  Using fluorescence 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 $\Delta{\cal F}\sim \gamma A_d\gg k_B T$ induced by
high interfacial tension ($\gamma \sim 10^{-2}$ N/m) and nanoscale surface
defects with characteristic areas $A_d \simeq$ 10--30 nm$^2$. For the studied
water/water interface with ultra-low surface tension ($\gamma \sim 10^{-4}$
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
($A_d \simeq$ 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/kinetics of microparticles at
water/water interfaces in bio-colloidal systems.
