Technical Brief

Two-Pronged Jet Formation Caused by Droplet Impact on Anisotropic Superhydrophobic Surfaces

[+] Author and Article Information
J. T. Pearson, D. Bilodeau

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602

D. Maynes

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602
e-mail: maynes@byu.edu

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 10, 2015; final manuscript received December 30, 2015; published online April 22, 2016. Assoc. Editor: John Abraham.

J. Fluids Eng 138(7), 074501 (Apr 22, 2016) (5 pages) Paper No: FE-15-1396; doi: 10.1115/1.4032596 History: Received June 10, 2015; Revised December 30, 2015

When a liquid droplet impacts a superhydrophobic surface with anisotropic surface patterning in the form of alternating ribs and cavities, the rebounding droplet may exhibit a unique two-pronged jet emission. Droplet impact experiments with 11 different fluids of viscosity that varied by more than three orders of magnitude were conducted, and this paper quantifies the Capillary number, Ca, and Ohnesorge number, Oh, ranges over which the two-pronged phenomenon occurs. For Oh > 0.0154, the behavior was never observed, while at lower values of Oh, the behavior occurs for an intermediate range of Ca that depends on Oh.

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Grahic Jump Location
Fig. 1

Example images of droplet impingement on anisotropic superhydrophobic surfaces under conditions where the two-pronged jet emission occurs (left) and where it does not (right)

Grahic Jump Location
Fig. 2

SEM image of a superhydrophobic surface comprised of alternating rib and cavity structures. The cavity width, wc, and the combined rib and cavity width, w, are shown.

Grahic Jump Location
Fig. 3

Shown are images for (a) the Ca regime just before two-pronged jets, two-pronged jets from (b) the transverse view, (c) the longitudinal view, (d) the top view, and (e) the regime just after two-pronged jets. For all image sets, Oh = 0.013. Images show, from top to bottom, a droplet at impact (time = 0 ms), as the jet is first visible (7.5 ms), and two frames showing jet progression (8.2 and 10 ms). Parts (a) and (e) show the transverse view.

Grahic Jump Location
Fig. 4

Plot of Capillary number, Ca, against Ohnesorge number, Oh. Impingement events that resulted in a two-pronged jet are shown in red and the scenarios that did not result in two-prong rebound are shown in blue. Equations for the upper and lower lines bounding the two-pronged jet regime are noted on the plot, with the fingering and/or splashing regime existing above the upper bounding line. Also shown is Eq. (1), a prior correlation from Ref. [17] to predict the onset of splashing for impact onto a dry surface.

Grahic Jump Location
Fig. 5

Probability, P, of observance of two-pronged jets as a function of Oh. Probability is taken within the region of Ca where two-pronged jets are observed.

Grahic Jump Location
Fig. 6

Plot of maximum relative diameter in the transverse spread direction, Dm/2, as a function of Oh. Maximum diameter data was taken at three Weber numbers, as shown. Also shown are least-squares power law fits to the data for We = 100 and 200 and a line corresponding to Eq. (2), where We = 200.



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