posted on 2021-03-23, 12:50authored byParisa Bazazi, S. Hossein Hejazi
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 ∼ tα. 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.