土壤水的密度存在较大的变化范围,但目前还没有一个完整的理论来全面描述这种变化规律。本文利用微米直径的石英管模拟多孔介质的孔隙系统,采用质量-体积法测量了直径在50~530 μm之间的8个不同直径石英管中水的密度。结果表明:当石英管直径小于75 μm时,水的密度大于体相水的密度,最高密度为1.19 g/cm³;而直径为100~250 μm时,水的密度略小于体相水,最低为0.98 g/cm³。水密度随石英管径而变化的规律可以用类似纳兰-琼斯势的经验公式来表达。研究表明:能使水密度增大或减小的水化作用、水-固界面作用、毛细作用或空化机制均不能解释石英管中出现的水密度变化。分析认为,毛管中的复杂的水动力学和流变学,特别是管嘴的剪切增稠及其逆过程可能是不同直径石英管中水密度变化的物理机制。该机制不同于解释土壤水密度变化的传统理论,为土壤水密度变化研究提供了一个崭新的视角。如果把石英管中水密度变化规律与基于多孔介质毛管束概念的土壤含水率模型结合,可以预测不同含水率土壤中水的密度。进一步的研究应该从流变学基本理论出发,建立剪切速率与黏度和黏度与密度之间的定量关系,从理论上构建毛管水和土壤水密度变化模型。

The density of soil water exhibits significant variation, yet no comprehensive theory currently exists to fully explain this pattern. In this study, quartz tubes with micrometer-scale diameters were used to simulate the pore systems of porous media, and the mass-volume method was employed to measure the density of water in eight quartz tubes with diameters ranging from 50 μm to 530 μm. The results indicate that when the diameter of the quartz tube is less than 75 μm, the density of water exceeds that of bulk water, reaching a maximum of 1.19 g/cm. Conversely, when the diameter ranges between 100 μm and 250 μm, the density of water is slightly lower than that of bulk water, with a minimum of 0.98 g/cm. The variation in water density with quartz tube diameter can be described using an empirical formula similar to the Lennard-Jones potential. The findings suggest that conventional mechanisms such as hydration effects, water-solid interfacial interactions, capillary effects, or cavitation cannot fully account for the observed changes in water density within the quartz tubes. Instead, the analysis indicates that the complex hydrodynamics and rheology within the capillaries: particularly shear thickening at the tube nozzle and its reverse process may be the primary physical mechanism driving the changes in water density across quartz tubes of different diameters. This mechanism represents a departure from traditional theories used to explain variations in soil water density and offers a novel perspective for understanding the phenomenon. It becomes possible to predict the density of water in soils with varying water contents by integrating the observed variation in water density within quartz tubes with soil water content models based on the concept of capillary bundles in porous media. Future research should focus on the fundamental principles of rheology to establish quantitative relationships between shear rate and viscosity, as well as between viscosity and density. Such efforts would enable the theoretical construction of models to describe the density variation of capillary water and soil water.