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The heat generated during wildfires modifies soil characteristics, including soil water repellency (SWR) and the water stability of aggregates, which are known to be interrelated. SWR lowers the rate of water entry into aggregates, minimizing aggregate disruption and subsequent erosion. This study aimed to examine these aggregate characteristics (SWR, water stability of aggregates) of thermally heated water-repellent soil aggregates under laboratory conditions. Water-repellent aggregates were collected from Eucalyptus grandis forest soil separately from four soil depths (0–5, 5–10, 10–15, and 15–20 cm) with varying initial repellency levels. Using an automated programmable muffle furnace, aggregates were separately exposed to three heating temperatures, TH (150, 200, 250 °C), three rates of heating (speed of rising temperature to reach relevant TH), RH (200, 400, 800 °C h−1), and three durations of exposure to relevant TH, ED (30, 60, 120 min). The molarity of an ethanol droplet test was used to measure the contact angle (contact angle>90°). The water drop penetration time (WDPT) was also measured. The SWR of aggregates declined with the increasing TH and ED. All aggregates were wettable once exposed to 250 °C. At the lowest TH and ED (150 °C, 30 min), the contact angle was <90° only in the least repellent aggregates collected from 10–15 and 15–20 cm depths. Although RH indicated the least influence on the measured parameters, the slowest RH (200 °C h−1) caused a comparatively greater decline in SWR. Water stability of aggregates increased with heating irrespective of decreasing SWR. Further investigations on heat-induced changes in organic compounds at molecular levels would be necessary to understand the theories for the behavior of aggregates. [ABSTRACT FROM AUTHOR]

Clay is an important factor that affects soil water repellency. The purpose of the present study was to assess the effects of clay content on contact angle and water drop penetration time (WDPT), and to examine their relationship. Model soils were prepared with silica sand and kaolinite, and hydrophobized using stearic acid (SA). The effect of kaolinite on the contact angle changed with the hydrophobic organic matter content of the model soils. The contact angles of organic matter free samples increased with increasing kaolinite content. This was explained by a decrease in the surface-free energy of kaolinite with the adsorption of water at 57% relative humidity. In samples with SA content above 1.6 g kg−1, the contact angle decreased with increasing kaolinite content because the hydrophobizing effect of SA was lower at higher kaolinite content owing to the lower extent of hydrophobic organic coatings. Kaolinite increased the WDPT in non-repellent soils, but decreased the WDPT in slightly to severely repellent soils. Clay effects on the contact angle and WDPT were not comparable and, consequently, the relationship between contact angle and WDPT was found to be non-uniform at different clay contents. [ABSTRACT FROM AUTHOR]

Soil water repellency is a transient soil property varying with soil–water contact time. The purpose of the present study was to determine the time dependence of the sessile drop contact angle and its relation to repellency persistence estimated using the water drop penetration time (WDPT) test with hydrophobized sand. The contact angle decreased exponentially and almost reached apparent equilibrium after 20 min of soil–water contact time. Time dependence of the contact angle can mainly be attributed to the adsorption of water molecules onto low-energy hydrophobic organic matter surfaces. Contact angles initially greater than 90° decreased to less than 90° within about 40 s. However, the WDPT of these samples was longer than 3600 s. The WDPT responded to the initial contact angle, but not to the contact angle decreased with soil–water contact time. This was considered to be caused by differences in the surface free energy between the surface and the lower layers. Repellency persistence, or the WDPT, can be considered to be the time taken to increase the surface free energy to overcome water repellency, not only on the surface in contact with the droplet, but also in the adjacent layers below the surface. [ABSTRACT FROM AUTHOR]

Soil water repellency affects the hydrological functions of soil systems. Water repellency is associated with the content and the composition of soil organic matter. In the present study, we examined the effects of hydrophobic and hydrophilic organic matter contents, the hydrophobic/hydrophilic organic matter ratio and the total organic matter content on water repellency using model sandy soils. Stearic acid and guar gum were used as the hydrophobic and hydrophilic organic compounds, respectively. Water repellency was estimated using the sessile drop method. Hydrophobic organic matter content was found to be the dominant factor affecting soil water repellency. Hydrophilic organic matter was found to increase the contact angle to some extent without the presence of hydrophobic organic matter. With the presence of both hydrophobic and hydrophilic organic matter, the effects of the hydrophilic organic matter content on contact angle were found to be dependent on the hydrophobic organic matter content of the soil. This relationship was explained by the differences in the surface free energies of different organic matter and mineral surfaces. The contact angle increased with increasing hydrophobic/hydrophilic organic matter ratio when the hydrophilic organic matter content was constant. When the hydrophobic organic matter content was constant, contact angles were roughly comparable, irrespective of the hydrophobic/hydrophilic organic matter ratio. The contact angles were not comparable at each total organic matter content. Accordingly, the hydrophobic/hydrophilic organic matter ratio and the total organic matter content in soil may not provide satisfactory information about soil water repellency. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the effects of ambient relative humidity (RH) on contact angle and water drop penetration time (WDPT) using a series of sand samples hydrophobized with stearic acid. The contact angle was estimated using the sessile drop method. The contact angle increased first sharply and then slightly with increasing stearic acid content up to about 5.0 g kg−1. The contact angle did not change with increasing stearic acid content above 5.0 g kg−1. The contact angle increased with increasing RH from 33 to 94%. The RH did not affect the contact angle of samples with a stearic acid content above 5.0 g kg−1, where the particles were considered to be completely coated by hydrophobic organic material. The contact angle increased with increasing RH in sands partially coated with hydrophobic material, which might have resulted from an increase in adsorbed water molecules on the remaining high-energy mineral surfaces. The WDPT increased with increasing RH. The higher the RH, the lower the stearic acid content at which extreme repellency (WDPT ≥ 3600 s) appeared. Samples with a contact angle below 75° or with surface free energy above 72 mN m−1 (equivalent to the surface tension of water) were non-repellent (WDPT < 1 s). Samples with a contact angle above 90° or with surface free energy below 50 mN m−1 were extremely repellent. There was no effect of RH on the relationship between WDPT and contact angle (or surface free energy), indicating that the effect of RH was comparable on both measurements. [ABSTRACT FROM AUTHOR]

The accuracy and the reproducibility of water repellency measurements can be improved with the use of proper measurement techniques. The purpose of this study is to compare the contact angles obtained using three different methods and to examine the relationship between contact angle and water drop penetration time (WDPT) using non-repellent to extremely repellent sands. Fine silica sand, artificially hydrophobized with stearic acid, was used for the measurements. Contact angles were estimated using the molarity of an ethanol droplet (MED) test, the capillary rise method (CRM) and the sessile drop method (SDM). The CRM was effective for soils with contact angles < 90°. The MED test was suitable for soils with contact angles ≥ 90° using 10 s as the ethanol drop penetration time. The SDM was effective for soils with contact angles ranging from very low to very high (11–105°). Directly measured contact angles using the SDM were in good agreement with indirectly obtained contact angles using the MED test and the CRM. The WDPT was < 1 s for contact angles ranging from 11 to 69°, gradually increased from 69 to 93°, and exceeded 3600 s beyond that. The WDPT was most sensitive for contact angles within the range of 88–93°. Extreme repellency was observed where the solid surface free energy was ≤ 45 mN m−1. [ABSTRACT FROM AUTHOR]