Measurement of the Yield Stress of Gellike Protein Layers on Liquid Surfaces by Means of an Attached Particle

We propose a new method for determination of the yield stress of adsorbed protein layers on an air−water interface. A small spherical particle is attached to the surface, and the latter is deformed by pulling up a barrier attached to it. When the tangential projection of the gravity force exerted on the particle exceeds a certain threshold, then the layer starts to undergo significant elastic deformation. This is registered as a difference between the experimentally measured particle displacement and that calculated by solving the Laplace equation of capillarity for the liquid boundary. After the yield threshold, there is a linear dependence of the strain on the applied stress. We propose a modified Bingham model (two springs and a plastic element) to describe this particular rheological behavior. The elastic moduli found by a linear regression analysis of the stress (strain) relationship agree well with literature data for globular proteins. The corresponding yield stress turns out to be very sensitive to changes in the protein layer structure (caused either by the presence of surfactant molecules (Tween 20) or by differences in the bulk protein concentration). The threshold stress monotonically increases with rising protein content, which suggests a possible reinforcement of the adsorbed layer. The addition of Tween 20 brings about surface fluidization. Tween 20 can replace part of the adsorbed protein molecules and/or attach to them breaking the intermolecular linkages.