Wave-selected centrifugal fingering at a locally miscible interface.

HYPOTHESIS: 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. EXPERIMENTS: 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. FINDINGS: 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.