Leaping shampoo glides on a lubricating air layer

When a stream of shampoo is fed onto a pool in one's hand, a jet can leap sideways or rebound from the liquid surface in an intriguing phenomenon known as the Kaye effect. Earlier studies have debated whether non-Newtonian effects are the underlying cause of this phenomenon, making the jet glide on top of a shear-thinning liquid layer, or whether an entrained air layer is responsible. Herein we show unambiguously that the jet slides on a lubricating air layer. We identify this layer by looking through the pool liquid and observing its rupture into fine bubbles. The resulting microbubble sizes suggest this air layer is of submicron thickness. This thickness estimate is also supported by the tangential deceleration of the jet during the rebounding.

Figure 1

(a) Typical leaping shampoo jet and (b) definition sketch.

Figure 2

Shear-thinning properties of the shampoo at 21.5 ${}^{\circ }$C.

Figure 3

Overall view of the shampoo jet bouncing off an inclined pool of the same liquid. The scale bar is 3 mm long. The white arrow points at the breakup of the air layer into threads and bubbles. The relative times of the frames are $t=0$, 124, 287, 296, 308, 324, and 335 ms. The incoming jet is 0.30 mm in diameter, approaching the pool at 1.20 m/s. See also the video in the Supplemental Material [10].

Figure 4

Breakup of the air layer under the jet. Times relative to first observed rupture are $t=-717,-189$, 20, 31, 40, and 132 ms. The incoming jet is 0.56 mm in diameter and approaches the pool at 1.59 m/s. See also the video in the Supplemental Material [10].

Figure 5

(a) The trajectory of typical particles within the jet. The solid curve marks the air layer, and the dashed curve indicates the upper side of the jet if the cross section remains circular. (b) The velocity vs horizontal location along the curving jet.