A criterion for buoyancy induced drop detachment based on an analytical approximation of the drop shape

The detachment of small oil drops from metal surfaces placed in an aqueous medium is examined. It is observed that buoyancy induced drop break-up and not 'roll-up' is responsible for most of the oil removal. The aqueous phases used in this study are concentrated aqueous solutions of anionic surfactants and the oils used are lighter than water. Roll-up occurs but is not able to cause detachment of the oil drop completely on its own. A quantitative view of the above system is developed leading to a stability criterion. This stability criterion can be used to predict buoyancy induced partial drop detachment. The quantitative criterion is developed using an analytical approximation for the pendant drop profile. This approximation results in much smaller errors as compared with the 'spherical cap assumption. The quantitative criterion is used to predict interfacial tensions (IFT), density differences and three phase contact angles that are required to induce partial drop removal from solid surfaces. Predictions from the equations match the observations. The above criterion is used to compute the stability boundaries for different drop volumes in the space defined by the above variables. (C) 2001 Elsevier Science B.V. All rights reserved.