Spontaneous Pattern Formation of Colloidal Particles by Evaporation-Induced Self Assembly

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Bottom-up self-organization approaches are promising for fabricating higher-order patterned surfaces composed of submicron particles. The first example among the patterns that have been extensively studied would be stripes; however, the formation of stripe patterns has so far been confined to partially or fully hydrophobic surfaces. By contrast, we have succeeded in preparing well-defined stripe patterns even on strongly hydrophilic substrates via a convective self-assembly technique. By using this technique, a stripe pattern was produced simply by suspending a substrate in a dilute suspension, without any complicated procedure; the stripes spontaneously aligned parallel to the contact line. We reveal that neither stick-slip motion nor dewetting, which control processes on hydrophobic surfaces, is relevant to this technique; instead, negative feedback of particle concentration, induced by a concavely curved shape of the meniscus, governs the stripe formation. Then we examined the effect of ion concentration in a suspension. In a low concentration region, stripes formed vertical to the contact line, which would be due to the fingering instability of the tip of the meniscus. On the other hand, a gasket pattern formed in a high concentration region, which would be because ions play a role as a lubricant between particles and a substrate.