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Electrokinetic instabilities in a shear thinning fluid start at a smaller electric field and an earlier location than in a Newtonian fluid but with a smaller wave amplitude and frequency. 

Thin-film flow down a fibre exhibits rich dynamics and is relevant to applications such as desalination, fibre coating and fog harvesting. These flows are subject to instabilities that result in dynamic bead-on-fibre patterns. We perform an experimental study of shear-thinning flow down fibres using 20 different xanthan gum solutions as our working liquid. The bead-on-fibre morphology can be oriented either symmetrically or asymmetrically on the fibre, and this depends upon the surface tension, fibre diameter and liquid rheology, as defined by the Ostwald power-law index. For highly shear-thinning liquids, it is possible for the pattern to be complex and exhibit simultaneously both asymmetric large beads and symmetric small beads in the isolated and convective flow regimes. We quantify the transition between flow regimes and bead dynamics for the asymmetric morphology, and compare with Newtonian flow, as it depends upon the experimental parameters. Finally, the dimensionless bead frequency is shown to scale with the Bond number for all of our experimental data (symmetric and asymmetric). 

Surfactants are often added to particle suspensions in the flow of Newtonian or non-Newtonian fluids for the purpose of reducing particle-particle aggregation and particle-wall adhesion. However, the impact on the flow behavior of such surfactant additions is often overlooked. We experimentally investigate the effect of the addition of a frequently used neutral surfactant, Tween 20, at the concentration pertaining to microfluidic applications on the entry flow of water and three common polymer solutions through a planar cavity microchannel. We find that the addition of Tween 20 has no significant influence on the shear viscosity or extensional flow of Newtonian water and Boger polyethylene oxide solution. However, such a surfactant addition reduces both the shear viscosity and shear-thinning behavior of xanthan gum and polyacrylamide solutions that each exhibit a strong shear-thinning effect. It also stabilizes the cavity flow and delays the onset of flow instability in both cases. The findings of this work can directly benefit microfluidic applications of particle and cell manipulation in Newtonian and non-Newtonian fluids.