Enhanced spreading of surfactant-containing, self-rewetting fluids in pulmonary drug delivery

We study the enhanced spreading and internal diffusion of a cold, self-rewetting droplet laden with both surfactant and medicine that is placed over a hot liquid film. Spreading is induced by solutocapillary and thermocapillary effects simultaneously. A numerical simulation based on Stokes flow is performed, and the internal velocity map is obtained. The horizontal velocity flux and total medicine absorption are calculated to examine the internal diffusion and transport behaviors for a low-viscosity case and a high-viscosity mucus case. The results show that solutocapillary and thermocapillary effects contribute to droplet spreading positively and negatively, respectively. Self-rewetting fluids enhance spreading by increasing the surface tension gradient and prolonging the time required for spreading to reach a steady regime. For the self-rewetting fluid case at the final calculation time, the thermo-Marangoni number ΣT = 0.03, and the soluto-Marangoni number ΣS = 0.9, the internal diffusion and medicine absorption are enhanced by 9.1% and 8.3% relative to the ordinary fluid, respectively. When a droplet spreads on a high-viscosity mucus at the same Marangoni numbers, both spreading and diffusion are hindered. The spreading enhancement provided by self-rewetting fluids is much smaller than in low-viscosity cases. However, medicine absorption still increases by 11%.