Direct simulations of cells motions and deformations in flow

Direct numerical simulations (DNS) are used to study the motions and deformations of blood cells, especially leukocytes, in pressure driven flows in parallel plate channels with both smooth and uneven walls under adhesion force between the leukocytes and the channel wall. Leukocytes are represented by two composite fluid models. The first model is the compound-drop model in which the cytoplasm and the nucleus are modeled as fluids, and the second one is the drop-rigid-particle model in which the cytoplasm is modeled as a fluid and the nucleus as a rigid particle. The adhesion force is computed using two adhesion force models. In the first model, the adhesion force is given by a potential, and in the second model it is given by Dembo\u27s kinetic adhesion model. The numerical code is based on the finite element method and the level-set technique is used to track the cell membrane position. In the absence of the adhesion force, in a pressure driven flow the leukocyte moves away from the wall to an equilibrium location. In presence of the adhesion force, provided it is located within the range of the force, the leukocyte is attracted to the layer of endothelial cells and it flattens under the action of hydrodynamic forces. It is found that for the normal parameter values and flow rates the adhesive force given by the kinetic model is too small to capture the leukocyte. The time at which all bonds are broken and the leukocyte moves away from the wall increases when the capillary number is increased, and decreases with increasing Reynolds number. The former suggests that the adhesion tendency of a leukocyte increases as its cortical tension is reduced. The distance traveled by a leukocyte before all bonds are broken increases with the Reynolds and capillary numbers. The rolling velocity of the leukocyte near an uneven wall varies in the sense that it appears to slip when its lower surface is in the gap between the spheres and stick when it comes close to the spheres\u27 surfaces, which is in qualitative agreement with the experimental data