Contact Line Pinning and Depinning Can Modulate the Rod-Climbing 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. First, an enhancement in the magnitude of climbing height for any particular value of the rod rotational speed and second, a decrease in the threshold rod rotational speed required for the appearance of the rod-climbing effect were observed. Observed phenomena are explained by considering the contact line behavior at the rod−fluid interface. Transient evolution of the meniscus at the rod−fluid interface revealed that the three-phase contact line was pinned for a bare rod and depinned for an oil-coated rod. We modeled the subject fluid as a Giesekus fluid to predict the climbing height. The differences in the contact line behavior were incorporated via the contact angle at the rod−fluid interface as a boundary condition. Agreement was found between the observed and predicted climbing height, establishing that contact line behavior may modulate the rod-climbing effect.

Contact angle hysteresis can modulate the Newtonian rod climbing effect

“…Since there is no actual contact of the interface with the rod surface, pinning and hence CAH cannot occur. This is analogous to the de-pinned CL on the oil coated rod surface of our previous work related to the rod climbing effect of viscoelastic liquids 6 . This explains that the contact angle will not change further and assumes a constant saturated value.…”

“…It is already shown that the climbed interface experiences a sharp dip extremely close to the rod (Figure 4) therefore, climbing height cannot be defined in terms of the vertical displacement of the three-phase CL on the rod surface as done in our previous work 6 . Here we define the climbing height, H as the vertical distance between the points of maximum elevation of the steady state interface at a given ω and the static condition as shown in are shown for two different cases-first, assuming a flat meniscus near the rod surface, i.e., θ w = 90 • as used in the existing literature 2 .…”

“…The essential criteria for climbing is that the lighter liquid should have higher viscosity otherwise a dip is observed instead of a climb 1,2 . The name "rod climbing effect" comes from a similar phenomenon classically observed at the air-liquid interface of viscoelastic liquids like polymer melts 3 and polymeric solutions [4][5][6] . Although the stretched interface has a similar shape in both, the Newtonian and the viscoelastic rod climbing effect, the governing mechanism is completely different.…”

“…Although our previous work 6 outlines the role of CAH due to CL pinning, but for an entirely different system of gas-liquid-solid interface in the rod climbing of viscoelastic liquid. Dynamics of a liquid-liquid-solid CL as opposed to gas-liquid-solid CL, presents a more complex system because the interaction of the two liquids with the solid surface is comparable 9 .…”

Elasticity can affect droplet coalescence

“…The far away bridge profile should match the slope of the contact angle h. Hence, the far away boundary condition is expressed as Convincing agreement between the experimental and theoretical profiles suggests that the lubrication model is capable of predicting the coalescence dynamics to an acceptable level. However, some deviation between the experimental and theoretical profiles can be observed, which may have resulted due to pinning 52 on the needle surface or the continuous volume growth of the droplets.…”