The dark-field microscope may be used to observe directly the characteristics of composite films. The liquid phases, one or both of them containing suspended solid particles as test objects (in these experiments bacteria were used), are spread between slide and cover-glass and examined with any desired lenses. The liquid-liquid interfaces appear as bright lines and the solid particles as shining motes.
An interfacial kinetic mechanism has been observed in films of all composition studied. The bacteria are transported along the phase boundary lines in a striking and characteristic manner and quite independently of movements in the adjoining organic or aqueous phases. These movements in the interface are interpreted as essentially due, according to the composition of the films, to local inequalities in interfacial surface tension, or to minute currents from mixing of the two phases across and along their boundary line, or to both forces acting together.
The bacteria (non-motile in these experiments) reached the interface by brownian movement or currents or shifts in the position of the boundary line. Once in the interface they tended to remain, and accumulated there, in instances where the liquid-liquid interfacial tension was high at least, in higher concentration than in the contiguous phases. Bacteria could, however, escape from the interface in a variety of ways detailed above.
With liquids which differ markedly in interfacial tension and miscibility with water, these properties may be correlated with the characteristics of the preparation. With cyclohexane-water films, for instance, (immiscible, interfacial tension high), the boundary was less readily drawn out into projections, the interfacial trapping mechanism was more efficient, and brownian movement of bacteria in the interface was less free than with cyclohexanol-water films (miscible, interfacial tension low).
Analysis of the mechanism of the phenomena herein described will be given in the paper following.