A TECHNIQUE FOR DIRECT OBSERVATION OF PARTICLE MOTION UNDER SHEAR IN A LANGMUIR MONOLAYER

We have used fluorescence microscopy to observe the effect of shear on rigid two-dimensional structures residing in a Langmuir monolayer. Mo nolayers of 12-NBD stearic acid undergo a two-dimensional liquid-to-so lid transition on the water surface. The domains of the solid phase th at form in the coexistence region are elongated (needle-shaped) crysta llites. We observe the effect of shear within the monolayer in two geo metries: two-dimensional Poiseuille flow and simple shear flow created by moving bands. The technique allows us to determine the average ori entational order parameter as well as the details of the rotational ki nematics, which are, as expected, well-described by a Jeffery orbit. W e propose that this technique of direct observation in Langmuir monola yers will be a useful method for the study of systems of rigid particl es under flow. (C) 1997 The Society of Rheology.