STABILITY OF NEWTONIAN AND VISCOELASTIC DYNAMIC CONTACT LINES

The stability of the moving contact line is examined for both Newtonia n and viscoelastic fluids. Two methods for relieving the contact line singularity are chosen: matching the free surface profile to a precurs or film df thickness b, and introducing slip at the solid substrate. T he linear stability of the Newtonian capillary ridge with the precurso r film model was first examined by Troian et al. [Europhys. Lett. 10, 25 (1989)]. Using energy analysis, we show that in this case the stabi lity of the advancing capillary ridge is governed by rearrangement of fluid in the flow direction, whereby thicker regions develop that adva nce more rapidly under the influence of a body force. In addition, we solve the Newtonian linear stability problem for the slip model and ob tain results very similar to those from the precursor film model. Inte restingly, stability results for the two models compare quantitatively when the precursor film thickness b is numerically equal to the slip parameter alpha. With the slip model, it is possible to examine the ef fect of contact angle on the stability of the advancing front, which, for small contact angles, was found to be independent of the contact a ngle. The stability of an Oldroyd-B fluid was examined via perturbatio n theory in Weissenberg number. It is found that elastic effects tend to stabilize the capillary ridge for the precursor film model, and thi s effect is more pronounced as the precursor film thickness is reduced . The perturbation result was examined in detail, indicating that visc oelastic stabilization arises primarily due to changes of momentum tra nsfer in the flow direction, while elasticity has little effect on the response of the fluid to flow in the spanwise direction. (C) 1996 Ame rican Institute of Physics.