Thin Films in Unsaturated Porous Media - Effects on Flow and Transport

9805409OrFlow在部分饱和的多孔介质中，即使在中等非饱和条件下(例如，重力排水介质)，也可以通过小于几微米的孔隙尺寸进行渗流。因此，实际的非饱和溶质输运模式总是涉及到靠近活性表面的薄膜中的水流动。传统的非饱和多孔介质中的液体分布、流动和溶质运移模型由于将介质孔隙空间几何表示为“柱状毛细管束”而存在偏差。这一图景被“空填充”方法进一步扭曲，在这种方法中，假设毛孔(毛细血管)完全减饱和，基于毛细管的潜力。实验和理论证据表明，对于非饱和条件，液体配置完全不同，充满液体的角落和摆动空间构成水力连接的主干，并通过覆盖暴露的固体表面的薄液体膜进一步连接。因此，与传统模型相比，界面力在传输和反应中起着更大的作用。由于界面作用力的范围可能超过膜厚，以至于可能不存在整体溶液，所以流体力学和电化学方面是密不可分的，必须同时考虑。提出了一种新的理想化的单胞，它代表了介质的孔隙空间几何形状和表面积，作为柱状毛细管束模型的替代。为了考虑膜吸附和毛细冷凝，提出了一种统一的方法来模拟多孔介质中的这两个过程，并将其应用于新假定的基本孔隙空间几何结构。界面科学形式主义被用来计算作为热力学条件、孔空间几何形状和比表面积的函数的膜厚度。新的概念框架将改进低含水率下液体形态的表示以及相关的水力和运输特性。此外，它还为考虑液体薄膜中粘性、电性和分子作用力之间的重要相互作用提供了合理的依据。对这些重要的孔隙尺度过程的理解对于在更大的感兴趣的尺度上对溶质的复杂行为进行基于物理的解释是必不可少的。薄膜流体动力学和诱导界面现象的细节，以及它们在低饱和水平下对反应溶质迁移的影响将是未来研究的主题。

9805409OrFlow in partially saturated porous media, even at moderately unsaturated conditions (e.g., gravity-drained media) takes place through pore sizes smaller than a few micrometers. Hence, practical unsaturated solute transport regimes invariably involve water flow in thin films adjacent to active surfaces. Conventional models for liquid distribution, flow, and solute transport in unsaturated porous media are biased by the geometrical representation of media pore space as a "bundle of cylindrical capillaries". The picture is further distorted by the "empty-filled" approach where a complete desaturation of pores (capillaries), based on the capillary potential, is assumed. Experimental and theoretical evidence suggests a radically different liquid configuration for unsaturated conditions whereby liquid-filled corners and pendular spaces form the backbone for hydraulic connectivity and are further connected through thin liquid films coating exposed solid surfaces. Consequently interfacial forces play a larger role in transport and reactivity than assumed by conventional models. Because the ranges of interfacial forces may exceed film thicknesses to the extent that bulk solution may not be present, hydrodynamic and electrochemical aspects are inseparable and must be considered simultaneously.A new idealized unit cell representing media pore space geometry and its surface area is proposed as an alternative to the bundle of cylindrical capillaries model. To account for film adsorption and capillary condensation, a unitary approach to modeling these two processes in porous media is proposed and applied to the newly assumed basic pore space geometry. Interface science formalism is used to calculate film thickness as a function of thermodynamic conditions, pore space geometry, and specific surface area. The new conceptual framework should improve the representation of liquid configuration at low water contents and the associated hydraulic and transport properties. In addition, it provides a rational basis for considering the important interplay among viscous, electrical and molecular forces operating within thin liquid films. The understanding of these important pore-scale processes is essential to physically-based interpretation of complicated behavior of solutes at larger scales of interest. Details of thin film hydrodynamics and induced interfacial phenomena, and their effects on reactive solute transport at low saturation levels will be a subject of future studies.