Contact angle hysteresis can modulate the Newtonian rod-climbing effect

Abstract

The present work investigates the role of contact angle hysteresis at the liquid-liquid-solid interface (LLS) on the rod climbing effect of two immiscible Newtonian liquids using experimental and numerical approaches. Experiments revealed that the final steady state contact angle, θw at the LLS interface varies with the rod rotation speed, ω. For the present system, θw changes from ∼69◦ to ∼83◦ when the state of the rod is changed from static condition to rotating at 3.3 Hz. With further increase in ω, the θw exceeds 90◦ which cannot be observed experimentally. It is inferred from the simulations that the input value of θw saturates and attains a constant value of ∼120◦ for ω > 5 Hz. Using numerical simulations, we demonstrate that this contact angle hysteresis must be considered for the correct prediction of the Newtonian rod climbing effect. Using the appropriate values of the contact angle in the boundary condition, an excellent quantitative match between the experiments and simulations is obtained in terms of- the climbing height, the threshold rod rotation speed for onset of climbing, and the shape of liquid-liquid interface. This resolves the discrepancy between the experiments and simulations in the existing literature where a constant value of the contact angle has been used for all speeds of rod rotation.

Rod-climbing rheometry revisited

“…22 The analysis presented in this section and our measurements for the 0.3 wt% solution, which undergoes rod-descending, extend the validity of rod-climbing rheometry in principle to a much wider range of complex fluids, provided special care is taken in selecting a rigid rod constructed from a solid material that the fluid wets. 25…”

“…22 The analysis presented in this section and our measurements for the 0.3 wt% solution, which undergoes rod-descending, extend the validity of rod-climbing rheometry in principle to a much wider range of complex fluids, provided special care is taken in selecting a rigid rod constructed from a solid material that the fluid wets. 25 Finally, we note that a special (redundant) case arises when b = 0, i.e., we can either have c 0 = 0.25 or an inelastic fluid with C 2,0 = C 1,0 = 0. In this case, if the fluid is viscoelastic, a finite value of C 1,0 can first be measured independently as discussed in Section 4.1 using normal force measurements, or from the asymptotic scaling of a viscoelastic master curve for G 0 (o r ).…”