Frequency structure of the nonlinear instability of a dragged viscous thread

TL;DR

This paper investigates the complex nonlinear instability patterns of a viscous fluid thread falling onto a moving surface, revealing multiple regimes, bifurcations, and a novel multifrequency state with locked frequencies.

Contribution

It introduces detailed experimental analysis of the frequency structure and bifurcations in the nonlinear instability of a viscous thread on a moving surface.

Findings

Identification of four distinct motion regimes.

Discovery of a multifrequency state with 3:2 frequency locking.

Observation of sharp bifurcations without hysteresis.

Abstract

A thread of viscous fluid falling onto a moving surface exhibits a spectacular variety of types of motion as the surface speed and nozzle height are varied. For modest nozzle heights, four clear regimes are observed. For large surface speed, the thread is dragged into a stretched centenary configuration which is confined to a plane. As the surface speed is lowered, this exhibits a supercritical bifurcation to a meandering state. At very low surface speeds, the state resembles the usual coiling motion of a viscous thread falling on a stationary surface. In between the meandering and coiling regimes, a window containing a novel multifrequency state, previously called "figures of eight" is found. Using an improved visualization technique and a fully automated apparatus, we made detailed measurements of the longitudinal and transverse motion of the thread in all these states. We found that the multifrequency state is characterized by a complex pattern of motion whose main frequencies are locked in a 3:2 ratio. This state appears and disappears with finite amplitude at sharp bifurcations without measurable hysteresis.