Are standard values the best choice? A critical statement on rheological soil fluid properties viscosity and surface tension

The rheological properties of a fluid in terms of viscosity eta and surface tension gamma are often neglected in studies on hydrostatic and hydrodynamic phenomena. However, their measurement seems to provide us with more benefit than it takes time and effort. Especially regarding transport of fluids in soil, the exact definition of the fluid properties is necessary for predicting water transport correctly. In this paper we review and summarize the available scientific knowledge on rheological soil fluid properties and supplement it with our own investigations on viscosity and surface tension of salt and soil solutions. Our results showed neither a clear linear relationship between eta and salt concentration in aqueous solutions nor to the kind of cation or anion of the dissolved salt whereas the surface tension of salt solutions generally increased linearly with molar concentration though at different rates. As an example, a 1 M MgSO4 solution doubled eta but increased gamma only by 3%, whereas 1 M MgCl2 caused 4% increase in gamma, but still 50% increase in eta. Viscosity of soil solutions depended on soil-water ratio as well as fertilization of the soil. The largest deviation to standard eta of water was about 6% at 40% gravimetric water content. Concluding from the literature review and our own findings, we recommend to additionally measure rheology (namely viscosity and surface tension) of the soil solution in order to improve modeling of hydrostatic and hydrodynamic phenomena in soils by utilizing more realistic fluid properties which might deviate significantly from the usually employed standard values. The mathematical error cumulates with increasing salt concentration as a deviation of 10% to the standard value also causes an error of 10% in linear relationships derived from that parameter, e.g. the equation of Hagen-Poiseuille. Future investigations should focus on the manifold single effects as well as on the interactions of different dissolved components in the soil solution on viscosity and surface tension and how these are influenced by temperature, pressure (i.e. shear rate) and concentration. (c) 2012 Elsevier B.V. All rights reserved.