(753e) Interfacial Rheological Responses of Langmuir Monolayers to Magnetic Probes: Exploring Non-Newtonian Behavior

Various methods have been employed to measure the interfacial rheological properties of surfactant monolayers, but it is common to find disagreement in the literature in the values obtained by these techniques. Many of these measurements cannot fully account for effects such as history-dependence, yield stress, and viscoelastic recovery which are known to exist in some monolayers. We use electromagnets to drive microfabricated, micron-scale magnetic probes of prescribed shape, embedded within insoluble surfactant monolayers at the air-water interface. Small amplitude oscillatory shear experiments provide a linear response, recovering purely shear moduli from the angular displacement of circular probes under applies magnetic torques. We describe a series of experiments to investigate the different rheological responses to shear, dilation, and extension that arise under different forcings with different probes. During each of these experiments, the response of the condensed domains to the magnetic probe is simultaneously visualized with fluorescence microscopy. Thus we correlate the measured response of the monolayer under different driving forces/torques with the observed microstructural deformations, rearrangements, and behavior more generally.