A commercially available technique for assessing thequality of liquid aluminium alloys is the Prefil-Footprintertest in which a quantity of liquid metal is forced to passthrough a fine filter. Normally the filtration rate is mea-sured and compared with recorded information. A cleanmelt retains the open pore structure of the filter keeping itsresistance to a minimum and achieving a high rate of fil-tration. In contrast, a low flow rate would indicate the buildup of inclusions leading to partial choking of the filter [1].The curve of filtrate weight versus time provides a measureof the relative cleanliness of an aluminium melt by com-parison with a wide variety of benchmark results [2]. Thefilter and its deposit can be subsequently sectioned and theinclusions identified by metallographic techniques.Some of the most important and common types ofinclusions detectable by the test are those entrained fromthe surface as films, such as oxides. The entrainmentmechanism is a folding action, ensuring that all suchentrained films are double, and have been called bifilms[3a].Shortly after a period of bulk turbulence such asimmediately after pouring, the bifilms are in a convolutedand compact shape, and so a high proportion is expected topass through most filters [3a]. However, the filter used inthe Prefil test is particularly fine, with a pore size in theregion of only 60 lm so that a higher proportion of thecoarser bifilms is expected to be retained. Due to theirdouble nature, the thin layer of residual air trapped betweenthe dry faces of the films ensures that bifilms are naturalbarriers to the growth front during solidification. Con-versely however, the wetted outer faces of the double filmare seen to constitute excellent favourable substrates for theformation and growth of many intermetallic and secondphases [3a].During the course of work with the Prefil technique, andexamination of a metallographic section of a filter revealeda curious feature: regions of very fine dendrite arm spacing(DAS) were observed immediately adjacent to regions ofcoarse DAS (Fig. 1a–f). Since DAS is typically taken as anindication of the freezing time of the alloy [3c], theimplication of such extremes of cooling rate in such aconductive metal at such close spacing appeared impossi-bly improbable.Optical examination and scanning electron microscope(SEM) microanalyses show that these regions possessedthe same average composition as the original alloy (andthus were not any kind of curious inclusion or secondphase).On closer examination (Fig. 1a–j), it was noted that theregions of contrasting DAS were separated by thin films,many decorated with silicon or other intermetallic parti-cles, sometimes on both sides of the films; an indication ofan occasional symmetry suggesting that the films werealmost certainly oxide bifilms (not all bifilms are sym-metrical because sometimes one film is thicker, or of adifferent composition or structure than the other, so thatsometimes precipitates occur on one side but not the other[3a]). Thus, it was concluded that the regions of differingDAS constituted pockets of liquid alloy isolated from eachother by bifilms.The phenomenon of a liquid Al alloy in the form ofdroplets, isolated from its neighbour by oxide films