Uncoupling evaporation kinetics and particle depositions of sessile suspension droplets on soft substrates*

In present work, the evaporation of water droplets containing 0.125wt% SiO₂ nanoparticles on PDMS surface were recorded by a CCD camera and then the particle depositions left were imaged by a laser scanning confocal microscope. The PDMS was fabricated with various curing ratios of monomer to cross-linker, n=10:1∼160:1. It is found that when n<60:1, the droplet evaporation starts with constant contact radius (CCR) mode, followed by constant contact angle (CCA) mode and then mixed mode, and finally end up with a short CCR mode.The “coffee-ring” pattern is left behind with cracks and deposition tail. Whereas, when n≥60:1 the dying droplet spread first and its duration decreases with softness of substrate. Then the evaporation kinetics is dominated by CCR mode almost till the end of the evaporation at n=60:1 and 80:1 while mixed mode at n≥100:1. As for particle deposition, at n=60:1 and 80:1 a continuous annular deposition pattern is formed with a lateral section of sharp ox horn. When n≥100:1 there is residue dimple after droplet evaporation due to the plastic deformation of substrates. The particles would precipitate on this dimple and the finger instability at the outer edge of the deposition is visible.