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What Do Drops Do? Surface Wetting and Network Geometry Effects on Vadose-Zone Fracture Flow
Pages 243-270

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From page 243...
... Does ABSTRACT Capillary conceptual models of vadose zone flow predict that large pores and open fractures should transmit water only when the rock as a whole approaches saturation. Observations, such as bomb-pulse radionuclides at Yucca Mountain, indicate more rapid transport in fractures at lower overall saturation values than capillary theory would suggest.
From page 244...
... The dominant conceptual model for vadose zone flow can be called the capillary conceptual model. The capillary conceptual model of flow holds that water flows in pore spaces that contain continuous films held in place by capillary tension (Richards, 1931~.
From page 245...
... Pruess (1999) defines the fundamental paradox of unsaturated fracture flow: how can fast flow occur in the presence of the strong matrix imbibitions of partially saturated rock?
From page 246...
... (d) FIGURE 8-1 Conceptual models of vadose fracture flow processes: (a)
From page 247...
... as yet another alternative conceptual model. Another difference between drop flows and film flows is the influence of wetting properties.
From page 248...
... This is an important starting point, as wetting properties and contact angles control, in part, whether liquids spread in films in zero contact angle systems, or isolate themselves into drops in systems with nonzero contact angles. Measurements of contact angles on quartz and on materials commonly used for laboratory simulation of fracture flow, such as glass or epoxy, are not zero.
From page 249...
... Depending on those properties, one may expect a range of processes that are important for vadose zone flow. Wetting is classically viewed in terms of surface energies that are associated with interfaces between liquids, solids, and gases.
From page 250...
... Some authors use "wetting" to refer to any system where there is adhesion, that is, contact angles less than 180° (Padday, 1993) , while others, including Richards (1931)
From page 251...
... The zero contact angle thus provides a separator between regimes where film flows will dominate, from those where drops will be more prevalent at lower saturations. Contact Angles for Geologic Materials and Surface Roughness Effects The previous section reviewed the fundamentals of wetting, spreading, and adhesion.
From page 252...
... (2000) determined dynamic contact angles on quartz capillaries and found that contact angles varied between approximately 30° and 70° depending on capillary radius and flow velocity.
From page 253...
... FLUID MOVEMENT ON SINGLE SURFACES WITH NONZERO CONTACT ANGLES As discussed above, the energy relationships between the liquid and the surface influence the fundamental flow processes on that surface. Current flow theories assume continuous adsorbed water films and a contact angle of zero.
From page 254...
... , and the difference between these values is the contact angle hysteresis. Or ~ \ FIGURE 8-3 Advancing and receding contact angles for drops.
From page 255...
... Equation 8.8 shows that the resistance to sliding is based on the width of the drop and not on its total contact area with the surface. If two drops have the same Drops Slide When Mass Drop Sliding Exceeds Sliding Criterion Direction AL ~ )
From page 256...
... showed that the contact angle hysteresis (the difference between the cosines of the advancing and receding contact angles) varies with drop shape, and the retentive forces could vary with the drop length.
From page 257...
... FIGURE 8-6 Critical drop radius for sliding between two parallel plates.
From page 258...
... These analyses suggest that the heights of critical drops on single surfaces will be on the order of a millimeter or less. Hence, we may expect that singlewalled drop flow will occur for fractures larger than 1 mm in aperture, and twowalled flow will occur in fractures with less than 0.5 mm aperture for contact angles consistent with air, water, and quartz or glass.
From page 259...
... . In terms of boundary conditions, drop flows are best described by flow rate or flux boundary conditions, as head boundary conditions do not apply.
From page 260...
... report fracture flow experiments using epoxy replicas of natural fractures. The epoxy is reported to have a contact angle of 20° on flat surfaces.
From page 261...
... In some cases, drops fell freely without touching the fracture walls. Film flows occurred when the aperture of the fracture was larger than the film thickness and may have also transported fluid to the saturated islands.
From page 262...
... Water Resources Research 35: 1019-1037. Copyright by American Geological Union.
From page 263...
... One observation in reviewing experiments is the importance of measuring and reporting the contact angles of the fluids and the surfaces that are used. FRACTURE NETWORK GEOMETRY AND VADOSE FLOW As mentioned in the introduction, the occurrence of fast flow in fractures in the presence of strong matrix imbibition is a major paradox in unsaturated fracture flow systems.
From page 264...
... If we assume that the driving forces for flow are dominantly gravitational, and flowis in the dip direction of fractures, we can develop several mechanisms for flow localization arising from the geometries of fracture networks. These include flow diversion, flow focusing, and flow connectivity.
From page 265...
... Fracture intersections in sparsely fractured rock also may serve as focusing agents (Figure 8-10~. Two other conceptual models involving focused flow in fracture networks deserve mention.
From page 266...
... combined the "weeps" approach with the results of dual-permeability modeling to arrive at the wetted and active portions of a fracture network at Yucca Mountain. They started with the fracture-matrix reduction factor that comes out of a dual-permeability model that had been calibrated to matrix saturation and geochemical data.
From page 267...
... SUMMARY AND CONCLUSIONS The observation of fast flow in vadose fracture networks encourages a reexamination of the capillary model of vadose-zone flow for fractures. One important aspect of capillary flow models is the assumption that flow ceases when the saturation drops to the point where the liquid no longer forms a continuous phase.
From page 268...
... Laboratory experiments on simulated fractures show drop and rivulet effects. These effects may arise from the use of modeling materials that have nonzero contact angles with the water.
From page 269...
... Estimating the consequences of significant fracture flow at Yucca Mountain. High Level Radioactive Waste Management, Proceedings of the Third Annual International Conference, American Nuclear Society, pp.
From page 270...
... Transient film flow on rough fracture surfaces. Water Resources Research 36: 1737-1746.


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