Wetting behavior of oil droplets on CO2 bubbles influenced by interfacial shear elasticity

Abstract

Hypothesis

Wetting dynamics and behaviors of droplets and bubbles are often considered with respect to the interfacial/surface tensions of the system, with contributions from the interfacial shear rheology largely unexplored. We hypothesize that the interfacial shear elasticity acts as a resistive force to droplet wetting, with the equilibrium wetted state more influenced by the interfacial shear rheology than the interfacial/surface tensions.

Experiments

An oil droplet of DecTol (4:6 v/v) containing asphaltenes is brought into contact with a CO₂ bubble in brine. The shear viscoelasticity of the oil-brine interface is varied by changing the asphaltenes concentration (150 and 15,000 mg/L) and aging time of the system (10 and 120 min). Wetting behaviors of the oil droplet on the CO₂ bubble are captured using high speed imaging.

Findings

Considering the interfacial/surface tensions of the three phases, the equilibrium wetted state would be that of an oil droplet fully encapsulating a CO₂ bubble. This is true for the low asphaltene concentration and short aging time system. When aged for longer, or for the high asphaltene concentration, partial encapsulation is observed. The change in wetting behavior is a result of the shear elasticity of the asphaltene film at the oil-brine interface. As the asphaltene film fractures/yields, the oil droplet begins to wet the CO₂ bubble, causing the interfacial film to be stretched. With the asphaltene film resistive force acting opposite to the wetting force, complete encapsulation is prevented, and for the highest film elasticity, partial dewetting of the oil droplet on the CO₂ bubble is seen, indicating a stronger resistive force than the wetting force (Gibbs energy).