Contact angles for hydrophilic and hydrophobic soils. The contact angle ( ) is the angle from the solid surface to the liquidgas interface, passing through the liquid.

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... a soil exhibiting hysteresis, the equilibrium water content associated with any particular matric potential will be lower for a wetting curve than for a drying curve ( Fig. 3-8). The initial water content for the wetting or drying process also plays a role. Notice in Fig. 3-8 the clear difference in the drying curve for the silty clay loam soil when the drying process began from full saturation compared to when the drying process began at a lower water content indicated by the point labeled "B". in the soil ...

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... a soil exhibiting hysteresis, the equilibrium water content associated with any particular matric potential will be lower for a wetting curve than for a drying curve ( Fig. 3-8). The initial water content for the wetting or drying process also plays a role. Notice in Fig. 3-8 the clear difference in the drying curve for the silty clay loam soil when the drying process began from full saturation compared to when the drying process began at a lower water content indicated by the point labeled "B". in the soil water retention curve has multiple possible causes including: air entrapment, contact angle ...

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... soil is rewetted and small pockets of air become trapped in the interior pore spaces. This entrapped air cannot easily be removed, even if the soil is submerged underwater. As a result, higher water contents occur along the primary drainage curve from a fully saturated condition than those that occur during subsequent re-wetting (e.g. Fig. 3-8). Due to air-entrapment during re-wetting, the soil water content approaches a maximum value below the true saturated water content and this lower value is sometimes called the satiated water content. The image in Fig. 3-9 was generated by X-ray computed tomography and shows air-entrapment in the complex macro-pore network of a ...

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... along the primary drainage curve from a fully saturated condition than those that occur during subsequent re-wetting (e.g. Fig. 3-8). Due to air-entrapment during re-wetting, the soil water content approaches a maximum value below the true saturated water content and this lower value is sometimes called the satiated water content. The image in Fig. 3-9 was generated by X-ray computed tomography and shows air-entrapment in the complex macro-pore network of a satiated soil column. Soil chemical, physical, and biological processes can alter the amount and distribution of entrapped air over time, so the impact of air-entrapment on soil water retention can change with each subsequent ...

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... of entrapped air over time, so the impact of air-entrapment on soil water retention can change with each subsequent rewetting cycle A second potential cause of hysteresis in the soil water retention curve is a phenomenon known as contact angle hysteresis. The contact angle is the angle at which a liquid-gas interface meets a solid surface (Fig. ...

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... soils in which much of the surface area becomes covered with organic coatings can have contact angles >90 , making them hydrophobic, i.e. tending to repel water. To visualize contact angle hysteresis and how it may affect soil water retention, a thought experiment may help. Imagine if we added a sufficiently small volume of liquid to the drop in Fig. 3-10a, the edge of the drop would not move but the contact angle would increase slightly. Likewise if we removed a sufficiently small amount of liquid, the contact angle would decrease slightly. Thus, contact angles for wetting and drying processes are different, i.e. contact angles exhibit hysteresis. The larger contact angles during ...

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... a sufficiently small amount of liquid, the contact angle would decrease slightly. Thus, contact angles for wetting and drying processes are different, i.e. contact angles exhibit hysteresis. The larger contact angles during wetting versus drying lead to higher (less negative) pressure potentials for the same water contents, consistent with Fig. 3-8 University A third potential cause for hysteresis is the ink bottle effect, which refers to the way in which drainage from a relatively large cavity, such as the body of an old-fashioned ink bottle, can be restricted if the fluid must drain through a relatively narrow opening, such as the neck of an inverted ink bottle. The analogy is ...

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... the fluid density (kg m-3), g is the acceleration due to gravity (m s-2), and r is the radius of the capillary (m). Thus, the smaller the radius of the capillary, the greater the height of the capillary rise. To better understand this equation,. The pressure potential just below the capillary meniscus is simply the negative of the capillary rise. Fig. 3-11, two capillary tubes have been inserted into water. The height of the resulting capillary re was greater for the uniformly narrow tube on the right than for the non-uniform tube on the left. Fig. 3-11. Illustration of the "ink bottle effect" during ...

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... rise. To better understand this equation,. The pressure potential just below the capillary meniscus is simply the negative of the capillary rise. Fig. 3-11, two capillary tubes have been inserted into water. The height of the resulting capillary re was greater for the uniformly narrow tube on the right than for the non-uniform tube on the left. Fig. 3-11. Illustration of the "ink bottle effect" during ...

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... used. When the sample reaches equilibrium with the imposed matric potential, i.e. when water stops flowing, the water content of the sample can be determined by the change in the mass of the sample. For matric potentials between -10 and -100 Kpa, small pressurized chambers often called Tempe cells work well, particularly for intact soil samples (Fig. 3-12b). A special porous ceramic plate at the bottom of the chamber, when saturated, allows water, but not air, to flow out of the chamber. The air pressure is increased to the absolute value of the desired matric potential, and once equilibrium is reached, the water content of the sample is determined based on the volume of water which ...

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... to the absolute value of the desired matric potential, and once equilibrium is reached, the water content of the sample is determined based on the volume of water which flowed out of the sample or the change in mass of the sample. For matric potentials between -100 and 1500 Kpa, specialized pressure plates in larger chambers have often been used (Fig. 3-12c). The principle of operation is similar to that of Tempe cells, but smaller samples of homogenized soil are used with each chamber housing multiple samples, and sometimes even multiple pressure plates. At these low matric potentials, true equilibrium may take many weeks or may never be reached, and a growing body of research suggests ...

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... with each chamber housing multiple samples, and sometimes even multiple pressure plates. At these low matric potentials, true equilibrium may take many weeks or may never be reached, and a growing body of research suggests that data from pressure plate measurements may be unreliable at matric potentials below -100 Kpa. Dew point potentiometers (Fig. 3-5) offer one alternative measurement approach in this matric potential range. ...