Colloidal Particle Adsorption at Water-Water Interfaces with Ultralow Interfacial Tension

Using fluorescence confocal microscopy we study the adsorption of single latex microparticles at a water-water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to "long-lived" metastable states caused by significant energy barriers ΔF∼γA_{d}≫k_{B}T induced by high interfacial tension (γ∼10^{-2}  N/m) and nanoscale surface defects with characteristic areas A_{d}≃10-30  nm^{2}. For the studied water-water interface with ultralow surface tension (γ∼10^{-4}  N/m) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects (A_{d}≃500  nm^{2}). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water-water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics or kinetics of microparticles at water-water interfaces in biocolloidal systems.