Role of surface active molecules and nanoparticles on the spreading dynamics of drops on solid surfaces

The characteristics of an aqueous drop spreading on solid surfaces are the keys to many deposition processes including coating, printing, and enhanced oil production. We study the wetting dynamics of aqueous drops, loaded with either silica nanoparticles or sodium dodecyl sulfate surfactants, on a hydrophilic glass surface that is submerged in high viscosity oil. The balance between capillary and viscous forces determines the early-time spreading dynamics in clean systems, characterized by a droplet radius that grows linearly with the time which finally evolves to the late-time Tanner regime. Along recovering the viscous and Tanner regimes, silica increases the early time spreading rate and the final wetted area. The drop affinity to glass and the osmotic forces driven by the silica concentration gradient dictate the wetting characteristics of silica drops. Unlike clean water and silica drops, surfactants impose an early time retardation regime. The non-uniform distribution of surfactants at the interface generates Marangoni stresses before the drop-solid contact, consequently suppressing the film drainage. The addition of both surfactants and nanoparticles also imposes an early time retardation regime where its duration is an increasing function of interfacial viscoelasticity.