Brownian dynamics of colloidal probes during protein-layer formation at an oil–water interface

Abstract

We employ the Brownian motion of spherical colloids to investigate the microrheology of layers of the protein β-lactoglobulin adsorbing at a decane–water interface. Using an experimental design in which the protein concentration in the aqueous subphase increases in a diffusion-limited manner, we obtain a time-resolved characterization of the interfacial rheology. After undergoing a period of increasing interfacial viscosity, the layers become viscoelastic with a rheology that indicates an approach to a sol–gel transition. This evolution, which depends on subphase pH, contrasts in several significant ways with layer formation by β-lactoglobulin at an air–water interface. Comparison of the diffusivity of colloids of two sizes as the interfacial viscosity increases illustrates the transformation in particle mobility from a three-dimensional character in which the surrounding bulk fluids dominate to a two-dimensional form dictated by the drag from the interfacial layer. This transformation is similarly revealed in the two-point correlations of the particle motion, which show a change in spatial dependence indicating a crossover from three-dimensional to two-dimensional hydrodynamics.