Spreading of volatile fluids on swelling hydrophobic polymer brush layers

Polymer brushes are highly responsive materials with application areas in a broad spectrum. Therefore understanding their interfacial behaviour is essential. Under good solvent conditions, drop spreading causes changes in wettability on the brush surface and results in a finite contact angle. This feature was observed by placing a hexadecane drop on a dry hydrophobic poly lauryl methacrylate (PLMA) brush layer. These polymer brushes were synthesized via ARGET-ATRP to obtain high and low grafting densities and different chain lengths. In a closed chamber, that contained an excess of hexadecane, the spreading of the drop on the polymer brushes was followed by an optical microscope. As a result of the complex dynamics between the drop and the substrate, a halo of a few millimetres in width forms around the macroscopic contact line. A numeric model was extended to simulate such a droplet spreading on a swollen brush for understanding the coupled spreading, evaporation, condensation and wicking dynamics. A continuous gradual flux from liquid to brush, liquid to vapor, and brush to vapor can explain the partially wetting of the polymer brushes in this non-equilibrium state. Understanding the spreading dynamics of a droplet on hydrophobic surfaces will be crucial to explaining the mechanism of the halo formation.