Theoretical and experimental studies on the contact line motion of second-order fluid

In this study, we studied the contact line motion of second-order fluids theoretically and experimentally. The theoretical study showed that the positive first normal stress difference (N₁) increases the contact line velocity while the second normal stress difference (N₂) does not affect the contact line motion. The increased contact line velocity is caused by the hoop stress acting on the curved stream lines near the contact line. The hoop stress increases the liquid pressure near the contact line, and the increased pressure changes the surface profile to have the smaller curvature and smaller dynamic contact angle. The contribution of N₁is 1 order of magnitude smaller than the contribution from the viscous component when the Deborah number remains O(1). For experiments, silicone oils of different kinematic viscosities (1,000-200,000 mm²/s) were used while eliminating the drying problem and shear-thinning effect near the contact line. The silicone oils were well fitted to the second-order fluid model with the positive first normal stress difference. The spreading rate of a silicone oil drop on a solid surface was faster than the spreading rate predicted by the theory for Newtonian fluids. As the theory predicts that N₁increases the contact line velocity and the experimental result confirms the theoretical prediction, the effect of N₁is established.