Container effects on the free drainage of wet foams

This work studies how drainage of a wet foam is affected by certain characteristics of its container: diameter, wettability and shape of the walls. Drainage is registered by three simultaneous techniques: (a) electrical conductance measurements for the evolution of the local liquid fraction, (b) close-up photos for the evolution of the local bubble size distribution and (c) volumetric measurements for the evolution of the global liquid fraction in the entire container. Electrical measurements are conducted at different heights along the foam with the aid of several pairs of non-intrusive ring electrodes. Three cylindrical Plexiglas containers of different diameter are used, before and after treatment of their walls to allow tests with hydrophobic and hydrophilic walls, respectively. To resemble the shape of common industrial containers, the lower part of the largest container has a gradually reducing diameter towards its bottom. Foam decay is slower in the hydrophobic containers, this being less evident as diameter increases. Moreover, the role of diameter and shape is complex since the highest drainage rate is measured at the largest container whereas the lowest one at the medium container. The possible effect of the container walls in promoting the macroscopic rigidity of the foam structure is discussed