(303i) In-Situ Observation of Meniscus Tip Behavior During Stripe Pattern Formation in Vertical-Deposition Convective Self-Assembly

Convective self-assembly is one of the attractive techniques providing one with simple, scalable, and economical routes to the fabrication of patterned structures of particles. In our previous study, we utilized the convective self-assembly technique and successfully fabricated striped colloidal arrays having well-ordered periodicity of the order of micrometers in their line width and spacing. Furthermore, conducting the second stripe formation after the rotation of the substrate with the stripes by 90°, we obtained grid network patterns of colloidal particles. However, the stripe formation mechanism has to be elucidated to further control the formation process. In the present study, we directly observe the stripe formation process in the convective self-assembly of colloidal particles to clarify the formation mechanism. A hydrophilic substrate is vertically immersed in a suspension and kept stationary at a constant temperature. As the solvent evaporates, dispersed particles are carried into the contact line by the convection which compensates for the solvent evaporation from drying region, and deposited on the substrate to form stripe patterned colloidal arrays. We monitor the stripe formation processes by using an optical microscope. The illuminating light is monochromatic with the wavelength of l = 475 ± 15 nm. The light reflected on the meniscus and substrate surfaces interferes with each other, resulting in the interference fringe pattern near the meniscus edge. Calculating the thickness profile of the meniscus from the fringe images, we clarify the stripe formation mechanism as follows. Under a fairly low particle concentration condition, the meniscus is gradually stretched downward to be concave toward a substrate as the solvent evaporation progresses, because the rate of particulate film growth is slower than that of liquid level lowering by the solvent evaporation. The convective flow toward the meniscus tip accelerates the deformation of the meniscus, resulting in the rupture of liquid film at the thinnest point of the meniscus. The rupture propagates in lateral directions, and then the meniscus tip shifts downwards and sticks to a new position which is determined by the equilibrium contact angle.