Dynamic wetting of soluble polymer surfaces
Physical Chemistry of Solids and Surfaces, Material Characterisation
Final Report Abstract
Wetting is widely studied for its relevance in many industrial applications. This project focused on understanding the molecular aspect of the wetting of a solvent on a soluble polymer surface and the structures generated on the polymer surface. The process can be decomposed in three different parts: first the initial spreading of the droplet and the subsequent swelling of the first interfacial layer of the surface due to the solvent diffusion, second the deformation of the surface when the surface is swollen by large quantities of the solvent, third the evaporation of the droplet which leaves microstructures in the deformed polymer surface. All these sub-processes happen at different time- and length-scales, it is therefore complicated to set up experiments that can contemporarily shed light on all of them. This project used molecular dynamics simulations, both at the atomistic and at a coarse-grained level to understand the molecular mechanism that controls the spreading of the droplet and the swelling of the surface. We used atomistic simulations to understand two different aspects of the spreading: on the one hand we analyzed the molecules motion inside a droplet spreading on a liquid subphase, on the other hand we studied the solvent diffusion in a polymer matrix and the surface swelling. By studying aqueous ethanol droplet, with different ethanol concentration, we were able to understand how the surface tension gradient between the droplet and the liquid phase effects the formation of swirls both within the droplet and in the liquid reservoir. Even if some of the results can be predicted either by theoretical approaches or by numerical methods, only atomistic simulations can capture the phenomena happening at the molecular scale during the coalescence. We also simulated the real time evolution of low-molecular-weight polystyrene, below the glass transition temperature, in contact with its solvent, toluene. As expected the solvent diffuses into the polymer leading to an immediate swelling of the surface and to a progressive dilution of the chains. However, in the very short time of the simulations polymers chains get solvated and, after detaching from the surface, move into the solvent. The very fast detachment of the short polymer chains is favoured by the intrinsic roughness of the polymer surface, which gets immediately swollen by the penetrating solvent.
Publications
- How does low-molecular-weight polystyrene dissolve: Osmotic swelling vs. surface dissolution. Soft Matter 10, 9059-9064 (2014)
V. Marcon, N.F.A. van der Vegt
(See online at https://doi.org/10.1039/c4sm01636j) - Vortex formation in coalescence of droplets with a reservoir using molecular dynamics simulation. J. Colloid Interface Sci. 479, 189-198 (2016)
F. Taherian, V. Marcon, E. Bonaccurso, N.F.A. van der Vegt
(See online at https://doi.org/10.1016/j.jcis.2016.06.059)
