Damping and eigenfrequencies of surface capillary—gravity waves greatly depend on the boundary conditions. To the best of our knowledge, so far no direct measurement has been made of the dynamic behaviour of the contact angle at the three-phase interface (fluid—vapour—solid walls) in the presence of surface oscillation. Therefore, theoretical models of surface gravity–capillary waves involve ad hoc phenomenological assumptions as far as the behavior of the contact angle is concerned. In this paper we report a systematic experimental investigation of the static and dynamic properties of surface waves in a cylindrical container where the free surface makes a static contact angle $\theta_{\rm c} = 62^{\circ}$ with the vertical walls. The actual boundary condition relating the contact angle to the velocity of the contact line is obtained using a new stroboscopic optical method. The experimental results are compared with the theoretical expressions to be found in the literature. Two different regimes are observed: (i) a low-amplitude regime, where the contact line always remains at rest and the contact angle oscillates during the oscillation of the free surface; (ii) a higher-amplitude regime, where the contact line slides on the vertical walls. The profile, the eigenfrequency and the damping rate of the first non-axisymmetric mode of the surface gravity waves are investigated. The eigenfrequency and damping rate in regime (i) are in satisfactory agreement with the predictions of the Graham-Eagle theory (1983) of pinned-end edge conditions. The eigenfrequency and damping rate in regime (ii) show a strongly nonlinear dependence on the oscillation amplitude of the free surface. All the experimental results concerning regime (ii) can be explained in terms of the Hocking (1987 a) and Miles (1967, 1991) models of capillary damping by introducing an ‘effective’ capillary coefficient $\lambda_{\rm eft}$. This coefficient is directly obtained for the first time in our experiment from dynamic measurements on the contact line. A satisfactory agreement is found to exist between theory and experiment.
Front-surface absorbance spectra of wheat flours in the 250-650 nm region can be obtained by measuring reflectance spectra with a conventional spectrofluorometer suitably set to detect light scattered from powder samples. The spectra recorded on flour samples, obtained from seeds of four bread and five durum wheats, show high-intensity absorption bands due to aromatic amino acids of wheat proteins and low-intensity bands due to chromophores bound to low-molecular-weight compounds. The intensity of these last bands is proportional to the concentration of the corresponding chromophores present in the flour; thus, it can be used to measure the content of the compounds containing the chromophore(s). In particular, a quantitative determination of the carotenoids actually present in the flours is made, obtaining information on the original content of the seeds. This determination is important, as, for example, xanthophylls are well-known antioxidants and free-radical scavengers involved in aging processes of seeds. Reflectance measurements on powder samples are far more economic in terms of time and materials consumption than methods such as extraction and HPLC analysis of extracts and, in addition, give an evaluation of the overall content of carotenoids with absorption bands in the spectral range 450-500 nm. Application of the technique to other food powders with low-intensity absorption bands in the near-UV and vis region is possible.
A double beam spectrophotometric apparatus, based on laser light sources and on photodiode detectors, was built in order to perform accurate measurements of small concentration changes as those produced in chemical relaxation experiments. Relaxation techniques, where small and fastly decaying signals have to be monitored, call for high analytical sensitivity. To improve this figure of merit, the choice of a laser source proved to be particularly advantageous, since the very high light intensity, providing a remarkable reduction of the shot noise, allows signals of good quality to be obtained without amplification problems.The measured noise was found to be close to the calculated noise, which represents the theoretical lower limiting value. Under the best conditions the signal-to-noise ratio was improved by a factor 40 with respect to a commercial apparatus equipped with conventional light source, monochromator and photomultipliers. It is shown that the circumstances where the use of the present apparatus is specially suitable are those where the dynamic techniques become superior to the static methods in the determination of thermodynamic parameters of reacting systems.
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