Early Time Spreading Dynamics of Nanobubble-Laden Drops

Nanobubbles, when dispersed in a liquid phase, may enhance mass transport, adsorption, and reactions in many industrial applications, such as fabrication of functional materials, drug delivery, water treatment, carbon dioxide capture, and surface decontamination. Here, we experimentally study the early time spreading dynamics of nanobubble-laden surfactant drops on a hydrophilic solid surface submerged in an oil phase. Along with recovering the retarding effects of surfactants on the early time wetting dynamics, we report that nanobubbles can weaken Marangoni stresses and consequently reduce the duration of the retardation regime. Remarkably, we find that the duration of this retardation regime (t_r) exponentially decays with the nanobubble concentration in the dispersion (N_b) according to N_b ∼ log(1/t_r). The micro-particle imaging velocimetry analysis of the flow field inside the drop indicates a large reduction in the magnitude of velocities in the presence of surface-active materials, confirming the existence of Marangoni flow that opposes droplet spreading. Our research introduces a simple approach to calculate the nanobubble concentrations in liquids and offers guidelines for controlling wetting dynamics.