Negative viscosity of a liquid crystal in the presence of turbulence

Abstract

We report on the discovery of enormous negative viscosity in a nematic liquid crystal in the presence of turbulence induced by electric fields. As the negative viscosity in this system is so large, we are able to observe several phenomena originating from it. For example, we observe a spontaneous shear flow that rotates the upper disk of a rheometer, as well as the reversal of the rotational direction upon applying an external torque in the opposite direction. Hysteresis loops are also observed in the shear-stress-shear-rate curves, which is reminiscent of those seen for ferromagnetic and ferroelectric materials. The similarities between the phenomena observed for our system and ferroic materials are comprehensively demonstrated, although the two systems are fundamentally different in that the former is out of equilibrium. We elucidate the origin of the negative viscosity and propose a simple model that reproduces the phenomena observed in this active fluid.

Negative viscosity of liquid crystals in the presence of turbulence: Conductivity dependence, phase diagram, and self-oscillation

“…The shear stress for EBBA(H) first increased to a maximum at about 0.5 V/μm, and then decreased monotonically with increasing electric field, and the shear stress became zero around 1.2 V/μm and negative as the electric field increased further. The behavior of EBBA(H) and EBBA(L) (results not shown) was similar to that of MBBA [1].…”

“…The negative viscosity is closely related to electric fieldinduced turbulence [1]. Figure 3 shows the structures of EBBA(H) and EBBA(P) observed with a microscope.…”

“…Taking advantage of the enormous negative viscosity of MBBA, we successfully observed several characteristic phenomena with a conventional rheometer [1]. We observed a spontaneous shear flow that rotated the upper disk of the rheometer, as well as reversal of the rotational direction upon application of an external torque in the opposite direction.…”

“…We recently discovered enormous negative viscosity of the nematic liquid crystal MBBA (p-methoxybenzylidenep -n-butylaniline) in the presence of turbulence induced by electric fields [1]. The viscosity of a fluid is a measure of its resistance to flow.…”

“…In suspensions of Escherichia coli, negative viscosity was first measured, although it was very low (approximately −10 −1 mPa s) [5]. In contrast, MBBA exhibited much larger negative viscosity of −40 mPa s [1].…”