Dynamics of interface instabilities in nonionic lamellar phases

The swelling and dissolution dynamics of a soluble (small miscibility gap) and an insoluble (large miscibility gap) lamellar phase (L-alpha) of nonion ic surfactant in water have been qualitatively and quantitatively investiga ted. In the case of an insoluble L-alpha, we observe classical myelinic ins tabilities at the interface. (These are absent in the soluble case.) By dop ing the excess water phase with colloidal tracer particles, we show that wa ter enters at the roots, rather than through the walls, of the myelins. We estimate the mean water flux and find it to be consistent with this picture . We also study the interface between a lamellar phase and: pure surfactant liquid (L-2), At this interface we observe a new dynamic instability in wh ich "onion jets" appear to be ejected from L-alpha into L-2, where they dis appear. Closer investigation shows these onions to be focal conic II defect s residing near the surface of a wedge of homeotropic L-alpha, which is in contact with the wall of the sample holder. The defects move much faster th an the wedge. We offer some theoretical discussion of how these observation s shed light on the mechanism of interfacial instabilities, particularly my elin formation, in surfactant dissolution.