  We show the theoretical and experimental combination of acoustic and optical
methods for the in situ quantitative evaluation of the density, the viscosity
and the thickness of soft layers adsorbed on chemically tailored metal
surfaces. For the highest sensitivity and an operation in liquids, a Love mode
surface acoustic wave (SAW) sensor with a hydrophobized gold coated sensing
area is the acoustic method, while surface plasmon resonance (SPR) on the same
gold surface as the optical method is monitored simultaneously in a single
set-up for the real-time and label-free measurement of the parameters of
adsorbed soft layers, which means for layers with a predominant viscous
behavior. A general mathematical modeling in equivalent viscoelastic
transmission lines is presented to determine the correlation between
experimental SAW signal shifts and the waveguide structure including the
presence of the adsorbed layer and the supporting liquid from which it
segregates. A methodology is presented to identify from SAW and SPR simulations
the parameters representatives of the soft layer. During the absorption of a
soft layer, thickness or viscosity changes are observed in the experimental
ratio of the SAW signal attenuation to the SAW signal phase and are correlated
with the theoretical model. As application example, the simulation method is
applied to study the thermal behavior of physisorbed PNIPAAm, a polymer whose
conformation is sensitive to temperature, under a cycling variation of
temperature between 20 and 40 oC. Under the assumption of the bulk density and
the bulk refractive index of PNIPAAm, thickness and viscosity of the film are
obtained from simulations; the viscosity is correlated to the solvent content
of the physisorbed layer.
