Cellulose Nanocrystal Laden Oil–Water Interfaces: Interfacial Viscoelasticity, Emulsion Stability, and the Dynamics of Three-Phase Contact-Lines

We study the influence of cellulose nanocrystals on the oil–water interfacial viscoelasticity and consequently on the emulsification and the three-phase (oil–water–solid) contact-line movement. Cellulose nanocrystals (CNCs) are used in combination with hexadecyl trimethyl ammonium bromide (CTAB) as the source of nanoparticle-surfactant dispersion. We use two samples of oils (i) heptane (representing a low viscosity system) and (ii) mineral oil (representing a high viscosity system). The emulsification stability map is developed for the heptane-water systems at various CNC-CTAB concentrations. The map displays stable emulsion formations when dilatational interfacial viscoelasticity is above ∼ 40 mN/m. The coupled interfacial rheology and microscale characterization analysis show that CNCs become surface active by the adsorption of surfactants, migrate to the oil–water interface, and create a viscoelastic interface. The CNC-CTAB interfacial layer prevents the drop coalescence, creating highly stable medium internal phase emulsions. The emulsification experiments are further performed with a high viscosity mineral oil sample, where similar stability trends as heptane emulsions are obtained. Furthermore, it is shown that the CNC-CTAB-laden drops wet a hydrophilic solid surface, submerged in the mineral oil, with a wetting radius growing according to r ∼ t^α. Similar to the spreading of the surfactant (CTAB-laden) drops, three regimes are identified, an initial retardation regime (α ≤ 0.5), a second early-time viscous-dominated regime (α ∼ 1), and the late-time Tanner regime (α = 0.1). However, the CNC-CTAB viscoelastic interfacial layer increases the duration of the early-time spreading regime by one order of magnitude.