Droplets, including single component and multicomponent droplets are very common in our daily life and industries. Evaporation of sessile droplets on a solid substrate is an important fundamental phenomenon in various applications such as coatings, droplet-based microfluidics, inkjet printing, film formation and even spotting DNA microarray data. Until now, the evaporation of a multicomponent droplet on a heated surface, which is more practical and important to industries and daily life compared to monocomponent droplet evaporation, was not well understood, especially the internal flow field during the evaporation processes. Besides, as the technology of surface modification has developed, it is also significant to investigate the influence of heterogeneous surface wettability o...[ Read more ]

Droplets, including single component and multicomponent droplets are very common in our daily life and industries. Evaporation of sessile droplets on a solid substrate is an important fundamental phenomenon in various applications such as coatings, droplet-based microfluidics, inkjet printing, film formation and even spotting DNA microarray data. Until now, the evaporation of a multicomponent droplet on a heated surface, which is more practical and important to industries and daily life compared to monocomponent droplet evaporation, was not well understood, especially the internal flow field during the evaporation processes. Besides, as the technology of surface modification has developed, it is also significant to investigate the influence of heterogeneous surface wettability on droplet evaporation including mono- and multi-component droplets, in order to gain fundamental understanding and enhance evaporation efficiency.

Motivated by the importance mentioned above, in this research we first study the evaporation processes of multicomponent droplets on a heated homogeneous surface utilizing particle image velocimetry (PIV). It is found that the process of an evaporating multicomponent droplet of water/ethanol mixture can be divided into downward vortices stage, transition stage and upward vortices stage sequentially, while a pure water droplet only has the upward vortices stage. The three stages of multicomponent droplet evaporation were analyzed utilizing experimental data and dimensionless parameters, such as the thermal Marangoni number (Ma_T) and Solutal Marangoni number (Ma_S). For downward vortices and transition stages, the Marangoni effects especially Solutal Marangoni were dominating, which means evaporation of ethanol resulted in the first two stages. The occurrence of the transition stage indicated the buoyancy effect becomes comparable to the Marangoni effect as ethanol continuously evaporated, and the occurrence of the upward vortices stage indicated most of the ethanol gasified and the buoyancy effect dominated. Evaporation processes were also analyzed with internal flow velocities, showing consistence with our dimensionless analysis. The influence of input power and composition on the evaporation processes was also revealed.

The evaporation of multicomponent droplets on heterogeneous surfaces, which were wettability-patterned by surface modification, was also investigated to understand the influence of heterogeneous surfaces and seek potential to enhance evaporation efficiency as previous simulation results showed that heterogeneous wettability can increase the evaporation rate. It is found that, compared to a homogenous surface, heterogeneous surface wettability can change the evaporation process from three mode for the homogenous surface including constant contact line (CCL) mode, constant contact angle (CCA) mode and mix mode (MM) to two regimes, i.e. constant contact line (CCL) and moving contact line (MCL) modes. The contact line movement and its mechanism were revealed by microscope to explain macro evaporation behaviors. In addition, for analyzing the critical receding contact angles on homogenous and patterned surfaces, an improved local force model on the contact line was employed. The analysis results agree well for both surfaces, and confirm that the transition from CCL to MCL modes indicated droplet composition was monocomponent. It is also found that the heterogeneous wettability surfaces were able to enhance evaporation by elongating the contact line and enhance diffusion across contact line which is formed on hydrophobic-hydrophilic patterns boundaries.

To further understand the evaporation enhancement introduced by heterogeneous wettability surfaces, evaporation of monocomponent droplets on homogeneous and heterogeneous wettability surfaces with the same pattern ratio but different pattern size were compared. It is found that evaporation modes on heterogeneous wettability surfaces were different from those on homogenous surfaces which contained CCL, CCA, pattern-pinned (PP) and MCL modes, sequentially. To quantitatively understand the mechanism that enhances evaporation, generalized models were developed to predict the duration of each mode and transition of modes (critical receding contact angle) on each surface. Our generalized models, which predict CCL and CCA, agree well with Rowan’s and Erbil’s models, and show no evaporation enhancement in these two modes. However, evaporation enhancement was shown in PP and MCL modes, and was successfully explained by our models. It should be mentioned that, this is the first time that evaporation duration on the heterogeneous wettability surfaces was successfully predicted by generalized models and our models will provide a useful tool to predict the evaporation duration and design of functional surfaces for evaporation enhancement.