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Provedor de dados:  ArchiMer
País:  France
Título:  Bulk-phase viscoelastic properties of seawater
Autores:  Jenkinson, Ian
Data:  1993
Ano:  1993
Palavras-chave:  RHEOLOGY
Resumo:  Deformation and mixing in any medium are controlled by its viscosity and elasticity. The present study provides preliminary information, at oceanic shear rates, on the viscosity and elasticity in the bulk phase of seawater. Thirty-two samples of seawater, obtained on cruises at four times of year in a meso-oligotrophic part of the Mediterranean, were measured for bulk-phase viscoelastic properties in an oscillating-shear Couette flow with measuring gap 0.5 mm, over shear rates gamma from 0.0021 to 0.286 s-1. Fifteen samples were obtained on a single cruise in the German Bight, North Sea, when Phaeocystis blooms were taking place, and these were similarly measured at gamma from 0.0021 to 0.973 s-1. The bulk-phase measurements suffered interference contributed by the surface film. The interference mechanism was investigated, and a method was developed to correct for it, which permitted extraction of the bulk-phase viscosity. While extraction of bulk-phase elastic effects was possible only in the most thickened Mediterranean samples, in the generally thicker North Sea samples bulk-phase elasticity was closely related to bulk-phase excess (polymeric) viscosity. At gamma = 0.0021 s-1, viscosity, eta, in the Mediterranean samples ranged from 0.17 to 19 (mean 3.5, n = 23) times the average solution viscosity, eta(w)BAR which is gamma-independent and contributed principally by water and salt. Corresponding values for the German Bight samples were 0.99 to 127 (mean 52, n = 5) times eta(w)BAR. For each cruise, the overall excess viscosity, eta(E) = eta - eta(w), showed a power-law relationship with gamma such that eta(E) = k.gamma(-P). For the five different cruises, P varied from 1.1 to 1.5. Such high values of P indicate that thickening is contributed principally by cross-linked polymer gel rather than by overlapping chains. The elastic modulus G' was very variable, with sample maxima of 100 and 300 muPa in the Mediterranean and North Seas respectively. Thickening (determined as both eta(E) and G') was heterogeneous, with a variability coefficient (SD/mean) from 1 to 3 for different cruises and values of gamma. In the three Mediterranean cruises in which duplicate measurements were made for the same samples, all the variability could be accounted for by in-sample variability, suggesting that centimetre-scale flocculation-type processes were responsible. Gaussian standard deviation SD(G) was constant (indicating self-similarity) for the higher values of eta(E) both the Mediterranean and North Sea studies a well as for G' in the North Sea study. At the lower end of the eta(E) and G' distributions, SD(G) was higher, due to experimental variability. Over a range of appropriate shear rates, the turbulent (Kolmogorov) length scale L was calculated using the mean viscosities measured. The calculations suggest that, when turbulence is low or biological activity high, L is considerably higher than values previously supposed, and mixing correspondingly reduced. The heterogeneous nature of both eta(E) and G' must furthermore increase intermittence in both turbulence and mixing. It is concluded that a significant part of the sea functions as a lumpy, biopolymeric gel in which small-scale (less-than-or-equal-to 100.L) flow and mixing are under strong biological influence.
Tipo:  Text
Idioma:  Inglês
Editor:  Gauthier-Villars
Formato:  application/pdf
Fonte:  Oceanologica Acta (0399-1784) (Gauthier-Villars), 1993 , Vol. 16 , N. 4 , P. 317-334

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