Air entrainment and granular bubbles produced by an impinging jet of grains into water

TL;DR

This study explores the formation and stability of granular bubbles created by a dry granular jet impacting water, revealing how grain size and impact conditions influence bubble characteristics and longevity.

Contribution

It introduces the concept of stable granular bubbles formed by a dry granular jet impacting water, a phenomenon previously unexplored in fluid dynamics.

Findings

Granular bubbles are stable due to a grain shell preventing coalescence.

Bubble size and air volume depend on grain size, liquid properties, and impact velocity.

Granular bubbles can sink or rise depending on buoyant and weight forces.

Abstract

A jet of water entering into a pool of the same liquid can generate air entrainment and form bubbles that rapidly raise to the surface and disintegrate. Here we report the equivalent phenomenon produced by a plunging dry granular jet, so far unexplored. For grains smaller than a critical size, the granular jet entering into the pool produces air entrainment that leads to bubbles formation. The bubbles formed are covered by a shell of grains attached to the bubble air-water interface due to capillary-induced cohesion. In contrast to classical air bubbles, these "granular bubbles" are stable over time because the granular shell prevents coalescence and keeps the air encapsulated either if the bubbles rise to the surface or sink to the bottom of the pool, which is determined by the competition of the buoyant force and the weight of the assembly. Experimentally, we show how the bubble size and volume of entrained air depend on the grain size, liquid properties and jet impact velocity.