| The understanding of wetting phenomena on cylindrically symmetrical objects is very important for spin finish application in the fiber/textile industry. Even when a solution easily wets a flat surface, it tends to breakup into droplets on a fiber surface due to the extra curvature. When a fiber is withdrawn from a liquid reservoir, the liquid film that initially wetted the fiber surface may remain stable or break into droplets. These phenomena can be considered in three different stages: dynamic wetting, film formation and film instability development. Film formation and its instability developments were investigated and related to various factors, such as fiber dimensions, fiber surface roughness/geometry, fiber surface energy, surface tension as well as the viscosity of the fluids to be coated, and the processing conditions of the finish application.; Initial film thickness measured with weight pick up method was shown as a function of capillary number (Ca). With Newtonian fluid (glycerol/water-PP fiber), the initial film thickness followed the LLD (Landau-Levich-Deryaguin) law at moderate Ca, while deviation from the LLD equation was observed at lower Ca in comparison with wetting systems reported by other groups. At higher coating velocity, inertial effects made the films thicker. Ethylene oxide/Propylene oxide (EO/PO) oligomeric fluids formed thicker films than predicted by the LLD equation and showed almost a linear dependence on Ca. Annular films developed instability in all investigated systems regardless of their wettability. Waves grew and transferred to a series of drops with the regular spacing. Then, the film broke down and drained into droplets while some droplet coalescence was observed. The rate of the instability development was related to the viscosity, surface tension of the liquid, fiber diameter and film thickness as expected by the theory.; Static wetting properties on a fiber surface was also studied with drop shapes, which is determined by capillary pressure. Contact angles of an axisymmertirc drops on fibers were calculated from length and radius of drops with a numerical scheme and well represented the wettability of fibers. |
