After long-time exposure, protein adsorption at fluid/fluid interfaces is documented to produce interfacial, gellike networks. Formation of this network apparently results from adsorption-induced conformational changes and subsequent interprotein aggregation at the interface. We utilize interfacial shear and dilatational rheology to probe the structure of a globular protein, lysozyme, and a disordered protein, β-casein, and the kinetics of network formation at the hexadecane/water interface. For the first time, we present a detailed comparison of the interfacial shear and dilatational responses. For lysozyme, the shear moduli grow with interface age indicating a transition from fluidlike behavior at early times to network formation (solidlike behavior). Conversely, the interfacial shear moduli of β-casein change very little with interface age; in addition, both G ′ and G ′′ for β-casein are an order of magnitude smaller than those of lysozyme. The strong protein intramolecular interactions that stabilize the native conformation of lysozyme act as kinetic barriers to conformational change and later become strong intermolecular interactions upon partial unfolding at the interface. Hence, interprotein linkages form (i.e., aggregation into an interfacial gel), resulting in the growth of G ′ with time. We find that the interfacial dilatational storage modulus, E ′, is comprised of a static response and a dynamic response. The static response corresponds to a change in the surface pressure upon interfacialarea change and is strain-rate independent. The dynamic contribution corresponds to rearrangement and reconfiguration of the protein molecules within the interface and is analogous to the shear storage response (i.e., a measure of the strength of interprotein linkages). The magnitudes of E ′ and G ′ for lysozyme and β-casein suggest that lysozyme initially adsorbs in a state similar to its native conformation. The native rigidity of the protein is linked to its kinetic stability at the interface. Globular lysozyme, once adsorbed, resists compression giving a high dilatational storage modulus. Contrastingly, native β-casein lacks tertiary structure, resulting in a small interfacial dilatational storage modulus relative to lysozyme. With increasing interface age, the static modulus of β-casein changes insignificantly, whereas it decreases substantially for lysozyme, indicating partial unfolding and loss of intrinsic rigidity. Upon unfolding, interprotein linkages form through hydrophobic peptide-peptide interactions. Correspondingly, G ′ and the recoverable dilatational storage modulus, δE ′, grow, signifying the onset of interfacial gelation.

We prepare solid-stabilized emulsions using paramagnetic particles at an oil/water interface that can undergo macroscopic phase separation upon application of an external magnetic field. A critical field strength is found for which emulsion droplets begin to translate into the continuous-phase fluid. At higher fields, the emulsions destabilize, leading to a fully phase-separated system. This effect is reversible, and long-term stability can be recovered by remixing the components with mechanical agitation.

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Complex fluid-fluid interfaces are common to living systems, foods, personal products, and the environment. They occur wherever surface-active molecules and particles collect at fluid interfaces and render them nonlinear in their response to flow and deformation. When this occurs, the interfaces acquire a complex microstructure that must be interrogated. Interfacial rheological material properties must be measured to appreciate their role in such varied processes as lung function, cell division, and foam and emulsion stability. This review presents the methods that have been devised to determine the microstructure of complex fluid-fluid interfaces. Complex interfacial microstructure leads to rheological complexity. This behavior is often responsible for stabilizing interfacial systems such as foams and emulsions, and it can also have a profound influence on wetting/dewetting dynamics. Interfacial rheological characterization relies on the development of tools with the sensitivity to respond to small surface stresses in a way that isolates them from bulk stresses. This development is relatively recent, and reviews of methods for both shear and dilatational measurements are offered here.

The behavior of monolayers of monodisperse prolate ellipsoidal latex particles with the same surface chemistry but varying aspect ratio has been studied experimentally. Particle monolayers at an air-water interface were subjected to compression in a Langmuir trough. When surface pressure measurements and microscopy observations were combined, possible structural transitions were evaluated. Ellipsoids of a sufficiently large aspect ratio display a less abrupt increase in the compression isotherms than spherical particles. Microscopic observations reveal that a sequence of transitions is responsible for this more gradual increase of the surface pressure. When a percolating aggregate network is used as the starting point, locally ordered regions appear progressively. When it reaches a certain surface pressure, the system "jams", and in-plane rearrangements are no longer possible at this point. A highly localized yielding of the particle network is observed. The compressional stress is relieved by flipping the ellipsoids into an upright position and by expelling particles from the monolayer. The latter does not occur for spherical particles with similar dimensions and surface chemistry. In the final stage of compression, buckling of the monolayer as a whole was observed. The effect of aspect ratio on the pressure area isotherms and on the obtained percolation and packing thresholds was quantified.

We report surface shear rheological measurements on dense insoluble monolayers of micron sized colloidal spheres at the oil/water interface and of the protein beta-lactoglobulin at the air/water surface. As expected, the elastic modulus shows a changing character in the response, from a viscous liquid towards an elastic solid as the concentration is increased, and a change from elastic to viscous as the shear frequency is increased. Surprisingly, above a critical packing fraction, the complex elastic modulus curves measured at different concentrations can be superposed to form a master curve. This provides a powerful tool for the extrapolation of the material response function outside the experimentally accessible frequency range. The results are discussed in relation to recent experiments on bulk systems, and indicate that these two-dimensional monolayers should be regarded as being close to a soft glass state.

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The p75 neurotrophin receptor (p75NTR) is expressed by neurons particularly vulnerable in Alzheimer's disease (AD). We tested the hypothesis that non-peptide, small molecule p75NTR ligands found to promote survival signaling might prevent Aβ-induced degeneration and synaptic dysfunction. These ligands inhibited Aβ-induced neuritic dystrophy, death of cultured neurons and Aβ-induced death of pyramidal neurons in hippocampal slice cultures. Moreover, ligands inhibited Aβ-induced activation of molecules involved in AD pathology including calpain/cdk5, GSK3β and c-Jun, and tau phosphorylation, and prevented Aβ-induced inactivation of AKT and CREB. Finally, a p75NTR ligand blocked Aβ-induced hippocampal LTP impairment. These studies support an extensive intersection between p75NTR signaling and Aβ pathogenic mechanisms, and introduce a class of specific small molecule ligands with the unique ability to block multiple fundamental AD-related signaling pathways, reverse synaptic impairment and inhibit Aβ-induced neuronal dystrophy and death.