Hydrodynamic interaction between two nonspherical capsules in shear flow.

The hydrodynamic interaction between two nonspherical capsules suspended in a simple shear flow is studied numerically using a front-tracking method. The capsules are enclosed by thin shells which develop in-plane tensions and bending moments due to a preferred three-dimensional unstressed configuration. Computations are performed for capsules with spherical, oblate spheroidal, and biconcave unstressed shapes for a wide range of dimensionless shear rates and initial separation distances between the two capsules. The bending modulus and viscosity ratio between the internal and surrounding fluids are chosen to be those of healthy red blood cells. Depending on the initial separation distance, we find that two spherical capsules in shear flow either cross over each other or undergo spiraling motion. In addition, the long-time interaction behavior of capsules also depends strongly on the unstressed shapes. More oblate or biconcave capsules exhibit two additional type of interactions, namely swapping and continuous rotation, which occur only when each capsule undergoes tumbling motion.