Effect of hydrogel particle additives on water-accessible pore structure of sandy soils: A custom pressure plate apparatus and capillary bundle model

TL;DR

This study introduces a novel pressure plate method and capillary bundle model to analyze how hydrogel additives alter the pore structure and water accessibility in sandy soils, revealing exponential decreases in accessible pore area with increased gel concentration.

Contribution

The paper presents a new experimental approach and model to quantify water-accessible pore distributions and demonstrates how hydrogel particles modify pore size and accessibility in sandy soils.

Findings

Water expelled depends on sample height and matches the capillary bundle model.

Pore size distribution is approximately log-normal with a total accessible area fraction of 0.34.

Hydrogel additives reduce accessible pore space exponentially and clog large pores while creating small pores.

Abstract

To probe the effects of hydrogel particle additives on the water-accessible pore structure of sandy soils, we introduce a custom pressure plate method in which the volume of water expelled from a wet granular packing is measured as a function of applied pressure. Using a capillary bundle model, we show that the differential change in retained water per pressure increment is directly related to the cumulative cross-sectional area distribution $f(r)$ of the water-accessible pores with radii less than $r$. This is validated by measurements of water expelled from a model sandy soil composed of 2 mm diameter glass beads. In particular, the expelled water is found to depend dramatically on sample height and that analysis using the capillary bundle model gives the same pore size distribution for all samples. The distribution is found to be approximately log-normal, and the total cross-sectional area fraction of the accessible pore space is found to be $f_0=0.34$. We then report on how the pore distribution and total water-accessible area fraction are affected by superabsorbent hydrogel particle additives, uniformly mixed into a fixed-height sample at varying concentrations. Under both fixed volume and free swelling conditions, the total area fraction of water-accessible pore space in a packing decreases exponentially as the gel concentration increases. The size distribution of the pores is significantly modified by the swollen hydrogel particles, such that large pores are clogged while small pores are formed.