You have finally gotten the grasp on viscosity, and now you're being told that you need to wrap your head around the idea of both zero viscosity and negative viscosity? We must be kidding, right? We are not! In fact, recent experiments have demonstrated not only that negative viscosity can be achieved, but also the impactful effect this can have. Researchers from Paris-Sud University have shown how bacteria, in particular E. coli, can alter the viscosity of a liquid by changing the hydrodynamic properties of the fluid they swim on Phys. Rev. Lett. 115, 028301 (2015).
Viscosity, in simple terms, is defined as 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. A viscosity measurement of zero means that the fluid will continue to flow indefinitely. This state allows mass to transfer without the transfer of energy and provides the superfluid the ability to overcome friction and therefore overflow its containers.
The surprise came around when they overfed these tiny bacteria with additional nutrients, and as a result, their rheometer started to measure not only a fluid viscosity of zero, but also negative values of viscosity. With the additional nutrients, 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.
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