Wetting Driven Globule-to-Brush Transition of Polymer-grafted Surfaces

Studying the behavior of liquid drops on surfaces of various complexities is critical in the design and fabrication of a wide range of surfaces. A proper understanding of the surface wettability can be employed in various energy and biomedical applications as well as applications such as enhanced oil-water separation, anti-biofouling etc. We employ Molecular Dynamics (MD) simulations to probe the spreading and imbibition of a liquid drop on a surface grafted by polymer molecules and observe their transition from an initially collapsed globule-like state, to a brush-like state upon wetting. We hypothesize that the drop spreading behavior is dictated by a balance between the inertial pressure and the viscoelastic dissipation associated with the polymer layer undergoing this transition. Our MD results indicate that the rheology of the polymer layer, behaving like a viscoelastic solid, can be described by a power-law index of n=2/3, ensuring that the scaling calculations recover the simulation predictions. We also observe that the dynamics of the polymer layer swelling depend on the grafting density and is faster for sparser grafting, and we recover the equilibrium polymer brush height scaling with grafting density, thus confirming a brush-like configuration of the polymer layer.