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Anomalous Behavior of Ultra-Low-Amplitude Capillary Waves. A Glimpse of the Viscoelastic Properties of Interfacial Water?
journal contribution
posted on 2017-05-18, 00:00 authored by Antonio Raudino, Domenica Raciti, Mario CortiWe
investigate, both theoretically and by a differential interferometric
technique, the behavior of large-wavelength capillary waves (of the
order of 10–4 m) selectively excited at the surface
of drops and bubbles with typical eigenfrequencies of the order of
102 Hz. The resonance peaks of gas bubbles or hydrocarbon
drops in water (radius less than 1 mm) highlight anomalously small
dissipation in the region of ultralow (sub-nanometric) oscillation
amplitudes, reaching a plateau at higher amplitudes. This is in sharp
contrast to the usual oscillating systems, where an anomalous behavior
holds at large amplitudes alone. Dissipation is strongly dependent
on the excited vibrational modes and, in spite of remarkable numerical
differences, water-vapor and water-hydrocarbon interfaces exhibit
the same overall trend. A phenomenological model was developed, based
on the assumption that water possesses a threshold viscoelasticity,
above which it behaves like a regular viscous fluid. The well-known Deborah number was then estimated within the anomalous region
and found to lie in the range of viscoelastic fluids. In agreement
with previous studies of nanohydrodynamics (e.g., atomic force microscopy
measurements with sub-nanometric tip motions), the present one lends
support to the idea that every self-aggregating fluid exhibits yield
stress behavior, including classical Newtonian fluids like water.
The essential requirement is that the applied perturbation lie below
a critical threshold, above which viscous behavior is recovered. Our
differential interferometric technique seems particularly suitable
for this type of studies, as it allows measurement of long-wavelength
capillary waves with sub-nanometric resolution on the oscillation
amplitudes.