Particle self-assembly in sessile droplet on patterned substrate

Abstract

Colloidal self-assembly on pattern substrate has grasped the attention of many researchers in understanding and developing a new fluidic system to explore discoveries that could prosper the industrial application like coating, inkjet printing, biosensors, microchip fabrication, and so forth. Therefore, the objective of this project is to investigate the particle self-assembly process of suspension droplets on micro pyramid cavities patterned substrate. Two different sizes of polystyrene particles dissolve in deionized water to create four different concentrations for the experiments. This research focused on the wettability and the evolution of the sessile droplets, the particle movement within the fluid, and the dried patterns. The PMMA patterned substrate is a hydrophobic surface as all the droplets formed a contact angle close to 100°. All the droplets evaporated in CCR mode and exhibit an octagonal wetting area except 0.1(wt%) for both the 3μm and 10μm particles with depinning of contact line take placed. This indicates that the particle is the pinning force that prevents contact line withdrawal. The droplet diameter measured along the 45° was bigger than the measurement taken along 90° by about 10% for 3μm and 5% for 10μm. The difference is caused by the difference in the contact area between the droplet and the substrate at each length hinting that the size of the drying pattern can be controlled by manipulating the parameters of the structures of the substrate. The size of a 3μm particle solution is bigger than the 10μm particle solution because smaller particles are more affected by surface tension over the gravity effect. The experimental results also show droplets with bigger particles tend to form uniform deposition patterns as compare to small particles drying patterns that are influenced by the capillary outward flow. An advanced confocal microscope is used to inspect the particle distribution on the substrate. Both 3μm and 10μm patterns at 2.6(wt%) show outer-ring patterns which means particles stacking above the cavities structures are easily drifted. The octagonal wetting area with sharp turning points forces more particles to accumulate at the octagon corners.