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  • 1
    In: Water Resources Research, June 2017, Vol.53(6), pp.4709-4724
    Description: The relaxation dynamics toward a hydrostatic equilibrium after a change in phase saturation in porous media is governed by fluid reconfiguration at the pore scale. Little is known whether a hydrostatic equilibrium in which all interfaces come to rest is ever reached and which microscopic processes govern the time scales of relaxation. Here we apply fast synchrotron‐based X‐ray tomography (X‐ray CT) to measure the slow relaxation dynamics of fluid interfaces in a glass bead pack after fast drainage of the sample. The relaxation of interfaces triggers internal redistribution of fluids, reduces the surface energy stored in the fluid interfaces, and relaxes the contact angle toward the equilibrium value while the fluid topology remains unchanged. The equilibration of capillary pressures occurs in two stages: (i) a quick relaxation within seconds in which most of the pressure drop that built up during drainage is dissipated, a process that is to fast to be captured with fast X‐ray CT, and (ii) a slow relaxation with characteristic time scales of 1–4 h which manifests itself as a spontaneous imbibition process that is well described by the Washburn equation for capillary rise in porous media. The slow relaxation implies that a hydrostatic equilibrium is hardly ever attained in practice when conducting two‐phase experiments in which a flux boundary condition is changed from flow to no‐flow. Implications for experiments with pressure boundary conditions are discussed. What happens to fluids in a porous medium after pumping is stopped? Fast X‐ray tomography shows that even in a sample smaller than a sugar cube fluid interfaces continue to move for hours until an optimal fluid configuration is reached. The pace is limited by slow relaxation of dynamic contact angles. Therefore hydrostatic equilibrium, which is the state at which all fluid interfaces come to rest, is hardly ever attained in practice when conducting two‐phase flow experiments where the flow is stopped in much larger soil or rock samples. Relaxation dynamics through internal redistribution of fluids after fast drainage occurs in two stages A quick dissipation within seconds is followed by slow relaxation within several hours due to relaxation of dynamic contact angles Fluid configurations during relaxation are very different from those during quasi‐static drainage and imbibition
    Keywords: Two‐Phase Flow ; Dynamic Effects ; Hydraulic Nonequilibrium ; Dynamic Contact Angle ; Fluid Configuration ; Fluid Topology
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 2
    Language: English
    In: Soil Science Society of America Journal, 2013, Vol.77(2), p.403
    Description: The influence of clay content in soil-pore structure development and the relative importance of macroporosity in governing convective fluid flow are two key challenges toward better understanding and quantifying soil ecosystem functions. In this study, soil physical measurements (soil-water retention and air permeability) and x-ray computed tomography (CT) scanning were combined and used from two scales on intact soil columns (100 and 580 cm super(3)). The columns were sampled along a natural clay gradient at six locations (L1, L2, L3, L4, L5 and L6 with 0.11, 0.16, 0.21, 0.32, 0.38 and 0.46 kg kg super(-1) clay content, respectively) at a field site in Lerbjerg, Denmark. The water-holding capacity of soils markedly increased with increasing soil clay content, while significantly higher air permeability was observed for the L1 to L3 soils than for the L4 to L6 soils. Higher air permeability values observed for 580- than 100-cm super(3) soil columns implied a scale effect and relatively greater importance of macropores in convective fluid flow at larger scale. Supporting this, x-ray CT showed that both interaggregate pores and biopores (pores formed by earthworms and plant roots) were present at L1 to L3 in decreasing order, whereas only interaggre- gate pores were observed at L4 to L6. Macroporosity inferred from x-ray CT to quantify pores 1 mm decreased from 2.9 to 0.1 % from L1 to L6. A progressive improvement was observed in the linear relationship (R super(2) increasing 0.50-0.95) of air permeability with total air-filled porosity, CT-inferred macroporosity, and CT-inferred limiting macroporosity (minimum macroporosity for any quarter of soil column). The findings of this study show the immense potential in linking x-ray CT-derived soil-pore parameters with classical soil physical measurements for quantifying soil architecture and functions. [PUBLICATION]
    Keywords: Soil ; Permeability ; Earthworms ; Soil Structure ; Clay ; Porosity ; Computed Tomography ; Denmark ; Air Pollution;
    ISSN: Soil Science Society of America Journal
    E-ISSN: 0361-5995
    E-ISSN: 14350661
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