerical results in the regime of large deformations and with the theory in the regime of moderate deformations.

Domaines: Dynamique des Fluides [physics.flu-dyn]; Mécanique des fluides [physics.class-ph]

Clément de Loubens, Julien Deschamps, Florence Edwards-Lévy, Marc Leonetti. Tank-treading of microcapsules in shear flow. Journal of Fluid Mechanics, 2016, 789, pp.750-767. ⟨10.1017/jfm.2015.758⟩. ⟨hal-02020215⟩