Rod climbing effect modulated by the three-phase contact line behavior*

We present an exposition on the rod climbing effect by replacing the bare rod with a silicone oil-coated rod. This revealed two key differences: (i) For a fixed rod rotation speed, an enhancement in the magnitude of the climbing height. For instance, climbing height increased from ~1.3 mm to ~2.6 mm at a rod rotation speed of 600 RPM. (ii) There exists a threshold rod rotation speed, ω_th for rod climbing to occur and this ω_th decreases on introduction of the oil-coated rod. We hypothesize that these differences can be explained by the altered behavior of the three-phase contact line at the rod-fluid interface in presence of an oil layer. Experimental evidence suggested that the contact line exhibits pinned mode with the bare rod and de-pinned mode with the oil-coated rod. We propose a new set of boundary conditions in terms of altered contact angle at the rod-fluid interface to incorporate the role of the contact line behavior. An agreement between the observed and the predicted climbing height using the Giesekus model confirms our hypothesis that the contact line behavior can modulate the rod climbing effect.