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Polymer Dynamics and Rheology
Published in Richard A. Pethrick, Gennadi E. Zaikov, Teiji Tsuruta, Naoyuki Koide, Polymer Yearbook 13, 2019
The self diffusion coefficient Ds can not be measured by DLS at finite concentration, but is conveniently measured by either forced Rayleigh scattering (FRS) or pulsed field gradient NMR. K. Adachi, H. Yu et al. (3626, E1276) measured Ds of bulk polyisoprene as a function of molecular weight. The result of Ds ~ M−3 in the entangled region does not agree with the prediction of the tube model of Ds ~ M−2. Similar discrepancy have been found for semidilute and concentrated polystyrene solutions. The tube model translates entanglement dynamics, being a mathematically intractable many body problem, into a one body problem on physically reasonable grounds. The above diffusion data clearly indicate that the theory only qualitatively explains dynamical behavior of highly entangled polymeric system, and also that simple modification of the theory like the constraint release model does not lead to final solution of entanglement dynamics related to topological interaction between long polymer chains.
Diffusion and Thermodiffusion in Hydrocarbon Mixtures
Published in Devrim Balköse, Ana Cristina Faria Ribeiro, A. K. Haghi, Suresh C. Ameta, Tanmoy Chakraborty, Chemical Science and Engineering Technology, 2019
Cecília I. A. V. Santos, Valentina Shevtsova, Ana C. F. Ribeiro
In the thermodiffusion field, it’s possible to find several techniques applied in the experimental front. These embrace from the classical Soret cell,31 the thermogravitational column,32 the two-chamber thermodiffusion cell,33,34 the thermal field-flow fractionation,35 to optical methods like the microfluidic fluorescence method,36,37 laser-beam deflection technique,38,39 thermal diffusion forced Rayleigh scattering,40–42 as well as the thermal lens technique.43 Accurate measurements of both molecular and thermal diffusion coefficients for binary mixtures are obtained from optical methods with very good accuracy and reliability since no perturbation is introduced into the diffusive process. Still, many efforts are being directed to study thermodiffusion in multicomponent mixtures.44,45 The situation with ternary mixtures is more complicated as the sign of the Soret coefficients of the various components could be different, and it destabilizes the system.46–48 In this respect, orbital laboratories provide an ideal environment for the measurements due to the absence of buoyancy driven convection.
Light Scattering Studies of Microemulsion Systems
Published in Promod Kumar, K. L. Mittal, Handbook of Microemulsion Science and Technology, 2018
It is possible to create in the samples a refractive index gradient by using a light fringe pattern of spacing i. For instance, if photochromic probes are dissolved in the sample, the refractive index will vary in the light fringes. This is the principle of the forced Rayleigh scattering technique: This scattering produces a Bragg spot in the scattering angle associated with i. If the light fringes are suppressed, the refractive index gradient progressively disappears because of the diffusion of the photochromic probes. The time dependence of the intensity scattered in the Bragg direction allows determination of the diffusion coefficient. For instance, if the dye is in the microemulsion droplets (and stays there), then the diffusion coefficient measured is the droplet self-diffusion coefficient Ds [17]. Figure 2 shows the forced Rayleigh scattering determinations of Ds together with light scattering determinations of D and Ds with the same samples.
Soret separation of species in a salt solution under varying transient thermal field: an interferometric study through sensitivity analysis
Published in Experimental Heat Transfer, 2021
Yogesh M. Nimdeo, Atul Srivastava
In the context of the application of optical techniques, Mialdun et al. [14] carried out thermodiffusion studies in a binary mixture of water-isopropanol using optical digital interferometry, beam deflection technique, and thermogravitational column along with sliding symmetric tubes. The concentration and temperature dependence of mass diffusion, thermodiffusion, and Soret coefficient were studied with different techniques and compared with the same measurements carried out under microgravity conditions. Dueramae et al. [15] used a beam deflection technique for investigating the effect of hydrophobicity on thermodiffusion through an aqueous solution of acetylated dextran at varying temperatures. The author proposed that an increase in hydrophobicity can change the sign of the Soret coefficient from negative to positive. Colombani and Bert [16] recorded the changes in phase due to the evolution of concentration and temperature gradients in the fluid medium under constant thermal field using holographic interferometry. They recommended this technique for thermodiffusion as well as convection flow studies in multicomponent mixtures by conducting a single experiment on water-lithium chloride solution under temperature difference of 6°C. Polyakov and Wiegand [17] used a thermal lens and thermal diffusion forced Rayleigh scattering method to investigate thermal diffusion behavior of three binary systems including ionic and nonionic solutes in water at varying concentrations. The author preferred the use of a thermal lens method to avoid the use of dye required in Rayleigh scattering for mixtures with weak absorption capacity at a given wavelength. But the results obtained through the thermal lens technique showed large uncertainty of up to 30% at higher concentration. Hence, authors proposed that the thermal lens technique is reliable at large Soret effect and high optical contrast. Torres et al. [18] measured Soret coefficient in binary solutions and biological suspensions by developing two phase-shift interferometry set up combined with a single rotating polarizer. The authors found the method to be productive for the extraction of whole field concentration distribution and Soret coefficient measurement. But it is also observed in the literature that in two wavelength interferometry, deviation occurs in the coefficients measurement due to change in the phase at two wavelengths as well as distribution of the data over a broad timescale. The concentration and temperature gradient also differ at two wavelengths, which do not match with thermocouples-based data. Recently, Tomaru et al. [19] tried to minimize the above errors in data analysis. The authors individually measured the concentration and temperature changes at a different wavelength and determined thermodiffusion properties by matching temperature and concentration gradients at two wavelengths where thermocouples reading were used as a reference point. Still, it seems that the enhancement of accuracy in the interferometric measurement requires sufficient attention.