Evaporation-induced flow motion and particle deposition in sessile droplets
Final Report Abstract
This project has investigated experimentally the internal and superficial flow of evaporating droplets, with the main objective to improve the understanding of the role of the evaporation-driven flows in the deposition of final stain in water droplets. The results contributed to a significant advancement of the current knowledge of the physics of evaporating droplets with the following breakthrough observations: 1) The magnitude of the thermal-Marangoni flows predicted by current state-of-the-art numerical simulations is more than two orders of magnitude larger than what observed in experiments. Our experiments showed that this discrepancy cannot be ascribed to contaminants, as often reported in the literature, and that a more complex thermofluidic scenario at the water-air interface must be considered to achieve accurate theoretical predictions. 2) In droplets with a low content of salt, such as commercially available mineral waters, there is a surprising coexistence of two flow patterns. In the outer part of the droplet, we observed the conventional flow directed toward the contact line in the fluid bulk (capillary flow) and directed toward the drop summit in the water-air interface (thermal-Marangoni flow). In the central part of the droplet, we observed a reversal bulk flow directed toward the center. This resulted in stagnation region in the water-air droplet resulting in accumulation ring in the deposition stain at 0.6-0.7 droplet radii. 3) In droplets with a large content of salt, the flow is reversed with the bulk flow pointing toward the droplet center and the interfacial flow pointing toward the contact line. We demonstrated that this flow can be responsible of the formation of ring-shaped stains, contrary to the common belief that a capillary flow could be the only possible mechanism for formation of ring-shaped stains. This research also provided a large database of velocity patterns, investigating a large parameter space of evaporating conditions (humidity and temperature) and droplet sizes and provides a precious reference for quantitative comparisons of future improved numerical simulations.
Publications
- Interfacial flows in sessile evaporating droplets of mineral water, Phys. Rev. E 100, 033103 (2019)
M. Rossi, A. Marin, and C. J. Kähler
(See online at https://doi.org/10.1103/PhysRevE.100.033103) - Particle distribution and velocity in electrokinetically induced banding, Microfluid. Nanofluid. 23, 67 (2019)
M. Rossi, A. Marin, N. Cevheri, C. J. Kähler, and M. Yoda
(See online at https://doi.org/10.1007/s10404-019-2227-9) - Solutal Marangoni flow as the cause of ring stains from drying salty colloidal drops, Phys. Rev. Fluids 4, 041601 (2019)
A. Marin, S. Karpitschka, D. Noguera-Marín, M. A. Cabrerizo-Vílchez, M. Rossi, C. J. Kähler, and M. A. R. Valverde
(See online at https://doi.org/10.1103/PhysRevFluids.4.041601) - Flow profiles near receding three-phase contact lines: influence of surfactants, Soft Matter 17, 10090-10100 (2021)
B. B. Straub, H. Schmidt, P. Rostami, F. Henrich, M. Rossi, C. J. Kähler, H.-J. Butt and G. K. Auernhammer
(See online at https://doi.org/10.1039/D1SM01145F)
