Evaporation and morphological patterns of bi-dispersed colloidal droplets on hydrophilic and hydrophobic surfaces
Year: 2018
Journal: Soft Matter, Volume 14, DEC 28, page 9901–9909
Authors: Iqbal, R.; Majhy, B.; Shen, Amy Q.; Sen, A. K.
Organizations: OIST; IIT Madras [MEE1516843RFTPASHS]; Japan Society for the Promotion of Science [18H01135]
Understanding the formation of different morphological patterns depending on the particle size and surface wettability has great relevance in the separation, mixing and concentration of micro/nano particles and biological entities. We report the evaporation and morphological patterns of evaporating bi-dispersed colloidal droplets on hydrophilic and hydrophobic surfaces. To explain the underlying mechanisms of various particle distribution patterns, we propose a phenomenological model that accounts for the drag force, van der Waals and electrostatic interaction forces, and surface tension force acting on the particles. In the case of the hydrophilic surface ( approximate to 27 degrees), there is a competition between the frictional force arising due to the van der Waals (approximate to 10(-8) N) and electrostatic interaction forces (approximate to 10(-10) N) and the surface tension force (approximate to 10(-7) N) that depends on the particle size. Consequently, the smaller particles (0.2 and 1.0 m in diameter) are found to be pinned and form an outer ring at the contact line whereas the larger particles (3.0 and 6.0 m in diameter) move inward, either forming an inner ring or flocculating depending on the particle size. Interestingly, a completely different morphological pattern of the micro/nano particles is observed on a hydrophobic substrate ( approximate to 110 degrees): contact line pinning is no longer observed and particles form a centralized deposition pattern. The order of the magnitude of the surface tension force is higher as compared to the frictional force (approximate to 10(-8) N); thus the particles are driven radially inward and accumulate at the center of the droplet. Owing to the mixed mode of evaporation toward the end of evaporation, only a fraction of smaller particles travel radially outward due to the coffee-ring effect. Scanning electron microscopy images reveal that smaller particles are present mostly at the center with a small fraction of smaller particles at the edge of the pattern, whereas larger particles are uniformly distributed throughout.
