(470f) A Micro-Swimmer Model with Dipolar, Quadrupolar and Rotlet Dipolar Flows

To study the collective swimming of a
peritrichous bacterium such as Escherichia coli, we model a
micro-swimmer using a Low-Order Multipole Swimming (LOMS) model.  This model consists of an ellipsoidal
particle that migrates at a constant velocity along its long axis and generates
a flow of a superposition of a Stokeslet dipole, a Stokeslet quadrupole and a
Rotlet dipole. The magnitude of these three flow components are found by
modeling a single E.coli cell using a bead-spring model containing 120
beads. The bead-spring model accounts for: 1), the
hydrodynamic and the mechanical interactions among the cell body and multiple
flagella; 2), the reversal of the rotation of a flagellum in a tumble; and 3),
the associated polymorphic transformations of the flagellum. Because a flexible
hook connects the cell body and each flagellum, the flagella can take
independent orientations with respect to the cell body. This simulation
reproduces the experimentally observed behaviors of E. coli, including the steady
clockwise swimming near a wall. The reduction of this 120-bead model to a
simple LOMS is therefore the first multi-scale simulation study
on micro- swimmers. On the contrary to the previous study using dipolar flow
swimmer, it is found that the hydrodynamic interaction between micro-swimmers
gives a significant contribution to the cell-cell scattering, or rotational
diffusion of swimmers, mainly because of the higher-order flows,
i.e. the Stokeslet quadrupole and Rotlet dipole.