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Issue 9, 2015
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Dynamics of high-speed micro-drop impact: numerical simulations and experiments at frame-to-frame times below 100 ns

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Abstract

Technologies including (3D-) (bio-)printing, diesel engines, laser-induced forward transfer, and spray cleaning require optimization and therefore understanding of micrometer-sized droplets impacting at velocities beyond 10 m s−1. However, as yet, this regime has hardly been addressed. Here we present the first time-resolved experimental investigation of microdroplet impact at velocities up to V0 = 50 m s−1, on hydrophilic and -phobic surfaces at frame rates exceeding 107 frames per second. A novel method to determine the 3D-droplet profile at sub-micron resolution at the same frame rates is presented, using the fringe pattern observed from a bottom view. A numerical model, which is validated by the side- and bottom-view measurements, is employed to study the viscous boundary layer inside the droplet and the development of the rim. The spreading dynamics, the maximal spreading diameter, the boundary layer thickness, the rim formation, and the air bubble entrainment are compared to theory and previous experiments. In general, the impact dynamics are equal to millimeter-sized droplet impact for equal Reynolds-, Weber- and Stokes numbers (Re, We, and St, respectively). Using our numerical model, effective scaling laws for the progression of the boundary layer thickness and the rim diameter are provided. The dimensionless boundary layer thickness develops in time (t) according to Image ID:c4sm02474e-t1.gif, and the diameter of the rim develops as Image ID:c4sm02474e-t2.gif, with drop diameter D0 and inertial time scale τ = D0/V0. These scalings differ from previously assumed, but never validated, values. Finally, no splash is observed, at variance with many predictions but in agreement with models including the influence of the surrounding gas. This confirms that the ambient gas properties are key ingredients for splash threshold predictions.

Graphical abstract: Dynamics of high-speed micro-drop impact: numerical simulations and experiments at frame-to-frame times below 100 ns

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Publication details

The article was received on 07 Nov 2014, accepted on 16 Dec 2014 and first published on 16 Dec 2014


Article type: Paper
DOI: 10.1039/C4SM02474E
Citation: Soft Matter, 2015,11, 1708-1722
  • Open access: Creative Commons BY license
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    Dynamics of high-speed micro-drop impact: numerical simulations and experiments at frame-to-frame times below 100 ns

    C. W. Visser, P. E. Frommhold, S. Wildeman, R. Mettin, D. Lohse and C. Sun, Soft Matter, 2015, 11, 1708
    DOI: 10.1039/C4SM02474E

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