(360j) Dynamic Collision of Water Drops Coated with Food Grade Surfactant Poly-Glycerol-Poly-Ricinoleate (PGPR) in Canola Oil

The dynamic collision of emulsified water drops in the presence of non-ionic surfactants plays a crucial role in many practical applications. We measured the interaction forces between colliding water drops coated with food grade non-ionic surfactant PGPR (polyglycerol polyricinoleate) in Canola oil at concentrations well below typically used to form stable emulsions. PGPR is a fat soluble non-ionic surfactant widely used in chocolate industry as it is known to be the best emulsifier for stabilizing water-in-oil emulsions. Even though the use of PGPR is widespread in food industry, there is a lack of understanding of fundamental mechanisms (and forces) that contribute to the stability of emulsions. The experiments indicated that the interaction forces were dominated by steric forces. Interfacial tension measurement and adsorption isotherm fitting showed that the water-canola oil interface is almost saturated with PGPR for concentrations above 0.2 wt%. To probe meta-stability of emulsified water drops for concentrations below 0.2 wt%, we measured the interaction force as a function of PGPR concentration and velocity of collisions. The hysteresis between the approach and retraction curves increased with increase in the maximum force to which the water drops are pushed to. We observed that during the retract phase of drop collision there was a kink in the force curves and sudden jump out. We attributed such a force jump outs to bridging and extension of adsorbed PGPR molecules on water drops. In order to rule out the role of water transport in the form of PGPR reverse micelles to observed force jump outs, we carried out water drop collision experiments with 1 wt% salt in water. Salt was introduced to arrest the water transport across oil layer. However, we found similar force jump outs even in the presence of salt. This observation indicated that bridging and extension of PGPR molecules was the mechanism behind the force jumps observed during retraction phase of drop collision. The Stokes-Reynolds-Young-Laplace (SRYL) model was used to calculate theoretical force curves. For higher collisions velocities, the agreement between the calculated and experiment data was acceptable, but the SRYL model failed to describe the extension or kink feature observed at slower velocities below. Both the AFM data and the comparison to the model calculation indicated the presence of a short-range attractive force, not of a hydrodynamic origin, attributed to the bridging and extension of PGPR molecules on the surface of water drops below saturation of the interface.