[Paper Review] Nonspherical armoured bubble vibration

Gaurav Prabhudesai, Ilyesse Bihi+3 more|LillOA (Université de Lille (University Of Lille))|Jan 19, 2017

Particle Dynamics in Fluid Flows2 references374 citations

TL;DR

The paper experimentally demonstrates that cylindrical armoured bubbles transition stepwise to spherical shapes under vertical vibrations, releasing surface-bound particles, with the transition being frequency-dependent and metastable in nonspherical form. It explains energy considerations and shows that particle dissemination is more power-efficient in the nonspherical state, especially near resonance.

ABSTRACT

In this paper, we study the dynamics of cylindrical armoured bubbles excited by mechanical vibrations. A step by step transition from cylindrical to spherical shape is reported as the intensity of the vibration is increased, leading to a reduction of the bubble surface and a dissemination of the excess particles. We demonstrate through energy balance that nonspherical armoured bubbles constitute a metastable state. The vibration instills the activation energy necessary for the bubble to return to its least energetic stable state: a spherical armoured bubble. At this point, particle desorption can only be achieved through higher amplitude of excitation required to overcome capillary retention forces. Nonspherical armoured bubbles open perspectives for tailored localized particle dissemination with limited excitation power.

Motivation & Objective

• Investigate how cylindrical armoured bubbles respond to vertical mechanical vibrations.
• Characterize shape evolution from cylindrical to spherical with increasing vibration amplitude.
• Understand how vibration facilitates particle release from the bubble surface.
• Develop an energy-based explanation for the metastable nonspherical state and transition to spherical shape.

Proposed method

• Produce cylindrical armoured bubbles with tailored radius and length using a capillary-tube method.
• Excite bubbles with a vertical vibration exciter and measure amplitude and frequency.
• Record bubble shape evolution with high-resolution imaging and quantify using the roundness descriptor Ro.
• Analyze how Ro changes with excitation amplitude at different frequencies.
• Use energy balance arguments to compare interfacial energies in spherical vs nonspherical configurations.

Experimental results

Research questions

• RQ1 How does the shape of armoured bubbles evolve under increasing vibration amplitude and varying frequency?
• RQ2 Is the nonspherical armoured bubble a metastable state, and what energy mechanisms drive its transition to a spherical state?
• RQ3 What are the conditions under which particle desorption occurs from armoured bubbles, and how do these differ between nonspherical and spherical shapes?
• RQ4 How does excitation frequency influence the threshold amplitude for shape transition and particle release?
• RQ5 Can smaller bubbles and particles alter the required energy for particle dissemination as predicted by scaling analyses?

Key findings

• Nonspherical armoured bubbles transition stepwise to spherical shapes as vibration amplitude increases, leading to reduced surface area and particle release.
• The critical amplitude for transition depends strongly on excitation frequency and is lowest near the Minnaert resonance.
• Nonspherical bubbles are metastable since converting to a sphere lowers interfacial energy, but the transition is enabled by vibration and kinetic energy release.
• Particle desorption from spherical bubbles requires overcoming capillary retention forces, with higher excitation power needed for massive release.
• A simple energy balance links migration energy ΔE to surface energies, explaining stability and desorption behavior; scaling suggests smaller bubbles/particles require higher relative oscillation amplitudes for release.