Beyond coffee ring: Anomalous self-assembly in evaporating nanofluid droplet on a sticky biomimetic substrate

Evaporation induced self-assembly is of paramount importance in many fields ranging from optoelectronic devices, blood spatter analysis, food industry, and thin film deposition. In this article, we report the evaporative drying of a nanofluid droplet on an inclined biomimetic sticky substrate obtained by soft lithographically replicating the structures of Rose petals on crosslinked Polydimethylsiloxane and demonstrate the influence of substrate inclination on the transitions in morphodynamics of the final deposit patterns. Based on experimental data and agglomeration kinetics, we present three unique morphologies induced by substrate inclination. First, buckling from the side in an upright droplet due to air cavity in the substrate. Second, sedimentation induced side buckling in an inclined droplet. Finally, cavity from the bottom in an inverted droplet. We provide a detailed physical explanation of the transition in the morphologies by exploring the coupling among droplet-substrate orientation, evaporation, internal flow and particle agglomeration.