The effect of shear flow on the structure and dynamics of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed. While undisturbed, the particles arrange themselves on a hexagonal lattice due to strong dipole-dipole repulsion resulting from ionizable sulfate groups on their surfaces. As the interface is subjected to shear flow, however, the lattice adopts a new semi-ordered, anisotropic state for which two distinct regimes are observed. At low particle concentrations or high shear rates, nearest neighbors in the lattice align in the flow direction and create strings of particles that slip past each other fairly readily. This results in a stretching of the overall structure and achievement of a steady state orientation in the system. In contrast, at high concentrations or low shear rates, the interparticle forces gain importance and tend to keep the particles more strongly in their lattice positions. As a result, domains within the lattice are forced to rotate, thus giving rise to movement of particles perpendicular to the flow direction. Thus a rotation, in addition to stretching, of the structure is apparent in this case.

Both physical surface rheology and computer simulation experiments were performed to understand the conformation of polymers in 2D systems. The interfacial stress rheometer was used to measure surface rheological properties of Langmuir monolayers of poly(tert-butyl methacrylate) with molecular weights between 81.5 and 780 kg mol-1. The Π−A isotherms for these monolayers show two transitions:  a plateau at Π ≈ 18 mN m-1 and a bend at Π ≈ 52 mN m-1. The properties of the films are shown to be substantially different above and below the plateau surface pressure, Πp. Below Πp, the monolayers are primarily viscous, and surface viscosity increases linearly with MW, while above Πp, the films are more elastic, and surface viscosity is MW-independent. Computer simulations of these systems produce qualitatively similar Π−A isotherms. The observed transition at Πp marks the changeover from polymer chains existing in a single layer to forming regions of multilayers.

Langmuir monolayers of mixtures of straight-chain and branched forms of hexadecanol were studied using surface pressure-area isotherms, Brewster angle microscopy, and interfacial rheology measurements. For mixtures containing less than 30% branched molecules, the isotherms show only a lateral shift to a lower area that is proportional to the percentage of straight chains. Above this fraction, the isotherms are qualitatively different. The surface viscosities of both straight and mixed monolayers show a maximum in the condensed untilted phase near Π ) 20 mN/m. The addition of branched molecules results in a non-monotonic increase in surface viscosity, with the maximum occurring near 12% branched molecules. Visualization of these immiscible monolayers using Brewster angle microscopy in the liquid condensed phase reveals the formation of discrete domains that first increase in number density and then decrease as surface pressure is increased.