Periodic stripe pattern morphology via evaporation of inclined microdroplets of Au nanocolloids

Periodic assemblies of functional nanoparticles hold critical value in nanodevice fabrication, offering diverse applications in biosensing, energy conversion, printing, and patterning technologies. Understanding the interplay between fluid dynamics (capillary and Marangoni flows) and nanoscale colloidal interactions are crucial for controlling the final deposition patterns. Regulating the three-phase-contact-line (TPCL) motion during evaporation of a colloidal droplet to repetitive pinning-depinning transitions lead to the formation of multiple rings or stripe patterns. This study explores a novel technique for patterning solid substrate with semi-circular stripe patterns by modulating the stick-slip motion of the TPCL, through gravity-induced droplet shape deformation. We show that when the droplet evaporates on a tilted substrate, its geometry changes, leading to curvature-driven particle transport along the interface and triggering the stick-slip behaviour of the TPCL. On the lower contact angle side, the TPCL recedes towards the centre of the droplet, adapting a spherical cap geometry from its initial asymmetrical shape. We will discuss the role of interplay between the surface tension and gravitational forces establishing the oscillatory motion of the TPCL, resulting in semi-circular stripe patterns at the receding TPCL site, while a coffee-eye structure associated with a depletion zone forms at the pinning TPCL site. Precise control of nanoparticle concentration and droplet drying configuration is essential for generating multiple stripe patterns with tunable length scale. Notably, the number of stripes per unit length can be controlled by adjusting the substrate's tilting angle and nanoparticle concentration. This simple and inexpensive control over surface patterning highlights the technique's potential for diverse applications such as optoelectronic device for analyte sensing, foreign body detection in blood droplets etc.