Wave-selected centrifugal fingering at a locally miscible interface

Interfaces between locally miscible fluids subjected to oscillatory shear are expected to destabilize through a mechanism analogous to Kelvin-Helmholtz instability, and we hypothesize that centrifugal forcing under rotational oscillation can subsequently amplify this wave state into radial fingers, coupling two distinct instability mechanisms within a single interfacial system.

We studied a stratified system of polydimethylsiloxane and acetone in a cylindrical vessel under sinusoidal rotational forcing at frequencies of 5 to 20 Hz, using top-view and side-view imaging to track interfacial wave onset, wavelength selection, and subsequent radial finger growth, complemented by pendant-drop tensiometry to quantify the effective interfacial tension of the locally miscible pair.

Oscillatory shear first organizes the interface into an azimuthally periodic wave state whose onset and wavelength match oscillatory Kelvin-Helmholtz theory once viscous stresses are included; centrifugal forcing then selectively amplifies wave crests into fingers, a process governed by a local radial mobility parameter contrasting centrifugal and viscous pressure drops, and all post-onset growth data collapse under a single centrifugal-viscous timescale, ruling out classical Saffman-Taylor, Faraday, and Rayleigh-Taylor mechanisms as the origin of the pattern.