Rheological and Tribological Behavior of Gels and Emulsions Containing Polymer and Phospholipid

Oil-in-water (O/W) emulsions are widely used in food, pharmaceutical, and personal care applications. These types of systems often contain hydrophobically modified polymers and phospholipids, where the polymer acts as a rheology modifier or emulsifier, while the phospholipid acts as a functional performance surfactant. However, the underlying mechanism through which the different components affect rheological and tribological characteristics is not well understood. To this end, we take a hierarchical approach, evaluating first the hydrophobically modified polymer alone, followed by polymer and phospholipid and then an O/W emulsion stabilized with the polymer or the phospholipid and polymer combined. We characterize the tribological behavior using a soft model contact consisting of polydimethylsiloxane (PDMS), which has elastic modulus similar to human skin. Bulk rheology results show that the studied systems are shear thinning and have gel-like behavior with elastic modulus increasing substantially upon phospholipids addition. Friction coefficients increase in the elastohydrodynamic regime with increasing sample viscosity. However, systems containing the hydrophobically modified polymer and phospholipids show a lower friction coefficient at the boundary regime. Adsorption studies with a quartz crystal microbalance with dissipation (QCM-D) measurements show that phospholipids are being adsorbed onto the PDMS surface. Confocal laser scanning microscopy of the PDMS surfaces before and after immersion in a hydrophobically modified polymeric suspension containing rhodamine B shows the presence of the polymer on the PDMS surface even after DI water rinse, indicating polymer adsorption, thus resulting in lower friction coefficients at low speeds.