Removal of organic films from rotating disks using aqueous solutions of nonionic surfactants: Film morphology and cleaning mechanisms

In this work, we consider the cleaning of an organic liquid film, consisting initially of a concentrated solution of abietic acid in isopropyl alcohol, from the surface of a rotating disk by using aqueous solutions of a nonionic surfactant, pentaethylene glycol mono-n-dodecyl ether. The results show that the removal process takes place in three consecutive stages. The first stage is controlled by the solubilization of the abietic acid by surfactant penetration and subsequent mass transfer from the interface to the bulk of the aqueous solution. During the first stage, the film absorbs water from the aqueous solution and breaks up into drops that leave portions of the surface exposed. The absorption of surfactant and water reduces the organic-phase viscosity, until the drops start to move on the disk surface under the action of shear forces. These drops aggregate into spiral-shaped continuous rivulets through which the organic phase flows until it comes off the disk edge. Such behavior occurs during the second stage of cleaning, which has a rate of removal appreciably faster than the first stage. The rivulets are shown to be tangent to the stress exerted by the aqueous solution on the surface of the organic phase. The rivulets eventually break, leading to a third stage with lower removal rates, in which the removal mechanism is apparently the roll up of organic-phase drops under the action of shear forces. In this work, we present experimental evidence that supports the described mechanism, based on photographs showing the morphology of the film structure in the different cleaning stages. A model is derived that relates the empirical observations of cleaning rates to physical parameters describing the solubilizing film hydrodynamics.