Extreme resolution transient deformation fields of a viscoelastic substrate during sessile droplet evaporation: Revealing a link between evaporation and hydrophilicity.

Droplets spreading, wetting, and drying on viscoelastic substrates is important in a host of technologies including 3D printing, coatings, and microfluidics. Previous research has shown that microscopic surface deformations can significantly alter macroscopic dynamics such as droplet spreading and evaporation; however, we still are lacking a complete picture of how the liquid receding kinetics—particularly for fast contact line velocities—on a solid are influenced by mechanical substrate properties. Here, we investigate experimentally the influence of substrate compliance on droplet evaporation, in particular, the receding contact line behavior, and show that above a critical receding contact line speed—a characteristic relaxation rate—the substrate exhibits enhanced hydrophilicity. To elucidate this behavior, we used 3D reference free traction force microscopy to map microscopic surface deformations near the contact line for various receding contact line velocities. Such microscopic deformations, i.e., wetting ridges, are known to contribute to viscoelastic dissipation and retard droplet spreading. We show that this viscoelastic “brake”, in conjunction with the sharp wetting ridge tip, acts to pin the contact line during fast evaporation giving rise to enhanced hydrophilicity.