Experimental and theoretical studies of fiber coating instabilities

A. G. GONZÁLEZ; J. DIEZ; R. GRATTON; D. M. CAMPANA; F. A. SAITA

Mar del Plata, Argentina

Congreso; Pan-American Advanced Studies Institute (PASI) on Interfacial Fluid Dynamics: From Theory to Applications; 2007

National Science Foundation, EE. UU.

The instability of a liquid layer coating the surface of a thin cylindrical wire is studied experimentally and numerically. The wire (diameter 110 micrometers) is vertically suspended and threaded through a syringe that injects drops of silicon oil. They are centred on the vertical axis of the wire and slide along it by gravity. This process coats the wire with a liquid film of uniform thickness that depends on the size of the drop. We use an anamorphic optical system that compresses the vertical scale to accommodate several wavelengths and widens the horizontal one to follow in detail the evolution of local minima and maxima. A primary mode grows in the early stages of the instability. Its wavelength may differ significantly from that corresponding to the maximum growth rate predicted by the linear theory. In a nonlinear stage, a second mode appears whose wavelength is half that of the primary mode. When the amplitude of the secondary mode becomes comparable with that of the primary one, the length measured between final drops is quite different of that seen between initial protuberances. Experimental timelines showing the growth and position of the maxima and minima are compared with the linear theory and simulations. Contrary to the predictions of previous widely used quasi-static theories, experiments show that the relation between maximum and minimum of the primary mode is better approximated by a linear dependence. Numerical simulations confirm this result.