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

TL;DR

This study investigates how contact line pinning and de-pinning influence the rod-climbing effect, showing that oil coating alters contact line behavior, thereby enhancing climbing height and reducing the threshold rotation speed.

Contribution

The paper demonstrates that contact line behavior at the rod-fluid interface significantly modulates the rod-climbing effect, supported by experimental observations and a predictive model.

Findings

Oil coating increases climbing height for given rotation speeds.

Oil coating decreases the threshold rotation speed for rod-climbing.

Contact line pinning and de-pinning are key factors in modulating the effect.

Abstract

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.