Published online by Cambridge University Press: 13 March 2013
The objective of this study is to investigate the motion of an ellipsoidal capsule in a simple shear flow when its revolution axis is initially placed off the shear plane. We consider prolate capsules with an aspect ratio of two or three enclosed by a membrane, which is either strain-hardening or strain-softening. We seek the equilibrium motion of the capsule as we increase the capillary number $\mathit{Ca}$, which measures the ratio between the viscous and elastic forces. The three-dimensional fluid–structure interaction problem is solved numerically by coupling a boundary integral method (for the internal and external flows) with a finite element method (for the wall deformation). For any initial inclination with the flow vorticity axis, a given capsule converges towards a unique equilibrium configuration which depends on $\mathit{Ca}$. At low capillary number, the stable equilibrium motion is the rolling regime: the capsule aligns its long axis with the vorticity axis, while the membrane tank-treads. As $\mathit{Ca}$ increases, the capsule takes a complex wobbling motion at equilibrium, precessing around the vorticity axis. As $\mathit{Ca}$ is further increased, the capsule long axis oscillates about the shear plane, while the membrane rotates around a capsule cross-section that also oscillates over time (oscillating–swinging regime). The amplitude of the oscillations about the shear plane decreases as $\mathit{Ca}$ increases and the capsule finally takes a swinging motion in the shear plane. It is found that the transitions from rolling to wobbling and from wobbling to oscillating–swinging depend on the mean energy stored in the membrane.
Rolling motion of a prolate capsule with an aspect ratio of 2 and a strain-hardening Skalak membrane (C2SK). The initial inclination ζ0 of the revolution axis with the vorticity axis of the shear flow is 85°; the capillary number is Ca = 0.1. The points M and N are the membrane points initially located on the short and long axis respectively. Point P is at the tip of the long axis at time t.
Wobbling motion of a prolate capsule with an aspect ratio of 2 and a strain-hardening Skalak membrane (C2SK). The initial inclination ζ0 of the revolution axis with the vorticity axis of the shear flow is 15°; the capillary number is Ca = 0.9. The points M and N are the membrane points initially located on the short and long axis respectively. Point P is at the tip of the long axis at time t.
Oscillating-swinging of a prolate capsule with an aspect ratio of 2 and a strain-hardening Skalak membrane (C2SK). The initial inclination ζ0 of the revolution axis with the vorticity axis of the shear flow is 60°; the capillary number is Ca = 1.5. The points M and N are the membrane points initially located on the short and long axis respectively. Point P is at the tip of the long axis at time t.