Effect of Mirco-/Nano-particle Size and Material on Self-Pinning Behavior for Coffee Ring Formation on Different Hydrophobic Surfaces
| Autor: | Kai-Chieh Yang, 楊凱傑 |
|---|---|
| Rok vydání: | 2017 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 105 When a colloidal droplet containing nonvolatile micro-/nano-particles dries out on homogeneous solid surfaces, a ring-like deposit of micro-/nano-particles develops at the edge of the droplet, known as the coffee ring effect. Self-pinning induced by the aggregation of nanoparticles at the drop edge is a pre-requisite for the coffee ring effect. The micro-/nano-particle concentration in the colloidal droplet increases during the evaporation process due to the solvent (water) evaporation and eventually reaches the pinning concentration of particles to initiate the self-pinning for the coffee ring formation on the alkylsilane coated surfaces. The effect of particle sizes, particle material and surface hydrophobicity on the pinning concentration was experimentally explored and examined at a fixed surrounding temperature (30 °C) and humidity (50 %). The pinning concentration of silica nanoparticles decreases along with an increase in the size of nanoparticles and is independent of the initial concentration of nanoparticles. The pinning concentration of silica nanoparticles of 400 nm in diameter linearly depends on the surface hydrophobicity, characterized by the receding contact angle of water on the surface. The linear regression curve between the pinning concentration of silica nanoparticles of 760 nm (and 1000 nm) in diameter and the receding contact angle of water on the alkylsilane coated surface remains intact but shifts downward and almost parallel to that of 400 nm in diameter, implying that the ability of self-pinning the three phase contact of lager silica nanoparticles is stronger than smaller ones. However, for the polystyrene nanoparticles (100, 500 and 1000 nm in diameter) on alkysilane coated surface, the tendency of pinning concentration affected by particle size totally opposite to that of silica particle system, although the linear dependence on the receding contact angle of the surface remains the same as that of silica particle system. In addition, the evaporation of colloidal droplet containing a mixture of bi-dispersed particles of two different sizes (400 + 1000 nm, 760 + 1000 nm in diameter silica particle and 100 + 1000 nm in diameter polystyrene particle) was also examined. The pinning concentration and the size of residual deposit of this bi-dispersed particle system are consistent with that of particles with lower pinning concentration (larger particle for silica particles and smaller particle for polystyrene particles), implying that the self-pinning is dominated by the particles with strong pinning effect to the three phase contact line. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
| Externí odkaz: |
|