| Knowledge of the rheological properties of leukocytes (white blood cells) is essential not only for the comprehension of microcirculatory flow dynamics, but also for the understanding of their functions and behavior in health and disease. These properties, together with the leukocyte's structural characteristics, determine the deformability of the cell, especially during large deformations, such as those involved in their release from the bone marrow or extravasation into the interstitium. Existing viscoelastic solid, Newtonian and non-Newtonian liquid drop models used to calculate the cell rheological properties do not adequately explain reported experimental observation. This is because such models do not take into account the morphology of the cell and its multi-layer structure.;In this work, the deformation and recovery of a compound liquid drop (three-layer fluid model) are investigated by means of a mixed Eulerian-Lagrangian computational methodology which allows large viscosity and capillarity differences between layers. Analysis of the deformation process confirms published results and clearly indicates that the history of the deformation is stored in the compound drop in terms of the shape of its components. The recovery process is found to provide information on the hydrodynamics of compound drops that is not supplied by the deformation analysis. It is discovered that a compound drop does not recover like a single phase Newtonian liquid drop except when the core is sufficiently deformed and its material properties are such that the core deformation/recovery time scale is compatible with that of the shell layer. A difference in the core and shell layer time scales causes an initial rapid recoil of the drop during which the shell fluid is the sole participant in the hydrodynamics, followed by a slower relaxation period during which the core and shell layer couple. The large deformation and subsequent recovery processes of compound drops also provide new insights into the complicated behavior of leukocytes. This three-layer fluid model describes the morphology of leukocytes better than existing ones. It consists of an outer interface (cell membrane), containing a shell layer (cytoplasm), and a core (nucleus).;Based on the compound drop dynamics, it is concluded that (1) the nucleus plays an essential role in the cell response, and (2) the energy stored in the deformed interfaces can be important. The present investigation indicates that unless the nucleus and its deformation are included in the analyses, neither Newtonian nor non-Newtonian drop models for leukocytes can yield a reliable picture of the hydrodynamics of such cells. |
