How Droplets Dry on Stretched Soft Substrates

Droplet evaporation on solid substrates is a ubiquitous phenomenon and is relevant in many natural and industrial processes. Whereas it has been reported that the evaporation process is sped up on soft substrates compared with that on hard substrates, no attempt has been made in exploring how substrate stretching affects droplet evaporation and evaporative deposition patterns. Here, we systematically investigate the contact line dynamics of droplets evaporating on substrates with different stiffnesses and stretching ratios and the structures of the evaporative deposition patterns of nanoparticles. We show that the shapes of the droplets and their contact line dynamics during evaporation are strongly affected by uniaxial stretching of the soft substrates. The droplet evaporates with an elongated noncircular contact line and switches the elongation direction during evaporation. The contact line recedes earlier and faster in the direction parallel to the stretching than in the perpendicular direction. This is rationalized by a direct visualization of the evolution of the surface deformation of the soft substrates, the so-called wetting ridge underneath the contact line. We show that this anisotropic contact line receding only occurs on stretched substrates that are sufficiently soft. The anisotropic contact line motion enables us to prepare diverse anisotropic noncircular patterns of nanoparticles, which are beyond the well-known coffee-ring, by evaporating colloidal suspension droplets on stretched soft substrates. Our results suggest that stretching soft substrates is a simple way to manipulate the evaporative wetting dynamics and particle deposition of droplets, which should be of great interest to a wide range of scientific and industrial applications.