  Glycerol-water mixtures were studied at molar concentrations ranging from
$x_\text{gly} = 1$ (pure glycerol) to $x_\text{gly}=0.3$ using shear mechanical
spectroscopy. We observed a low frequency mode in neat glycerol, similar to
what is usually reported for monohydroxy alcohols. This mode has no dielectric
counterpart and disappears with increased water concentration. We propose that
the hydrogen-bonded network formed between glycerol molecules is responsible
for the observed slow mode and that water acts as a plasticizer for the overall
dynamics and as a lubricant softening the hydrogen-bonding contribution to the
macroscopic viscosity of this binary system.
