Absorption of millimeter- and micrometer-sized droplets on nylon powder

Absorption of droplet by the powder media is a ubiquitous phenomenon in many natural and engineering processes, in which the diameter of droplets ranges from tens of micrometers (μm) to a few millimeters (mm) depending on the specific condition. In this study, we investigate the absorption process following the impact of the droplet with radii varying from mm- to μm-scale on nylon powder substrate. The fluid properties are varied by mixing the deionized water with isopropyl alcohol at different volume ratios. A syringe pipette tip is used to generate droplets of diameter ranging from 2.08 to 2.30 mm, whereas a drop-on-demand inkjet device is used to dispense droplets of diameter ranging from 88 to 140 μm. The absorption process of the single droplet is captured by a high-speed imaging system. We find that for mm- and μm-droplets, there exists different critical surface tension below which the imbibition into the nylon powder can occur. With the proposed nondimensionalization process, the scattered data of dimensionless absorbed volume V and dimensionless time τ for different fluids generally collapse into a power law relation V ~ τ^α, where the exponent α for mm-sized and μm-sized droplets is 0.856 and 0.518, respectively. The experimental data of μm droplets generally agree with the prediction of the Washburn equation and diffusion-type equation for the unidirectional capillary flow, i.e., V ~ τ^0.5. However, the results of mm-droplets favor more the prediction of the 3D radial flow (i.e., a hemispherically advancing infiltration front inside the powder), V ~ τ.