

	Mekanik In English

Doktorsdisputation

Simulation of individual cells in flow


Respondent	Huvudhandledare	Bihandledare	Datum
Lailai Zhu	Luca Brandt	Gustav Amberg	2014-03-28

Opponent
Michael Graham, University of Wiscosin-Madison, School of Engineering

Betygsnämd
Anne Juel, University of Manchester Anne-Virginie Salsac, Université de Technologie de Compiègne Anna-Karin Tornberg, Kungliga Tekniska Högskolan

Abstract

In this thesis, simulations are performed to study the motion of individual cells in ow, focusing on the hydrodynamics of actively swimming cells like the selfpropelling microorganisms, and of passively advected objects like the red blood cells. In particular, we develop numerical tools to address the locomotion of microswimmers in viscoelastic uids and complex geometries, as well as the motion of deformable capsules in micro- uidic ows. For the active movement, the squirmer is used as our model microswimmer. The nite element method is employed to study the in uence of the viscoelasticity of uid on the performance of locomotion. A boundary element method is implemented to study swimming cells inside a tube. For the passive counterpart, the deformable capsule is chosen as the model cell. An accelerated boundary integral method code is developed to solve the uid-structure interaction, and a global spectral method is incorporated to handle the evolving cell surface and its corresponding membrane dynamics. We study the locomotion of a neutral squirmer with an emphasis on the change of swimming kinematics, energetics, and ow disturbance from Newtonian to viscoelastic uid. We also examine the dynamics of dierent swimming gaits resulting in dierent patterns of polymer deformation, as well as their in uence on the swimming performance. We correlate the change of swimming speed with the extensional viscosity and that of power consumption with the phase delay of viscoelastic uids. Moreover, we utilise the boundary element method to simulate the swimming cells in a straight and torus-like bent tube, where the tube radius is a few times the cell radius. We investigate the eect of tube con nement to the swimming speed and power consumption. We analyse the motions of squirmers with dierent gaits, which signi cantly aect the stability of the motion. Helical trajectories are produced for a neutral squirmer swimming, in qualitative agreement with experimental observations, which can be explained by hydrodynamic interactions alone. We perform simulations of a deformable capsule in micro- uidic ows. We look at the trajectory and deformation of a capsule through a channel/duct with a corner. The velocity of capsule displays an overshoot as passing around the corner, indicating apparent viscoelasticity induced by the interaction between the deformable membrane and viscous ow. A curved corner is found to deform the capsule less than the straight one. In addition, we propose a new cell sorting device based on the deformability of cells. We introduce carefully-designed geometric features into the ow to excite the hydrodynamic interactions between the cell and device. This interaction varies and closely depends on the cell deformability, the resultant dierence scatters the cells onto dierent trajectories. Our high- delity computations show that the new strategy achieves a clear and robust separation of cells. We nally investigate the motion of capsule in a wall-bounded oscillating shear ow, to understand the eect of physiological pulsation to the deformation and lateral migration of cells. We observe the lateral migration velocity of a cell varies non-monotonically with its deformability.