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Negative Viscosity? You've Got to Be Kidding!

Posted by David Nieto on Aug 4, 2015 10:10:00 AM

In Rheometer, viscosity

We are not! In fact, researchers from Paris-Sud University have shown how bacteria, in particular E. coli, can contribute to flow by changing the hydrodynamic properties of the fluid they swim on Phys. Rev. Lett. 115, 028301 (2015). 

Viscosity is a fluid’s resistance to flow.The propelling motion of swimming micro-organisms can locally change the flow dynamics and lower the viscosity of the fluid by aligning themselves such that their pushing contributes to flow. It had been predicted that for low to moderate shear stress values these micro-organisms could reduce the viscosity of a fluid to zero, making it a superfluid, such as liquid helium. This means that the fluid will continue to flow indefinitely. 

The surprise came around when they overfeed these tiny bacteria, and as a result, their rheometer started to measure negative values of viscosity. The bacteria started propelling the fluid beyond the imposed shear! These results suggest that the bacterial motion can be used to drive micro-mechanical devices such as microfluidic pumps.