Contact line pinning and de-pinning can modulate the rod-climbing effect

Our experiments on the rod-climbing effect with an oil-coated rod revealed two key differences in the rod-climbing phenomena compared to a bare rod. On one hand, an enhancement in magnitude of climbing height for any particular value of rod rotation speed and second, a decrease in threshold rod rotation speed required for the appearance of the rod-climbing effect were observed. Observed phenomena is explained by considering the contact line behavior at the rod-fluid interface. Transient evolution of meniscus at the rod-fluid interface revealed that the three-phase contact line was pinned for a bare rod and de-pinned for an oil-coated rod. We modelled the subject fluid as a Giesekus fluid to predict the climbing height. The differences in the contact line behaviour were incorporated via the contact angle at the rod-fluid interface as a boundary condition. An agreement was found between the observed and predicted climbing height establishing that contact line behaviour may modulate rod-climbing effect.