Colloidal Mobility in Surfactant Films and its Application of the Shear Rheology of Protein Layers

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1033985LehenyUnder many circumstances proteins adsorb at air-water or oil-water interfaces, and at sufficient concentration such interfacial proteins can form layers with pronounced elasticity. The mechanical behavior of such layers is often key to their utility in current and developing technologies, particularly those related to the food, biomedical, and pharmaceutical industries. Interfacial microrheology, which uses colloidal probe particles to interrogate the mechanical properties of films at fluid interfaces, is emerging as a powerful approach to investigate interfacial layers. This proposal describes a set of experiments that will advance this approach significantly by providing new insights into the nature of colloid mobility at interfaces and will then exploit these insights to gain new understanding of the interfacial rheology of protein layers.Intellectual Merit:Knowledge of the rheological properties of protein layers is crucial both for understanding fundamental aspects of their formation and stability, as well as for enabling their adoption for technological applications. The study of protein layers can further provide unique perspectives on issues of protein denaturation, protein-protein interactions, and gel transitions. The proposed research program will be a joint effort involving three Investigators with a strong record of collaboration and with the complementary expertise needed to make substantial progress in this area. A central element of the proposed research will be the use of colloidal probes with different geometries that are employed both in active and passive interfacial microrheology measurements. As previous work by the Investigators has shown, this combined approach provides sets of complementary information that, when treated self consistently, can resolve ambiguities in the interpretation of any single measurement. The result is insight into both the nature of particle motion at interfaces and the interfacial rheology of the host layers implied by that motion. A key approximation in the analysis of the hydrodynamic forces on a particle confined to an interfacial layer is the incompressibility of the layer. Determining experimentally the range of validity of this approximation and the impact on particle mobility when it breaks down would have far reaching implications, and would help solidify the connection between particle motion and interfacial rheology that is at the center of the microrheology approach. Hence, the research will begin with experiments that investigate the limits of the incompressibility approximation. Informed by the resulting insights into the drag on colloids in interfacial layers, the proposed experiments will then address three key topics in protein layer rheology: (i) correlations in the evolution of surface viscosity and dynamic surface tension of protein solutions, (ii) the interfacial rheology of mixed solutions of proteins and small molecule surfactants, and (iii) the comparative mechanical properties of spread versus adsorbed protein layers.Broader Impacts:Through systematic investigation of the limits and consequences of a key property of surfactant layers, their incompressibility, the research will bring to the fore an issue relevant to a range of problems, such as multiphase flows at interfaces. By advancing a new, high sensitivity approach to interfacial rheology, the work will help expand the tools of interfacial science, potentially impacting a range of fields within materials and chemical engineering and the biosciences. For example, as microrheological techniques, the measurement approaches being developed require far smaller samples than conventional rheological methods. Thus, they could make feasible mechanical characterization of interfacial systems where large sample sizes are impossible or prohibitively expensive to synthesize. As part of this program, graduate students and undergraduates will receive research training in a highly interdisciplinary field that will prepare them for careers in academia and industry. Collaboration with a local science magnet high school will provide Baltimore City students with opportunities for research internships. To help promote the advancement of women in science and engineering, female high school students will also be recruited through the Women in Science and Engineering Program, a Johns Hopkins University outreach initiative, to participate in the research.