Evaporating multicomponent droplets

Abstract

Multicomponent droplets are ubiquitous in nature and technology. It is of great importance to understand their evaporation process. In this thesis, we have studied several multicomponent droplet systems, while focusing on the evaporation behavior, flow structures and subsequent particle deposition. We first studied the evaporation of 1,2-hexanediol-water binary droplets (Chapter 1), in which we observed and studied phase segregation. In Chapter 2, the flow in an evaporating glycerol-water binary sub-millimeter droplet with Bond number Bo ≪ 1 is studied. We measure the flow fields near the substrate during the evaporation process by micro-PIV for both sessile and pendant droplets, which surprisingly show opposite radial flow directions -- inward and outward, respectively. These observations clearly reveals that in spite of the small droplet size, gravitational effects play a crucial role in controlling the flow fields in the evaporating droplets. As a next step, we added 0.5 wt% silicone oil into the 1,2-hexanediol-water binary solution (Chapter 3). This minute silicone oil concentration dramatically modifies the evaporation process as it triggers an early extraction of the 1,2-hexanediol from the mixture. Surprisingly, we observe that the segregation of 1,2-hexanediol forms plumes, rising up from the rim of the sessile droplet towards the apex during the droplet evaporation. By orientating the droplet upside down, i.e., by studying a pendant droplet, the absence of the plume formation indicates that the flow structure is induced by buoyancy, which drives a Rayleigh-Taylor instability (i.e., driven by density differences & gravitational acceleration). In Chapter 4, we observed and studied the crystallization of sodium dodecyl sulfate (SDS) within an evaporating glycerol-water mixture droplet. The crystallization is induced by the preferential evaporation of water, which decreases the solubility of SDS in the mixture. In Chapter 5, we showed that control over particle deposition can be achieved through droplet evaporation on oil-wetted surfaces. We demonstrated by flow visualization, theory, and numerics that the final deposit of the particles is governed by the coupling of the flow field in the evaporating droplet, the movement of its contact line, and the wetting state of the thin film surrounding the droplet.