Particle-Tracking Studies of Protein Layer Formation at Fluid Interfaces

The tendency for proteins to adsorb at air-water or oil-water interfaces and to create stiff interfacial layers impacts current and developing technologies in the biomedical and pharmaceutical industries. Knowledge about the evolution of the rheology of protein layers is crucial for understanding the mechanisms driving layer formation, which can further provide a useful perspective on issues of protein denaturation, protein-protein interactions, and the gel transition. However, characterizing the rheology of such layers is difficult due to their confined geometry, their fragility, and the possibility of spatial heterogeneity. Particle tracking, which employs colloidal probes to interrogate the mechanical properties of soft materials, is a promising approach to investigate interfacial protein layers. This talk will describe particle-tracking studies of the time-dependent behavior of layers of the proteins beta-lactoglobulin and lysozyme adsorbing at air-water and oil-water interfaces. The experiments combine complementary passive and active techniques. In the passive measurements, the Brownian motion of ensembles of spherical colloids at the interface sheds light on incipient layers with modest interfacial viscosities and reveals transient mesoscale heterogeneity. In the active measurements, ferromagnetic nanowires at the interface rotate in response to magnetic torques, extending the range of measurements to stiffer layers and enabling characterization of nonlinear mechanical response. The talk will discuss approaches to connect the measured particle mobility with rheological parameters of the layers and the challenges of such interfacial microrheology.