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 ϕ>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 ϕ, 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 elastoplastic fluid exhibiting both strain-hardening and shear-thinning behaviors with Chebyshev coefficients e3>0 and ν3<0, respectively. We deduce that the disclination number density nd 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 Er determines the onset of flow, when Er>1, generating a strain-hardening response since Frank elasticity resists reorientation of molecular alignment within confined nematic domains. Above a critical frequency ωc, the loss modulus G″ increases slowly due to enhanced viscosity within confined nematic domains, G″∝ω1/2 [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)].
Highlights
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%
We explore, systematically, a)Author to whom correspondence should be addressed; electronic mail: tiffany.wood@ed.ac.uk the yielding behavior of these composites using smallamplitude oscillatory strain (SAOS) and large-amplitude oscillatory strain (LAOS) measurements using a straincontrolled rheometer, and we present a simple theoretical model to explain the behavior
We bring SAOS and LAOS experiments together with a simple theory to explore the dynamic behavior of an extraordinarily stable soft-solid formed upon dispersing colloids, of volume fractions 18%, f, 45%, in a nematic phase where nematogens are weakly oriented perpendicular to the colloid surface
Summary
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%. 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 . Results reveal that the frequency response of the composite is independent of volume fraction f, an indication that the dispersed phase increases the density of disclinations spanning the composite without further effect. Beyond the linear viscoelastic regime (LVR), LAOS experiments indicate the composite is an elastoplastic fluid exhibiting both strain-hardening and shear-thinning behaviors with Chebyshev coefficients e3 . More in-depth insights into yielding behavior can be revealed through LAOS measurements, having been applied to a wide variety of soft matter systems including polymer solutions [9], drilling fluids [10], dense suspensions [11], and wormlike micelles [12]. We present the first LAOS measurements and analysis on colloidal dispersions in nematic liquid crystals
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