Fragmentation in turbulence by small eddies

TL;DR

This study provides experimental evidence that in turbulence, bubbles are primarily broken by small eddies below their size, challenging the classical Kolmogorov-Hinze paradigm which assumes eddies of the same size cause fragmentation.

Contribution

The paper introduces an experimental approach to distinguish eddy scales and demonstrates that small sub-bubble-scale eddies dominate bubble fragmentation, revising existing turbulence models.

Findings

Bubbles are preferentially broken by sub-bubble-scale eddies.

Fragmentation involves rapid, intense deformation by small eddies.

Stress criteria alone are insufficient to predict fragmentation.

Abstract

From air-sea gas exchange, oil pollution, to bioreactors, the ubiquitous fragmentation of bubbles/drops in turbulence has been modelled by relying on the classical Kolmogorov-Hinze paradigm since the 1950s. This framework hypothesizes that bubbles/drops are broken solely by eddies of the same size, even though turbulence is well known for its wide spectrum of scales. Here, by designing an experiment that can physically and cleanly disentangle eddies of various sizes, we report the experimental evidence to challenge this hypothesis and show bubbles are preferentially broken by the sub-bubble-scale eddies. Our work also highlights that fragmentation cannot be quantified solely by the stress criterion or the Weber number; The competition between different time scales is equally important. Instead of being elongated slowly and persistently by flows at their own scales, bubbles are fragmented in turbulence by small eddies via a burst of intense local deformation within a short time.