High-speed wave-mixing laser Doppler imaging in vivo

An interferometric method for parallel optical spectroscopy in the kilohertz range is reported, as well as its experimental validation in the context of high-speed laser Doppler imaging in vivo. The interferometric approach enables imaging in the low light conditions of a 2 kHz frame rate recording with a complementary metal-oxide semiconductor camera. Observation of mice craniums with near-infrared (lambda=785 nm) laser light in reflection configuration is reported. Doppler spectral images allegedly sensitive to blood flow are sequentially measured at several optical frequency detunings, to shift the spectral range of analysis in the radio-frequency spectrum.     

Nondestructive investigation of the thixotropic recovery of unvulcanized heavily filled rubber

A dynamic method has been used to estimate the thixotropic change in the viscoelastic properties of materials. The thixotropic recovery of the structure of a heavily filled rubber, destroyed by processing under stationary conditions, has been investigated. It is found that the relaxation spectrum changes during the recovery process.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 561–564, May–June, 1969.     

Rheology and molecular structure of a polyethylene melt. 3. Relaxation spectra and characteristic relaxation time

By comparing the relaxation spectra respectively calculated on the basis of the iterative and approximation methods, it is confirmed that the latter may be used to describe the viscoelastic properties of polyethylene melts. A method is proposed for identifying the form of the frequency relaxation spectrum by three characteristic numerical quantities. The effect of the molecular-mass, the molecular-mass distribution, and the long-chain branching characteristics on the form of the frequency relaxation spectrum is considered. The question of the geometrical similarity of the relaxation spectrum is discussed, and it is shown that, in general, these spectra are not linearly similar. The asymmetry of the spectrum depends solely on the index representing the rate of fall of the dynamic viscosity with increasing frequency of deformation. Methods of determining the characteristic relaxation time without constructing a frequency relaxation spectrum are proposed; ways of determining the characteristic relaxation time from experimental data are indicated.For communication 2, see [10].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 322–330, March–April, 1976.     

Studying the longitudinal flow of polymers with periodic deformation

Conclusions 1. At low rates of elongation the state of a polymer melt depends neither on the strain nor on the strain rate. The frequency dependence of the components of the complex longitudinal modulus, measured by superposition of a periodic deformation on a steady longitudinal flow, resemble, at low elongation rates, the frequency characteristics of the components of the complex shear modulus.2. Relaxation processes due to steady longitudinal flow affect the components of the complex longitudinal modulus measured by periodic deformation.3. A melt of a polydisperse polymer under conditions of longitudinal flow begins, at some definite strain rate, to respond to periodic deformation as a rubber-like body and continues to do so as the amount of elongation increases, which is manifested by the trend of the frequency dependence of the modulus component E', this trend becoming similar to that for a solid body.4. As the strain increases during elongation, the upper boundary of the flow range, which is determined by the periodic deformation, shifts toward low frequencies.5. The viscoelasticity characteristics of a material subject to elongation can be accurately enough described by nonlinear equations with the aid of the linear shear-relaxation spectrum.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 507–513, May–June, 1978.     

NIRS and Aquaphotomics Trace Robusta-to-Arabica Ratio in Liquid Coffee Blends

Coffee is both a vastly consumed beverage and a chemically complex matrix. For a long time, an arduous chemical analysis was necessary to resolve coffee authentication issues. Despite their demonstrated efficacy, such techniques tend to rely on reference methods or resort to elaborate extraction steps. Near infrared spectroscopy (NIRS) and the aquaphotomics approach, on the other hand, reportedly offer a rapid, reliable, and holistic compositional overview of varying analytes but with little focus on low concentration mixtures of Robusta-to-Arabica coffee. Our study aimed for a comparative assessment of ground coffee adulteration using NIRS and liquid coffee adulteration using the aquaphotomics approach. The aim was to demonstrate the potential of monitoring ground and liquid coffee quality as they are commercially the most available coffee forms. Chemometrics spectra analysis proved capable of distinguishing between the studied samples and efficiently estimating the added Robusta concentrations. An accuracy of 100% was obtained for the varietal discrimination of pure Arabica and Robusta, both in ground and liquid form. Robusta-to-Arabica ratio was predicted with R²CV values of 0.99 and 0.9 in ground and liquid form respectively. Aquagrams results accentuated the peculiarities of the two coffee varieties and their respective blends by designating different water conformations depending on the coffee variety and assigning a particular water absorption spectral pattern (WASP) depending on the blending ratio. Marked spectral features attributed to high hydrogen bonded water characterized Arabica-rich coffee, while those with the higher Robusta content showed an abundance of free water structures. Collectively, the obtained results ascertain the adequacy of NIRS and aquaphotomics as promising alternative tools for the authentication of liquid coffee that can correlate the water-related fingerprint to the Robusta-to-Arabica ratio.     

Rheology and molecular structure of polyethylene melts. 2. Effect of molecular structure on the viscoelastic characteristics

The viscoelasticity of low-density polyethylene is investigated as a function of molecular structure. The viscoelasticity is characterized by three numbers — the zero-shear viscosity, the characteristic relaxation time, and the index of the rate of fall of dynamic viscosity with deformation frequency. The molecular structure is also characterized by three numbers — , , g_w. The zero-shear viscosity is shown to depend on the polydispersity of the molecules and a corresponding analytic expression is obtained. The method of determining the characteristic relaxation time from the maximum of the frequency relaxation spectrum is justified and an expression giving the characteristic relaxation time as a function of the principal characteristics of the molecular structure is derived. It is shown that long-chain branching of the molecules has an incommensurably greater (several orders) effect on the rheological characteristics of the branched polymer than the polydispersity of the molecular weight. The effect of the polydispersity of the molecules of commercial low-density polyethylenes on the viscoelastic characteristics cannot be established without taking the branching of the molecules into consideration.For communication 1 see [1].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 120–127, January–February, 1976.     

A contribution to the sintering model of viscoelastic liquid drops with solid kernels

A model for filled polymer sintering has been proposed. In this model the sintering process is reduced to coalescence of two viscoelastic drops with solid kernels. The mathematical calculations are based on the Ostwald empirical relation and the previous models for sintering of pure polymers. The results show that in the first approximation for calculations of filled polymer sintering we can use the same methods as for unfilled ones.Institute of Polymer Mechanics, Latvian Academy of Sciences, Riga, LV-1006 Latvia. Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 4, pp. 564–569, July–August, 1997.     

Asymptotics of a time correlation function in multiple recurrent scattering of scalar waves

We consider an integral that recently showed up in the calculation of the time-dependent field-field correlation function of the electric field inside polarizable (dielectric) particles. We derive new integral representations on which numerical algorithms can be based and which give information on the asymptotic behaviour for large values of a time parameter. We interpret the results of the paper in terms of the physical problem.