Emergence of capillary waves in miscible co-flowing fluids

TL;DR

This paper demonstrates the existence of capillary waves at the boundary of miscible co-flowing fluids, revealing a transition in wave behavior that enables measurement of transient interfacial tension on very short time scales.

Contribution

It uncovers the transition from inertial to capillary wave regimes in miscible fluids and introduces a method to measure rapidly decaying interfacial tensions.

Findings

Identification of capillary waves in miscible fluids

Observation of a transition to capillary wave scaling $k\sim\omega^{2/3}$

Measurement of rapid interfacial tension decay

Abstract

We show that capillary waves can exist at the the boundary between miscible co-flowing fluids. We unveil that the interplay between transient interfacial stresses and confinement drives the progressive transition from the well-known inertial regime, characterized by a frequency independent wavenumber, $k\sim\omega^{0}$, to a capillary wave scaling, $k\sim\omega^{2/3}$, unexpected for miscible fluids. This allows us to measure the effective interfacial tension between miscible fluids and its rapid decay on time scales never probed so far, which we rationalize with a model going beyond square-gradient theories. Our work potentially opens a new avenue to measure transient interfacial tensions at the millisecond scale in a controlled manner.