No abstract

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 GA, or in terms of the shear wave velocity and the attenuation coefficient. The solutions are quite elastic (G@ B GA) and both moduli increase with gelling. These e †ects are attributed to hydrogen bonding. The attenuation coefficient decreases abruptly at the onset of gelling. The e †ective penetration of the shear wave increases from 2.6 lm in solution to 3.4 lm in the gel.The moduli are greatly inÑuenced 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 e †ects 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 e †ect on the interpretation of the shear wave impedance and can be used to monitor changes in bulk properties through the gelling process.

SynopsisThe optical path difference of nitrocellulose fibers and pastes were measured on the polarizing microscope using three different compensation techniques, and a method is suggested for converting these path differences into birefringencies. By taking measurements on a sufficiently large number of fibers it is possible to estimate the average nitrogen content with an accuracy approaching that achieved by other methods on much larger samples. The distribution of nitrogen between individual fibers shows that material prepared by the displacement process differs from that made by the mechanical process and indicates that denitration occurs during displacement process manufacture. Solvents and plasticizers either increase or decrease the birefringence, and the structure of nitrocellulose pastes is discussed.

It is demonstrated that an AT-cut quartz crystal driven in the thickness-shear-wave mode and typically used as a sensor to monitor the viscoelastic shear-wave properties of a fluid also produce longitudinal pressure waves. Unlike the shear wave, these waves are capable of long-range propagation through the fluid and of reflection at its boundaries, notably at an outer fluid-air interface. They introduce a component into the measured electrical impedance and resonance frequency shift of the crystal, which reflects the setting up of cyclic pressure-wave resonances in the fluid. This has important implications for the practical employment of these crystal as sensors. Under appropriate conditions, as demonstrated for water and n-octane, it is possible to determine the propagating properties of sound waves in a fluid simultaneously with the viscoelastic shear-wave properties. These experiments are supported by an analysis of the appropriate hydrodynamic equations for waves in the crystal-fluid system, which predicts electrical characteristics in close agreement with those found experimentally.

New measurements on ultrasound absorption of aqueous solutions of the three globular proteins, bovine hemoglobin, horse heart myoglobin, and bovine serum albumin, at 20 degrees C are reported for the frequency range below 1.2 MHz and, in the case of hemoglobin and bovine serum albumin, to a limited extent for the range 2-30 MHz. The effect of protein conformation has been investigated by use of a range of denaturants and by change of pH. A much more detailed description of protein interactions emerges from the low-frequency studies than was possible hitherto. Specific absorption processes for myoglobin and bovine serum albumin peak in the neighborhood of pH 4.2. The process for myoglobin has a relaxation frequency between 1 and 2.6 MHz, while for bovine serum albumin the frequency is as low as 70 kHz. Both processes can be related to conformational changes in the molecules. In the case of hemoglobin, an absorption process with a relaxation frequency of 600 kHz is found that can be attributed to the dissociation equilibrium of the quaternary structure of the molecule and can be considerably enhanced by denaturants or pH change from neutral to acid conditions. Measurements at frequencies down to 200 kHz have also permitted a more thorough investigation of proton-transfer reactions at carboxyl and amino groups in these proteins and of the effect of pH on these reactions.