MHz Ultrasound Induced Roughness of Fluid Interfaces

Rym Boubekri (1) , Michel Gross (1) , Martin In (1) , Olivier Diat (2) , Maurizio Nobili (1) , Helmuth Moehwald (3) , Antonio Stocco (1)

1 L2C - Laboratoire Charles Coulomb
2 L2IA - Ions aux Interfaces Actives
3 Max Planck Institute of Colloids and Interfaces

The interface between two fluids is never flat at the nanoscale, and this is important for transport across interfaces. In absence of any external field, the surface roughness is due to thermally excited capillary waves possessing subnanometric amplitudes in the case of simple liquids. Here, we investigate the effect of ultrasound on the surface roughness of liquid-gas and liquid-liquid interfaces. MHz frequency ultrasound was applied normal to the interface at relatively low ultrasonic pressures (< 0.6 MPa), and the amplitudes of surface fluctuations have been measured by light reflectivity and ellipsometry. We found a dramatic enhancement of surface roughness, roughly linear with intensity, with vertical displacements of the interface as high as 50-100 nm. As a consequence, the effective contact area between two fluids can be increased by ultrasound. This result has a clear impact for enhancing interface based processes such as mass or heat transfer.

Domaines: Matière Molle [cond-mat.soft]; Chimie-Physique [physics.chem-ph]

Rym Boubekri, Michel Gross, Martin In, Olivier Diat, Maurizio Nobili, et al.. MHz Ultrasound Induced Roughness of Fluid Interfaces. Langmuir, 2016, 32 (40), pp.10177-10183. ⟨10.1021/acs.langmuir.6b02167⟩. ⟨hal-01397537⟩