Transient response of a capsule subjected to varying flow conditions:: Effect of internal fluid viscosity and membrane elasticity

The transient deformation of an axisymmetric capsule freely suspended in a pure straining flow is studied, for sudden or periodic variations of the intensity of the rate of strain. The particle Reynolds number is supposed to be very small and the problem is solved numerically by means of the boundary integral method. In the case of a sudden start of flow, the time response of the capsule can be approximated by an exponential function, and is thus characterized by only two parameters: the equilibrium deformation D-infinity and the characteristic response time tau(s). The respective influence of viscosity ratio, membrane elasticity, and initial particle geometry is analyzed. The dynamic response of the capsule subjected to periodic variations of the rate of strain is also studied. The response time tau(s) appears to be an appropriate parameter to estimate the capsule adaptability to changing flow conditions. (C) 2000 American Institute of Physics. [S1070-6631(00)00605-X].