Abstract
We study the formation of fine radial jets during the impact of a compound drop on a smooth solid surface. The disperse-phase droplets are heavier than the outer continuous phase of the main drop and sink to the bottom of the drop before it is released from the nozzle. The droplets often arrange into a regular pattern around the axis of symmetry. This configuration produces narrow high-speed jets aligned with every internal droplet. These radial jets form during the early impulsive phase of the impact, by local focusing of the outer liquid, which is forced into the narrowing wedge under each internal droplet. The pressure-driven flow forces a thin sheet under and around each droplet, which levitates and separates from the solid surface. Subsequently, surface tension re-forms this horizontal sheet into a cylindrical jet, which is typically as narrow as 35 μ m, while smaller droplets can produce even thinner jets. We systematically change the number of inner droplets and the properties of the main drop to identify the jetting threshold. The jet speed and thickness are minimally affected by the viscosity of the outer liquid, suggesting pure inertial focusing. The jets emerge at around eight times the drop impact velocity. Jetting stops when the density of the inner droplets approaches that of the continuous phase. The interior droplets are often greatly deformed and broken up into satellites by the outer viscous stretching, through capillary pinch-off or tip streaming.
| Original language | English (US) |
|---|---|
| Journal | Journal of Fluid Mechanics |
| Volume | 883 |
| DOIs | |
| State | Published - Nov 28 2019 |
Bibliographical note
KAUST Repository Item: Exported on 2020-10-01Acknowledged KAUST grant number(s): URF/1/2621-01-01, URF/1/3727-01-01
Acknowledgements: The work described herein was supported by King Abdullah University of Science and Technology (KAUST) research funding (URF/1/2621-01-01 and URF/1/3727-01-01). Some of the supplementary movies were submitted to the Gallery of Fluid Motions of the APS-DFD meeting held in San Francisco in November 2014 (V0042, https://doi.org/10.1103/APS.DFD.2014.GFM.V0042). E.Q.L. acknowledges the Thousand Young Talents Program of China, the National Natural Science Foundation of China (grants nos 11772327, 11972339 and 11621202) and Fundamental Research Funds for the Central Universities (grant no. WK2090050041). E.Q.L. also acknowledges Dr Y. Jin and the experimental Center of Engineering and Material Sciences at University of Science and Technology of China for using their Kirana camera at the revision stage of the manuscript. The authors thank Professor A. Marin of the University of Twente for valuable assistance.