Controlled Adsorption of Cellulose Nanofibrils on Fluid-Fluid Interfaces

Adsorption of particles at fluid-fluid interfaces is of great scientific and industrial importance, encompassing a variety of applications, from the stabilization of bubbles and emulsions to the fabrication of sophisticated materials with micro- and nanostructures. The enormous adsorption energy of a particle is believed to be the origin of adsorption, but it cannot fully explain many systems that do not adsorb, even with large adsorption energy. Here, it is systematically shown that the dynamic process of particle adsorption plays a critical role in determining whether particles are adsorbed at interfaces. Using a model system composed of a droplet and solid particle monolayer, the adsorption dynamics of cellulose nanofibrils are studied, focusing on the drainage time of a liquid thin film between the droplet and solid surface. Both the experimental and theoretical results indicate that the drainage time is regulated by the dimensionless capillary number. Controllable particle adsorption in conventional emulsions is further examined, confirming that results from this model system can be extended to large capillary numbers. This study provides important insight that it is not the adsorption energy, but the drainage process that governs particle adsorption, which can be quantitatively controlled by viscosity, interfacial tension, and particle approaching velocity.