Colloidal particle adsorption at water/water interfaces with ultra-low interfacial tension

TL;DR

This study investigates how latex microparticles adsorb at water/water interfaces with ultra-low surface tension, revealing equilibrium states, crossover dynamics, and unexpected damping behavior due to the diffuse interface and polymer entanglement.

Contribution

It provides the first detailed characterization of microparticle adsorption at ultra-low tension water/water interfaces, highlighting unique kinetic regimes and interface properties.

Findings

Observed equilibrium contact angles independent of particle size

Identified crossover from fast to slow adsorption dynamics

Detected position-independent damping coefficient

Abstract

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.