Drop impact on hairy surfaces

We investigate the impact of liquid drops on millimeter-scale hairy surfaces. By varying the speed of the drop, the spacing of the hairs, and the viscosity of the liquid, we observe a variety of behaviors. In some cases, the liquid drop can remain on top of the hair after impact, similar to a Cassie-Baxter superhydrophobic state. If the drop penetrates the hairy surface, the hairs can resist droplet spreading. Using this scenario as a reference case, we rationalize the role of the hairs in dissipating the kinetic energy of the impacting drop through a balance of inertia, viscosity, and surface tension. The various observed behaviors are classified according to scenarios in which kinetic energy is insufficient or in excess of this reference scenario, an argument that allows us to build and rationalize a phase diagram.

Figure 1

(a) Schematic of the drop impact experiment and the geometry of the hairs. (b) Drop not penetrating the hairy texture on impact (glycerol, $d=0.5$ mm, $V=0.6$ m/s). (c) Drop that partially penetrates the hairy texture (water $d=0.5$ mm, $V=0.6$ m/s). (d) Drop that penetrates the surface and is “captured” and prevented from spreading (water $d=0.75$ mm, $V=0.6$ m/s). (e) Drop that penetrates and spreads (water $d=2$ mm, $V=0.6$ m/s). (f) Drop that penetrates and forms long fingers and ejects droplets (water, $d=2,V=1.4$ m/s). The dashed white line outlines the contours of the fingers of the drop. The scale bars indicate 1 mm in all images.

Figure 2

(a) Phase diagram for drop impact experiments with water. Symbols defined in Fig. 1. (b) Phase diagram for drop impact experiments with glycerol. Symbols defined in Fig. 1. (c) Montage of water drop impact on a hairy surface with $d=0.75$ mm. The speed of impact is $V=0.6$ m/s. The time interval between frames is 2.8 ms. The corresponding point in the phase diagram (a) is surrounded by a dashed circle. (d) Montage of glycerol drop impact on a hairy surface with $d=1.5$ mm. The speed of impact is $V=2.0$ m/s. The time interval between frames is 0.83 ms. The corresponding point in the phase diagram (b) is surrounded by a dashed circle. The scale bars indicate 1 mm.

Figure 3

Phase diagram: Shown on the diagram are the various possible states observed in our experiments. Data are plotted in the $({\text{Re}}^{*},{\text{We}}^{*})$ phase space, as defined in Eq. (3). Superimposed on the diagram are the lines obtained by the balance of energy yielding Eq. (4). The dotted lines correspond to the values of $\beta =0.2,0.5,1$, and 5. These isolines are obtained using Eq. (4) and form boundaries between the observed states.