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Elastohydrodynamic interactions at small scales Nasouri, Babak


In this dissertation, the effects of elasticity on hydrodynamic interactions at small scales are investigated. In the microscale realm of microorganisms, inertia is irrelevant and viscous dissipation dominates the fluid motion and particles within it. As a result of this inertialess environment, microorganisms use non-reciprocal body distortions to facilitate locomotion and exhibit nontrivial behaviors in interacting with their surroundings; behaviors that have been shown to be intimately correlated to the elasticity of the cell body, or its small appendages called flagella (or cilia). Motivated by experimental observations, the effects of elasticity on hydrodynamic interactions of motile cells are investigated, using theoretical approaches. First, to model the flow field induced by microswimmers, a framework is given to account for the effects of the higher-order force moments. Specifically, the contribution of the second-order force moments of the flow field is evaluated, and explicit formulas are reported for the stresslet dipole, rotlet dipole, and potential dipole for an arbitrarily shaped active particle. For an elastic swimmer near a boundary, it is shown that the rotlet dipole bends the swimmer and results in qualitatively different swimming behaviors in comparison to the case of a rigid swimmer. Furthermore, it is demonstrated that elasticity can be exploited to evade the kinematic reversibility of the field equations in Stokes flow. A model elastic swimmer is proposed that despite the reversible actuation, can propel forward due to its nonreciprocal body deformations. The effect of elasticity in the formation of metachronal waves in ciliated microorganisms such as Paramecium and Volvox is also studied. Using a minimal model, it is shown that elastohydrodynamic interactions of cilia attached to a curved body lead to synchronization, with zero phase difference, thereby preventing the formation of wave-like behaviors unless an asymmetry is introduced to the system. Finally, the dynamics of capillary rise between two porous and elastic sheets are investigated. The liquid, as it rises, diffuses through the sheets and changes their properties. The significant drop in sheet bending rigidity due to wetting, causes the system to coalesce faster, compared to the case of impermeable sheets, and also remarkably reduces the absorbance capacity.

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