Suspensions of titania nanoparticle networks in nematic liquid crystals: rheology and microstructure

We study the influence of confinement on the rheology and structure of nematic liquid crystals (NLCs). NLCs get confined in networks of titania (TiO₂, primary particle size = 21 nm) nanoparticles in suspensions of TiO₂ and NLC, N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Suspensions with TiO₂ nanoparticle volume fraction (?) of 0.006–0.017, form viscoelastic solids with low elastic modulus (G′) of 10¹ Pa–10² Pa and short relaxation times. Increase in TiO₂ nanoparticle ? leads to a rise in G′ with TiO₂ nanoparticles forming a percolating network at a critical volume fraction (? _c) = 0.023, and G′ of ~10³ Pa. TiO₂/MBBA NLC suspensions at and above ? _c = 0.023 show G′ ~ ω ^x?1 scaling, where ω is the angular frequency and the minimum in loss modulus (G′′) with ω. The effective noise temperature, x decreases and approaches 1 with the increase in the TiO₂ nanoparticle ? from 0.023–0.035, is indicative of an increase in the glassy dynamics. Through the polarized light microscopy and differential scanning calorimetry experiments, we propose that the progressive addition of TiO₂ nanoparticles introduces a quenched random disorder (QRD) in the NLC medium which disturbs the nematic order. This results in metastable TiO₂/MBBA NLC suspensions in which NLC domains get confined in the network of flocs of TiO₂ nanoparticles. We also show that the salient rheological signatures of soft glassy rheology develop only in the presence of NLC MBBA and are absent in the isotropic phase of MBBA.