Micro-scale viscosity measurements of different thermotropic and lyotropic classes of liquid crystals by using ferrofluid inclusions

Dispersing micro-and nanoparticles in liquid crystals (LCs) is utilised to tune liquid crystal properties, to add functionality or to exploit the self-organization of the liquid crystals to transfer order onto dispersed particles. Dispersing ferrofluid droplets in LCs produces a system similar to LC-microparticle dispersions, because the ferrofluid droplets behave in first approximation as non-compressible, only slightly deformable particles. Such a LC-ferrofluid system adds a magnetic functionality, which can be exploited to measure the viscosity and its anisotropy on a microscopic scale when moving the ferrofluid inclusions through various liquid crystals of the thermotropic, lyotropic or colloidal kind. Effective viscosities can be determined as a function of changing environment, such as temperature or concentration, and for variation of other factors such as the pitch of chiral phases for instance. The viscosities are calculated using Stokes' Law together with the introduction of a boundary layer at the liquid crystal - ferrofluid interface. We present results for a variety of different liquid crystalline systems; thermotropic nematics, chiral nematics, smectics, lyotropic phases and chromonic systems, as well as colloidal LCs such as cellulose nanocrystals (CNC) and graphene oxide (GO) varying the viscosity over more than three orders of magnitude. The results are discussed in terms of the structures of respective phases.