Use of combined shear and pressure acoustic waves to study interfacial and bulk viscoelastic effects in aqueous polymeric gels and the influence of electrode potentials

AT-cut quartz crystals operating in the thickness-shear mode at 10 MHz have been used to study the viscoelastic changes occurring during the polymerization and gelling of acrylamide solutions. The gelling process is described in terms of the storage and loss moduli, G′ and G″, or in terms of the shear wave velocity and the attenuation coefficient. The solutions are quite elastic (G′≈G″) and both moduli increase with gelling. These effects are attributed to hydrogen bonding. The attenuation coefficient decreases abruptly at the onset of gelling. The effective penetration of the shear wave increases from 2.6 µm in solution to 3.4 µm in the gel. The moduli are greatly influenced by the application of a small bias voltage across the gel and this is related to the injection of ions into the shear wave sensing region. Abrupt switching of the bias induces large transient changes in the moduli that are attributed to Lippmann electrocapillary effects at the gel/crystal electrode interface. It is also demonstrated that the crystal produces longitudinal pressure waves, which propagate through the gel and produce secondary resonances. They have a marked effect on the interpretation of the shear wave impedance and can be used to monitor changes in bulk properties through the gelling process.