Water jet rebounds on hydrophobic surfaces : a first step to jet micro-fluidics

TL;DR

This paper demonstrates that water jets can rebound on hydrophobic surfaces without destabilization, revealing a new phenomenon with potential applications in micro-fluidics involving small water quantities.

Contribution

The study provides the first experimental evidence of true jet rebound on hydrophobic surfaces, exploring conditions that enable stable jet reflection.

Findings

Water jets can rebound on hydrophobic surfaces at specific angles and velocities.

Capillary forces dominate at sub-millimeter jet scales, enabling stable rebound.

Rebound behavior depends on substrate hydrophobicity and impact parameters.

Abstract

When a water jet impinges upon a solid surface it produces a so called hydraulic jump that everyone can observe in the sink of its kitchen. It is characterized by a thin liquid sheet bounded by a circular rise of the surface due to capillary and gravitational forces. In this phenomenon, the impact induces a geometrical transition, from the cylindrical one of the jet to the bi-dimensional one of the film. A true jet rebound on a solid surface, for which the cylindrical geometry is preserved, has never been yet observed. Here we experimentally demonstrate that a water jet can impact a solid surface without being destabilized. Depending on the incident angle of the impinging jet, its velocity and the degree of hydrophobicity of the substrate, the jet can i) bounce on the surface with a fixed reflected angle, ii) land on it and give rise to a supported jet or iii) be destabilized, emitting drops. Capillary forces are predominant at the sub-millimetric jet scale considered in this work, along with the hydrophobic nature of the substrate. The results presented in this letter raise the fundamental problem of knowing why such capillary hydraulic jump gives rise to this unexpected jet rebound phenomenon. This study furthermore offers new and promising possibilities to handle little quantity of water through "jet micro-fluidics"