Asymmetric instability in shear thinning flow down a fiber*

Thin film flow down a fiber is common in many industrial applications including fiber coating and heat/mass transfer processes. Such flows are subject to a number of instabilities including Plateau-Rayleigh breakup, isolated bead formation, and convective instabilities, which typically result in symmetric beading patterns. Recently, an asymmetric instability was observed in Newtonian fluids that is dependent upon surface tension and fiber diameter, but independent of viscosity. Here, we perform an experimental study of thin film flow down fibers with non-Newtonian polymer solutions and reveal how shear thinning behavior can also give rise to an asymmetric instability. We prepare several xanthan gum solutions, tuning their rheology by adding sodium chloride (NaCl) and surfactant (Tween 20). Experimental results show that increasing the intensity of shear thinning, with all other fluid properties held constant, leads to asymmetric beading patterns. We conclude by proposing a physical mechanism for these experimental observations.