Suspensions, nematics under shear

Aims and techniques:

Experimental and numerical studies were performed to study shear induced orientational phenomena in suspensions of elongated particles as well as in nematic liquid crystals.

Latest results:

Suspensions of neutrally buoyant elliptic particles were modeled in 2D using fully resolved simulations that provide two-way interaction between the particle and the fluid medium. Forces due to particle collisions were represented by a diffuse interface approach that allows the investigation of dense suspensions (up to 47% packing fraction). We focus on the role inertial forces play at low and high particle Reynolds numbers. The suspensions are characterized by the orientation distribution function (ODF) that reflects shear induced rotation of the particles at low Reynolds numbers, and nearly stationary (swaying) particles at high Reynolds numbers. In both cases, orientational ordering differs qualitatively from the behavior observed in the Stokesian-regime. The ODF becomes flatter with increasing packing fraction, as opposed to the sharpening previous work predicted in the Stokesian regime. The ODF at low particle concentrations differs significantly for the low Reynolds number and inertial regimes, whereas with increasing packing fraction convergence is observed. For dense suspensions, the particle–particle interactions dominate the particle motion. Soft Matter 16, 8925 (2020) (download pdf)

We studied the rheological and rheo-optical properties of suspensions of anisometric pigment particles in a non-polar fluid experimentally. Different rheological regimes from the dilute regime to an orientationally arrested gel state were characterized and compared with existing theoretical models. We demonstrate the intricate flow behaviour in a wide range of volume fractions. A unique combination of the optical properties of the particles results in a giant rheo-optical effect: an unprecedentedly large shear stress-induced birefringence was found in the isotropic range, exhibiting a sharp pre-transitional behaviour. J. Mol. Liq. 313, 113401 (2020) (download pdf)

Since the relaxation of the director distortions in nematic liquid crystals is much slower than the relaxation of velocity fluctuations, orientational instabilities take place at much smaller shear rate than the laminar-turbulent transition. We measured the threshold of the "roll instability" as a function of the frequency of the rectilinear oscillatory shear in a homeotropically oriented nematic layer. The results are compared with numerical linear stability analysis. [Phys.Rev.E. 58, 7419 (1998)]

In elliptic shear a director precession is observed that is much slower than the frequency of the excitation. It depends on the tilt of the director that is controlled by an electric field across the sample. A non-monotonic behavior is found experimentally where the different regimes correspond to different types of pattern formation [Phys. Rev. Lett. 84, 1934 (2000)].

In oscillatory compression (one of the plates is moving perpendicular to the plane of the sample with a typical frequency of f = 1-100 kHz) a similar slow precession of the director is generated if the director is tilted (homeotropic boundary conditions, externally induced tilt by an electric field for substances with negative dielectric anisotropy). By increasing the tilt anlge the precession reverses, which was reproducible in several samples using various materials. [Phys. Rep. 337, 171 (2000)].