Spontaneous localization of microspheres within drying droplets on wetting patterned substrates

During the drying of droplets containing microspheres (MSs), the coffee ring effect drives flow from the droplet’s center toward the contact line. Consequently, MSs form a circular deposit near the contact line after drying. Herein, we fixed the contact line using a triangular wetting patterned (WP) substrate. We investigated the flow field within droplets during drying and the resulting distribution of MSs. Our findings revealed that flow toward the tip of the triangular WP was faster than that toward the base. This asymmetric flow led to the spontaneous localization of MSs around the tip of the triangular WP during drying.