They may be simple bacteria, but they have an expert take on the principles of fluid dynamics.
Bugs such as Escherichia coli move by rotating a rod-like flagellum that trails from their cell wall. Experiments now show that this swimming stroke is no wild flailing, but a subtle technique to maximise thrust.
To model the movement of a bacterium鈥檚 flagellum through water, mechanical engineer Bian Qian and colleagues at Brown University in Providence, Rhode Island, lowered an elastic rod into oil 鈥� they used oil as it simulates the viscosity of water at the scale of bacteria. The top end of the rod was connected to a motor and the rod was set at a slight angle before being rotated.
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At slow speeds the rod remained rigid, tracing out the surface of a cone-as a weighted piece of string might if the weight swung in a circle. This simple motion provides no net thrust, but only forces that change direction and cancel each other out during each rotation.
At higher speeds, however, things get interesting. First, the flexible rod begins to bend a little because of the friction caused by its movement through the liquid. This creates a very small net force. But then, at a critical speed, the friction on the rod becomes large enough to deform it, abruptly, into a corkscrew formation. At this point, the experiments show, the thrust it produces increases sharply, due to the way fluid flows around the helical structure.
Powers says the results confirm earlier computer simulations suggesting that the deformation of a rotating appendage is key to producing significant thrust. 鈥淔lagella aren鈥檛 only flexible; they have the helical shape built in,鈥� says team-member Thomas Powers. This means flagella can exploit this trick of fluid dynamics even at slower rotation speeds. The research is available online ().
