  We analyze theoretically the effect of friction on quartz crystal
microbalance (QCM) measurements that probe soft (viscoelastic) films and
biomolecular layers adsorbed from aqueous solutions. While water provides a
natural environment for biomolecules, an interface with unknown rheological
properties forms between the adsorbed soft molecular layer and the quartz
substrate in the latter case. We investigate therefore the dynamics of soft
films adsorbed onto a solid quartz surface within a continuum mechanics
approach using both the Maxwell and the Voight/Kelvin models of viscoelasticity
and their combination. The rigorous expressions derived for the acoustic
response of a quartz crystal oscillator, accounting for both interfacial
(sliding) friction and internal friction (viscosity), demonstrate that the QCM
can be used as a sensor for quantitative characteristization of friction
effects as well as for ``in situ'' measurements of mechanical properties of
adsorbed biomolecular films.
