Scraping of foam on a substrate

Foam is not only an industrially important form of matter, but also one of soft jammed system such as colloidal suspensions and emulsion suspensions. Since foams are composed of gas bubbles and liquid, it is expected that the coupling of bubbles and liquid strongly affects rheology of foams, which is different from simple liquids. To reveal this coupling effect and considering the importance of foam applications, we studied the scraping of foam by a rigid plate on a substrate.

We mainly used 5.0 wt% solution of ionic surfactant TTAB (tetradecyltrimethylammonium bromide). We systematically investigate the scraping behaviors by the rigid plate as a function of scraping velocity, gap height, confinement length, amount of foam and wettability of the substrate.

The results show that there are three distinguishable scraping patterns: homogeneous scraping, no scraping, and slendered scraping. These behaviors are clearly different from those of simple liquid systems. Of these, the upper limit of homogeneous scraping could be explained theoretically by the competition between dewetting and shear, which is not previously discussed. Furthermore, the theory is applicable to the scraping of other soft jammed systems such as colloidal and emulsion suspensions.