Experimental study of the evaporation, interaction and deposition patterns from volatile droplets

Abstract

Evaporation of sessile droplets and the driven particles self-assembly are a common phenomenon in daily life, which has many applications in manufacturing, medicine and cooling in micro/nano- electronmechanical systems (MEMS/NEMS), amongst others. Therefore, a huge amount of research has been carried out in the field, particularly, on the evaporation behaviour of single droplets (of pure liquids or containing particles) on rigid substrates and the deposits left behind. However, the fundamental physics and mechanisms governing this phenomenon are not fully understood yet, especially for droplets on soft substrates and for multi-droplet scenarios. The aim of this thesis is to elucidate the evaporation of droplets on viscoelastic substrates and flexible films, as well as the evaporation of binary-droplet system by experimental investigation. In addition, deposition patterns formed by the evaporative droplets containing nanoparticles are also studied on viscoelastic substrates and in binary-droplet system.

Droplets containing nanoparticles are investigated to evaporate on viscoelastic substrates, on which wetting ridges are pulled up at the contact line by surface tension. The wetting ridge affects the evaporation behaviour in two ways. On the one hand, the growth of ridge height with increasing viscoelasticity facilitates the pinning of contact line and extends the evaporation mode of the constant contact radius. On the other hand, on very viscoelastic substrates, the wetting ridge can move horizontally due to the predominant viscous property. The moving ridge then carries the contact line to recede together, leading to a mixed evaporation mode. The resulting deposition patterns are also influenced by substrate viscoelasticity, varying from typical coffee-ring stains to ring stains of ox-horn-like profile, and finger-like deposits.

For another soft substrates i.e. thin films, the deposition of pure liquid droplets on them leads to capillary origami. The transient folding process of films, driven by evaporating water droplets is quantified and examined in detail. Additionally, experimental results show that different wettabilities of droplet liquids can trigger progressively or instantaneously complete folding of films. In turn, the folding of the films will weaken the evaporation of droplets and a linear dependence is found.

Lastly, the vapour-mediated evaporation of binary droplets, as well as the resulting nanoparticle deposits is systematically studied in the experiments by varying spacing between the two droplets, volatility of vapour-source droplet and wettability of substrate surfaces. Additionally, the evaporation of binary droplets entrapped in a confined environment is also investigated. The results show that the presence of an adjacent droplet will slow the evaporation of the other droplet, especially with small spacing or in a confined environment. In addition, the absorption of the vapour from adjacent droplets of a differing liquid type in the droplets containing nanoparticles will give rise to a nonuniform distribution of surface tension along the liquid-gas interface of the latter and trigger Marangoni flow, which results in a crescent deposition pattern on hydrophobic surfaces or a layered deposition structure on hydrophilic surfaces.