The yielding of defect-entangled dispersions in a nematic solvent

Abstract

Oscillatory rheology, at both small and large (LAOS) amplitudes, was performed to measure the dynamic response of a soft solid, formed on dispersing colloids into a thermotropic nematic liquid crystal at volume fractions of f. 18%. Due to weak homeotropic anchoring of nematogens at colloid surfaces, a Saturn-ring defect line, known as a "disclination," encircles each particle and entangles with neighboring Saturn-ring disclinations [Wood et al., Science 334, 79-83 (2011)]. We present the first experimental investigation of the yielding behavior of the resulting gel to reveal the underpinning physics. Results reveal that the frequency response of the composite is independent of volume fraction f, an indication that the dispersed phase simply increases the density of disclinations spanning the composite without further effect. Beyond the linear viscoelastic regime (LVR), LAOS experiments indicate the composite is an elasto-plastic fluid exhibiting both strain-hardening and shear-thinning behaviors with Chebyshev coefficients e 3. 0 and ν 3 , 0, respectively. We deduce that the disclination number density n d is constant until the strain amplitude is sufficient to break disclinations leading to shear-thinning behavior beyond the LVR. A simple theory is introduced revealing that the Ericksen number E r determines the onset of flow, when E r. 1, generating a strain-hardening response since Frank elasticity resists reorientation of molecular alignment within confined nematic domains. Above a critical frequency ω c , the loss modulus G 00 increases slowly due to enhanced viscosity within confined nematic domains, G 00 / ω 1=2 [Larson, R. G., The Structure and Rheology of Complex Fluids (Oxford University, 1999), p. 463]. Observation of this behavior in a small-molecule nematic solvent provides insights into the physics of flow behavior in other, more complex, defect-mediated liquid crystalline structures exhibiting similar properties [Colby, Europhys. Lett. 54, 269-274 (2001); Sahoo and Dhara, Liq. Cryst. 44, 1582-1591 (2017); and Romo-Uribe, Polym. Adv. Technol. 32, 651-662 (2021)].