Guilhem Poy

Unsteady flow and particle migration in dense, non-Brownian suspensions

We present experimental results on dense corn-starch suspensions as examples of non-Brownian, nearly hard particles that undergo continuous and discontinuous shear thickening (DST) at intermediate and high densities, respectively. Our results offer strong support for recent theories involving a stress-dependent effective contact friction among particles. We show, however, that in the DST regime, where theory might lead one to expect steady-state shear bands oriented layerwise along the vorticity axis, the real flow is unsteady. To explain this, we argue that steady-state banding is generically ruled out by the requirement that, for hard non-Brownian particles, the solvent pressure and the normal-normal component of the particle stress must balance separately across the interface between bands. (Otherwise, there is an unbalanced migration flux.) However, long-lived transient shear bands remain possible.

Continuous Rotation of Achiral Nematic Liquid Crystal Droplets Driven by Heat Flux

Suspended droplets of cholesteric (chiral nematic) liquid crystals spontaneously rotate in the presence of a heat flux due to a temperature gradient, a phenomenon known as the Lehmann effect. So far, it is not clear whether this effect is due to the chirality of the phase and the molecules or only to the chirality of the director field. Here, we report the continuous rotation in a temperature gradient of nematic droplets of a lyotropic chromonic liquid crystal featuring a twisted bipolar configuration. The achiral nature of the molecular components leads to a random handedness of the spontaneous twist, resulting in the coexistence of droplets rotating in the two senses, with speeds proportional to the temperature gradient and inversely proportional to the droplet radius. This result shows that a macroscopic twist of the director field is sufficient to induce a rotation of the droplets, and that the phase and the molecules do not need to be chiral. This suggests that one can also explain the Lehmann rotation in cholesteric liquid crystals without introducing the Leslie thermomechanical coupling-only present in chiral mesophases. An explanation based on the Akopyan and Zeldovich theory of thermomechanical effects in nematics is proposed and discussed.

Droplet relaxation in Hele-Shaw geometry: Application to the measurement of the nematic-isotropic surface tension.

Shape measurements after the coalescence of isotropic droplets embedded in a thin sample of a homeotropic nematic phase provides a tool to measure the nematic-isotropic surface tension. In addition, this experiment allows us to check the scaling laws recently given by Brun et al. [P.-T. Brun, M. Nagel, and F. Gallaire, Phys. Rev. E 88, 043009 (2013)] to explain the relaxation of ellipsoidal droplets in a Hele-Shaw cell.

Lehmann rotation of twisted bipolar cholesteric droplets: role of Leslie, Akopyan and Zel’dovich thermomechanical coupling terms of nematodynamics

ABSTRACT We show that the Leslie and texture-dependent Akopyan and Zel’dovich thermomechanical coupling terms of nematodynamics cannot explain the thermal Lehmann rotation of the twisted bipolar droplets observed in the coexistence region between a cholesteric phase and the isotropic liquid. On the other hand, these terms are pertinent below the transition temperature and can be determined by measuring the director rotation velocity in two different molecular configurations. In addition, a complete characterisation of the liquid crystal used (CCN-37) is also given. Graphical Abstract

Experimental relationship between surface and bulk rotational viscosities in nematic liquid crystals

Systematic measurements of the surface viscosity of several nematic liquid crystals in contact with a polymercaptan layer show that the latter scales like the bulk rotational viscosity . This result is interpreted in the framework of a ‘delocalized model’ for the surface viscosity.

On the existence of the thermomechanical terms of Akopyan and Zel’dovich in cholesteric liquid crystals

ABSTRACT We revisit a theoretical paper of Akopyan and Zel’dovich about the thermomechanical coupling terms in nematic liquid crystals. We show that the expressions of these terms given by these authors must be corrected to satisfy the Onsager reciprocity relations, a point already stressed by Pleiner and Brand in 1987. We then extend this calculation to the cholesteric phase and show that there are no additional terms in the uniaxial approximation of this phase. Finally, we give the correspondence between the Akopyan and Zel’dovich terms and those calculated by Pleiner and Brand in 1996 by making a different choice for the forces and the fluxes in the theory. Graphical Abstract

Fréedericksz transition under electric and rotating magnetic field: application to nematics with negative dielectric and magnetic anisotropies

ABSTRACT We study the action of a rotating magnetic field on the Fréedericksz instability under electric field of a homeotropic nematic sample sandwiched between two parallel electrodes. The liquid crystal (LC) is of negative dielectric and magnetic anisotropies and the magnetic field is parallel to the electrodes used to apply the electric field. We show that the sample destabilises above a critical voltage that depends on the magnetic field and its angular rotation velocity . The relation is calculated analytically in the synchronous regime, where the director rotates at the same angular velocity as the magnetic field. These predictions are compared to the experiment performed with the LC CCN-37. From this experiment, the values of the bend constant , of the rotational viscosity and of the magnetic anisotropy are deduced. Graphical Abstract