The tendency for proteins to adsorb at air-water or oil-water interfaces and to create stiff interfacial layers is vital to many current and developing technologies, particularly those related to the food, biomedical, and pharmaceutical industries. Further, the process of protein-layer formation provides a unique perspective on issues of protein denaturation, protein-protein interactions, and the gel transition. This proposal describes experiments to apply a new, high sensitivity approach to characterizing the interfacial shear rheology of protein layers by employing magnetic nanowires confined at the interface as active microrheology probes.
Intellectual Merit: A key difference between proteins adsorbed at interfaces and conventional small-molecule surfactants is the propensity of the proteins to form layers that are strongly viscoelastic. In many circumstances, this mechanical behavior can lead to superior properties, such as in stabilizing emulsions and foams. Consequently, knowledge of the rheological properties of protein layers is crucial both for understanding fundamental aspects of their formation and stability as well as for adopting them for technological application. Based on geometric considerations, the proposed microrheology approach using wire-shaped probes is naturally suited for measuring the shear rheology of nanometer-scale fluid films, and the nanowires should be significantly more sensitive than existing interfacial shear rheology techniques. The basis of the approach involves characterizing the drag experienced by nanowires confined to the air-water interface at which protein layers form as the wires are rotated by precise magnetic torques. Recent theory has predicted fundamental changes to the hydrodynamic behavior of an anisotropic object, such as a wire-shaped particle, when it is confined to such a thin film. Clarifying experimentally the validity of these predictions and their range of applicability would have far-reaching implications. Such clarification will also be necessary for a proper quantitative interpretation of the proposed interfacial rheology experiments on protein layers. The magnetic nanowires are an ideal system to investigate these theoretical ideas, and the project will include experiments to test the predictions. With the rotational drag on nanowires in films understood, the approach will then be applied to interfacial shear rheology studies of two protein layer systems (i) lysozyme solutions at low concentration for which layer formation is characterized by an extended induction period and (ii) films formed from solutions of lactoglobulin and small molecule surfactants for which the mechanical properties of the protein layer are highly sensitive to the presence of the surfactant. These systems are selected for the compelling scientific problems they present and for the opportunities to uncover significant new phenomena through the proposed experimental approach. However, an additional objective of the proposed experiments will be to establish more generally the technique of microrheology with magnetic nanowires as an important tool for studying the mechanical properties of interfacial systems.
Broader Impacts: As part of this project, a female high school student will be recruited from the Women in Science and Engineering (WISE) Program, a Johns Hopkins outreach initiative, to participate in the research. The broader impacts of the project will also include research training and education for a graduate student and an undergraduate student. In addition, the work will provide potentially significant impact to both the research on and the technology based on interfacial proteins layers, particularly with regard to systems with mesoscale heterogeneity that cannot be accessed with current techniques and to materials for which small sample quantity is currently a limiting factor. Stiff interfacial layers are vital to many current and developing technologies, particularly those related to the food, biomedical, and pharmaceutical industries.
