Bacterial Fluid Mechanics

( in collaboration with Prof. Tom Powers)

We have been engaged over several years in a program to understand the mechanics of flagellar motility. Many bacteria, such as E. coli or S. marascens propel themselves through water by means of several helical flagella, each of which are rotated by a nanoscale motor embedded in the cell wall.

Using macroscopic "flagella" immersed in a tank of high viscosity fluid, we can model the microscopic flow environment, matching the Reynolds number as well as the balance of viscous to elastic stresses, One experiment explores the process by which two flagella come together into a single bundle.

Macroscopic model of two bacterial flagella bundling.The video shows the front and side views, and illustrates how, when the helices start to rotate, they immediately begin to cross each other and eventually intertwine. This video is speeded up by a factor of twelve. (Kim et al, PNAS 2003) .

E. coli, swimming in water. Video courtesy of Howard Berg and Linda Turner (Harvard University)

Single flexlble filament, rotating in a viscous tank, viewed simultaneously from two angles. The root of the filament is set at an angle with respect ot the axis of rotation.

Three-dimensional reconstruction of experimental measurements of a flexible rod being deformed as it rotates in a viscous fluid. You can see the transition from the "splayed" form to the helical form. (Qian et al, PRL 2008)

A related experiment models the behavior of a single flexible filament as it is rotated in a viscous fluid. This is relevant for flagellar mechanics, but is also a possible mechanism for the propulsion of microscale swimming "robots". If the rod is at an angle to the axis of rotation, it initially is only bent back slightly by the viscous stresses. However, as the torque increases, the rod undergoes a shape transformation, adopting a helical shape, and generating significant proulsive thrust. This is a model for the behavior of flexible filaments such as cilia or flagella, although there are significant differences between the geometries in nature and this simple model problem.

Most recently, we have been looking at hydrodynamic synchronization, in which adjacent filament phase-lock due to hydrodynamic forces. This is of importance in bacterial motility, but also in other biological systems such as cilla in the lungs, and in embryo development.



In other work, we have also looked at the use of bacteria as actuators for microfluidic systems, and have demonstrated the enhancement of mixing and more excitingly, the generation of pumping systems that are powered by bacteria, and the ability of bacterial to push microscopic bits of PDMS around. This work (performed in collaboration with Howard Berg, Linda Turner and Nick Darnton at Harvard University) was the basis of Dr MinJun Kim's PhD thesis, and is now a major theme in the lab of MinJun Kim, who is now an assistant professor at Drexel University.

Our research is supported by NSF

Wedge of PDMS, being pushed by S. Marascens attached to the surface of the wedge. (Darnton et al, Biophyical J. 2004).