AN EXPERIMENTAL-STUDY OF RIVULET INSTABILITIES IN CENTRIFUGAL SPIN-COATING OF VISCOUS NEWTONIAN AND NON-NEWTONIAN FLUIDS

An experimental study on the onset and evolution of centrifugally driv en rivulets is presented, which aims to investigate the influence on t he instability of various experimental conditions (drop volume and rot ational frequency), the wetting properties of the liquid (surface tens ion and contact angle), and fluid viscoelasticity. The apparatus allow s continuous observation of the drop shapes following an impulsive spi n-up of the substrate, and these are analyzed by digital image analysi s. The flows exhibit an onset time, or, equivalently, a critical radiu s, before which the drop spreads axisymmetrically. Data on drop spread ing are compared with simple predictions of lubrication theory. The me asured azimuthal wave number and growth rate of the instability are in good agreement with the linear stability analysis of Troian et al. [E urophys. Lett. 10, 25 (1989)], as long as the critical radius is taken from the experiment itself. The most unstable wavelength is found to be independent of both drop size and rotation speed in the range of pa rameters investigated, as observed previously by Melo et al. [Phys. Re v. Lett. 63, 1958 (1989)]. On the other hand, a change in the wetting properties of the liquid significantly modifies the critical radius, w hich, in turn, affects the number of fingers, with the nonwetting flui d exhibiting a smaller critical radius. This trend is in agreement wit h the mechanism of instability that is linked to the presence of a cap illary ridge near the edge of the drop. No qualitative nor quantitativ e difference in behavior has been observed between a Boger fluid havin g a relaxation time of about 1 s, and its Newtonian solvent, in the ex perimental conditions considered.