We have examined the effect of surface contamination on the attenuation and
stiffness of compressional seismic waves in artificial cylindrical glass c
racks that are partially saturated with water. The compression of the gap p
erpendicularly to its plane reduces the gap volume and forces the water to
redistribute within the gap (conservation of volume of an incompressible li
quid). On clean surfaces, the water can flow without significant resistance
across the glass. This leads to a very low and almost constant attenuation
over a wide frequency range (approx. 3 mHz to 10 Hz), while the sample sti
ffness is constant. In the case of propanol contaminated surfaces, both the
attenuation and the stiffness are considerably higher than in the clean ca
se, and display a considerable frequency dependence. Both effects can be ex
plained with the Restricted Meniscus Motion Model. In this model, the redis
tribution of the liquid in the gap first leads to a change (increase) of th
e contact angle. The change of the meniscus curvature results in an increas
e of the pressure in the liquid and thus to a stiffening of the sample. Whe
n the resistive force, that prevents the contact line from sliding along th
e surface, is finally overcome, the contact line starts moving across the c
ontaminated surface. The motion against the resistive force dissipates ener
gy and increases the attenuation. The calculated data are in good agreement
for both the clean and the contaminated case; the model parameters fall in
the range that was established by independent experiments (e.g. WAITE et a
l., 1997).