2 results
Tank-treading of microcapsules in shear flow
- C. de Loubens, J. Deschamps, F. Edwards-Levy, M. Leonetti
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- Journal:
- Journal of Fluid Mechanics / Volume 789 / 25 February 2016
- Published online by Cambridge University Press:
- 26 January 2016, pp. 750-767
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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 long axis. The tracking of the distance between two close microparticles showed membrane contraction at the tips and stretching on the sides, a specific property of soft particles such as capsules. The 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 an S-like shape. When the viscous shear stress increased by three orders of magnitude, the short axis decreased by 70 % whereas the long 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 and numerics on capsules and from experiments, theory and 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 long 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 independent methods that compare experimental data both with numerical results in the regime of large deformations and with theory in the regime of moderate deformations.
Stretching of capsules in an elongation flow, a route to constitutive law
- C. de Loubens, J. Deschamps, G. Boedec, M. Leonetti
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- Journal:
- Journal of Fluid Mechanics / Volume 767 / 25 March 2015
- Published online by Cambridge University Press:
- 20 February 2015, R3
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Soft bio-microcapsules are drops bounded by a thin elastic shell made of cross-linked proteins. Their shapes and their dynamics in flow depend on their membrane constitutive law characterized by shearing and area-dilatation resistance. The deformations of such capsules are investigated experimentally in planar elongation flows and compared with numerical simulations for three bidimensional models: Skalak, neo-Hookean and generalized Hooke. An original cross-flow microfluidic set-up allows the visualization of the deformed shape in the two perpendicular main fields of view. Whatever the elongation rate, the three semi-axis lengths of the ellipsoid fitting the experimental shape are measured up to 180 % of stretching of the largest axis. The geometrical analysis in the two views is sufficient to determine the constitutive law and the Poisson ratio of the membrane without a preliminary knowledge of the shear elastic modulus $G_{s}$. We conclude that the membrane of human serum albumin capsules obeys the generalized Hooke law with a Poisson ratio of 0.4. The shear elastic modulus is then determined by the combination of numerical and experimental variations of the Taylor parameter with the capillary number.