Transitions in intra-droplet flow and wetting governing nanoparticle deposition patterns in inkjet-printed nanofluids

The deposition patterns of nanoparticles from evaporating inkjet-printed nanofluid droplets are determined by transitions in intra-droplet flow and wetting dynamics. These transitions are continuous and can be systematized when accurate measurement techniques, systematic samples, and appropriate dimensionless numbers are used.

This study investigated the roles of the Péclet number (Pe), Marangoni number (Ma), and the dimensionless evaporation rate ratio (τ_re/τ_ev) in the wetting dynamics and formation of nanoparticle deposition patterns. Using phase-shifting imaging ellipsometry, droplet shapes were measured with nanometer to micrometer precision, enabling the correlation of wetting dynamics with deposition patterns and accurately capturing their transitions.

The experiments demonstrated that deposition patterns-such as coffee rings, multi rings, spokes, and uniform films-emerge from specific intra-droplet flow and wetting dynamics. A high Pe, indicating dominant convective transport over diffusion, favors pronounced coffee ring patterns, whereas a low Pe results in more uniform deposition. Results showed that Marangoni-Bénard convections dominated at critical Ma values, whereas stick-slip motion governed the multi ring formation under low τ_re/τ_ev. These findings establish a predictive framework for tailoring the deposition patterns in inkjet-printed nanofluids. Furthermore, the high-precision measurements enabled new experimental observations, including the observation of larger spreading than pure liquids and coffee ring formation within nanoliquid films.