Approaches to characterize the degree of water repellency

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1 Journal of Hydrology (2000) Review Approaches to characterize the degree of water repellency J. Letey*, M.L.K. Carrillo, X.P. Pang University of California, 900 University Avenue, Riverside, CA 92521, USA Received 25 November 1998; accepted 28 December 1999 Abstract Measurement techniques that quantify the degree of soil water repellency are important for research and for the communication of research findings. The water drop penetration time (WDPT) is a commonly used measurement. If a water drop does not enter the soil spontaneously, the soil water contact angle is greater than 90 and the soil is considered to be water repellent. The time for the drop to enter the soil (WDPT) provides an indication of the stability of the repellency. The liquid air surface tension of an aqueous ethanol concentration series that enters the soil in approximately 5 s is identified as the ninety degree (ND) surface tension, g ND, of the soil. The g ND number can be used to calculate the solid air surface tension, g s,byg s ˆ g ND =4: The water soil contact angle can also be calculated from the g s value by the relationship cos u ˆ g ND =g w 1=2 1Š; where u is the contact angle and g w the water air surface tension. The water entry pressure, h p, which is a function of both the soil water repellency and pore size, is an important parameter for predicting infiltration and the stability of water flow in the field. Measurements of WDPT, g ND, and h p provide a complete characterization of the degree of water repellency Elsevier Science B.V. All rights reserved. Keywords: Water drop penetration time; Surface tension; Water entry pressure; Contact angle 1. Introduction Soil water repellency has become increasingly recognized as an important consideration in hydrology as evidenced by the attendance of participants from several countries at the International Workshop on Water Repellency held at Wageningen, Netherlands, September 1 3, Measurement techniques that quantify the degree of soil water repellency are important for research and for the communication of research findings. Tschapek (1984) reviewed criteria for determining the hydrophilicity hydrophobicity of soils. This paper reviews various approaches to characterizing soil water repellency. * Corresponding author. Fax: address: john.letey@ucr.edu (J. Letey). 2. Contact angle (0 90 ) An index of water repellency of plane surfaces is the contact angle between the liquid and solid. Soils do not provide planar surfaces that allow the geometric measurement of a contact angle. Thus, an alternative to geometric measurement is required. Soils have pores and occasionally have been represented as being composed of a bundle of capillary tubes. A capillary tube is a vast oversimplification of the complex geometric arrangement of soil pores. Nevertheless, helpful insight can be achieved by assuming that capillary tube model for soils. The capillary rise equation is h ˆ 2g l cos u=rrg 1 where h is the height of rise, g l the liquid air surface /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (00)

2 62 J. Letey et al. / Journal of Hydrology (2000) tension, u the liquid solid contact angle, r the capillary radius, r the liquid density and g the gravitational constant. According to Eq. (1), water will not spontaneously enter the soil if cos u is zero or a negative number (u equal to, or greater than, 90 ). A soil is commonly classified as being water repellent if a drop of water placed on the soil does not spontaneously enter the soil. By this convention, a water repellent soil is one which has a water solid contact angle equal to, or greater than, 90. Soils classified as being wettable by this approach may have differing contact angels between 0 and 90 which can affect soil water relationships such as infiltration rates. Letey et al. (1962) reported a technique for measuring the water solid contact angle for soils using a capillary rise approach. Assume that ethanol wets all soil with u equal to zero. According to Eq. (1) and using the subscript e to represent ethanol r ˆ 2g e =h e gr e : 2 This value of r can be substituted into Eq. (1) when the height of rise is measured with water in the same soil. This leads to the equation for calculation of u w, where the subscript w refers to water cos u w ˆ h w g e r w =g w h e r e : 3 The assumption that ethanol wets all soil materials at a contact angle equal to zero was used by Letey et al. (1962) and Tillman et al. (1989) to measure the water repellency of materials that had an initial contact angle less than 90. Tillman et al. (1989) measured sorptivity of water and ethanol in soil columns and used the ratio of ethanol to water sorptivity as a repellency index. Letey et al. (1962) measured infiltration rates of ethanol and water into soil columns to measure the water solid contact angles. These indirect procedures circumvent the need for a geometric measurement of u, which is impossible in soils. Since water repellent soils are usually identified as those having a u value greater than 90, the remainder of this paper will be devoted to procedures for characterizing the level of repellency for these soils. 3. Water drop penetration time (WDPT) This procedure involves placing a drop of water on the soil and measuring the time for it to penetrate. Because of its simplicity, this procedure is almost always used, even if other procedures are also invoked. This procedure separates soils which are classified as being water repellent from those which are not. Since water penetrates the soil if u is less than 90, WDPT is a measure of the time required for u to change from its original value, which was greater than 90, to a value approaching 90. Therefore, it is a measurement of the stability of the repellency and not necessarily an index of u. Marmur (1988) reported that a water drop could penetrate a capillary tube that had a u value greater than 90 if the radius of the drop was small relative to the capillary radius. For example, a water drop to capillary tube radius ratio of 20 could penetrate a capillary tube that had a u value equal to 93. However, even though the drop could penetrate the tube, a portion of the drop remained outside the tube if u was greater than Ninety degree surface tension The liquid surface tension which wets the soil material with a 90 contact angle was proposed as an index of water repellency by Watson and Letey (1970). This procedure employs the concept that a liquid can only completely enter the soil if u is less than 90. A series of aqueous ethanol solutions producing various surface tensions is prepared for this measurement. Drops of these solutions are placed on the soil. The higher surface tension solutions will set on the surface and the lower surface tension solutions will spontaneously penetrate the soil. The 90 surface tension (g ND ) is the surface tension of the solution where there is transition from penetration to setting on the surface. Five seconds was arbitrarily chosen as the reference time. In other words, the solution surface tension which penetrates in 5 s is assumed to be the solution surface tension which wets the soil at 90. King (1981) proposed a similar procedure as the 90 surface tension approach except he recommended measuring the molarity (rather than surface tension) of ethanol in a droplet of water required for soil infiltration within 10 s. This procedure has been referred

3 J. Letey et al. / Journal of Hydrology (2000) Fig. 1. Relationships between liquid air surface tension and the vol% of 95% ethanol in an ethanol water solution and the molarity of an ethanol water solution. to as the MED test. King proposed a classification where soils with a MED index 1 are not significantly water repellent and soils with a MED index 2.2 are severely water repellent. In other cases, for example Dekker and Ritsema (1994), the results of the ethanol drop test are reported in terms of the volumetric ethanol percentage. The 90 surface tension, MED index and volumetric ethanol percentage procedures are identical, only the reported numbers differ. One value can be converted to another, using the relationships depicted in Fig. 1. Butler and Wightman (1932) reported the experimental relationship between mole fraction of ethanol in water and the surface tension at 25 C. These data were used to construct the results presented in Fig. 1. Since the ethanol purchased for laboratory research is usually 95% ethanol, the vol% numbers in the figure are for the 95% ethanol material. Therefore no correction is required if 95% ethanol is used in the laboratory. The molarity was computed on the basis that the molecular weight of ethanol is 50 g mol 1 and its density at 25 C is 0.79 g cm 3. As will be detailed later, g ND is directly related to the solid air surface tension. Therefore, it characterizes a fundamental physical chemical property of the solid rather than merely providing an index. The relationship between g ND and MED index or volumetric ethanol percentages is not linear. This factor becomes important when statistical procedures are used to determine significant differences in water repellency between soils. The results of statistical analyses using MED index values can produce misleading conclusions. 5. Solid air surface tension The solid-air surface tension, g s, is a fundamental physical chemical property of a solid which affects its wetting properties. Therefore, characterizing the magnitude of water repellency by measuring the solid air surface tension would be valuable. Miyamoto and Letey (1971) derived an equation whereby g s could be determined by measuring the height of rise of liquids with various surface tensions. This procedure is cumbersome and requires the liquids to be in contact with the soil for some time to reach the equilibrium height of rise. This time of contact between the liquid and solid could modify the wetting behavior just as placing a water drop on the surface

4 64 J. Letey et al. / Journal of Hydrology (2000) does during the WDPT. Carrillo et al. (1999) used an alternative procedure for measuring g s based on combining theoretical relationships in a manner to obtain the relationship between g s and a measurable parameter. Assuming the solid vacuum surface tension is the same as the solid air surface tension, g s, and the liquid vacuum surface tension is the same as the liquid air surface tension, g l ; then the solid liquid surface tension, g sl, is (Good and Girifalco, 1960) g sl ˆ g s g l 2F g s g l 1=2 4 where F is a function of molecular properties of the solid and liquid. For a water hydrocarbon system, F is approximately unity. Young s (1805) equation is given by g l cos u ˆ g s g sl : Combining Eqs. (4) and (5) leads to cos u ˆ 2 g s =g l 1=2 1: 5 6 Selecting g l so that cos u ˆ 0 u ˆ 90 ; which is the ninety degree surface tension, leads to g s ˆ g ND =4: 7 Therefore the rather simple procedure of measuring g ND leads to a measurement of g s. This factor is a justification for reporting results in terms of g ND rather than the MED index or volumetric ethanol percentage. The numerical value of g s is useful for predicting the behavior between the solid and various liquids. For example, the liquid solid contact angle is zero when g s equals g l (Eq. (6)). If g s is greater than g l ; u is less than zero, which is geometrically impossible. Under this condition, spreading of the liquid over the surface occurs. Miyamoto and Letey (1971) graphically depicted the relationships between g s, u, g l and the spreading coefficient. 6. Contact angle (u greater than 90 ) The capillary rise technique previously described is not a good procedure for measuring u when u is greater than 90. Water will not enter the column so capillary rise will not occur. This shortcoming could be overcome by immersing the soil column in water such that the hydraulic pressure would force water into the column. The height of capillary rise could be measured relative to the water elevation outside the column (a negative value). The contact time with the water would likely modify the value of u so that the result would not be reliable. Another approach can be developed by substituting Eq. (7) into Eq. (6) leading to cos u ˆ g ND =g w 1=2 1Š: 8 The measurement of g ND leads to the computation of cos u as well as g s. Carrillo et al. (1999) experimentally verified the validity of these relationships. Therefore cos u can be determined for soils with u greater than 90 without extended exposure of the soil to water. 7. Water entry pressure head When u is greater than 90, pressure must be applied to force the water into the soil. The pressure head required to initiate water entry into the soil is a function of both u and the soil pore radius (Eq. (1)). This water entry pressure head (sometimes referred to as breakthrough pressure) has relevance to field cases because it affects infiltration. Carrillo et al. (1999) described a laboratory apparatus for measuring the water entry pressure. Basically the apparatus consists of a tube into which the soil material can be packed. The soil is retained at the bottom of the column by a screen or other materials that allows air to escape from the soil. Two electrodes are placed to protrude into the soil just below the soil surface. One electrode is connected to a 1.5 V battery and the other is connected to a data logger input. A pressure transducer is placed above the soil surface and its output monitored with a data logger. Water is applied at a constant flow rate to the column. The depth of the water, as measured by the pressure transducer, is recorded as a function of time. The electrodes are shorted out when the water penetrates the soil and the signal is transferred to the data logger input. The data logger simultaneously collects the voltage and pressure as a function of time. The water entry pressure head is the pressure when the voltage signal appears on the data logger input. To avoid preferential infiltration along the container walls, the walls should

5 J. Letey et al. / Journal of Hydrology (2000) be coated with a Teflon based dry film lubricant or other material to make the walls very water repellent before adding the soil. 8. Summary The degree of soil water repellency can easily be measured in the field or laboratory by measuring the ninety degree surface tension, g ND. Both the solid air surface tension, g S, and the water soil contact angle, u, can be computed from g ND by using Eqs. (7) and (8). These data provide information on the initial water repellency of the soil. The degree of water repellency changes with time after contact with water. Measurement of the water drop penetration time, WDPT, provides information on the stability of the repellency, measurement of both g ND and WDPT provides information on the degree and stability of water repellency. The water entry pressure head, which is a function of both water soil contact angle and pore size, is an important parameter for interpreting infiltration and water flow stability in the field. This measurement can be made in the laboratory by an apparatus such as described above. Development of a convenient reliable method of making this measurement in the field is important. References Butler, J.A.V., Wightman, A., Adsorption at the surface of solutions. Part I. The surface composition of water alcohol solutions. J. Chem. Soc. July, Carrillo, M.L.K., Letey, J., Yates, S.R., Measurement of initial soil water contact angle of water repellent soils. Soil Sci. Soc. Am. J. 63, Dekker, L.W., Ritsema, C.J., How water moves in a water repellent sandy soil. I. Potential and actual water repellency. Water Resour. Res. 30, Good, R.J., Girifalco, L.A., A theory for the estimation of surface and interfacial energies. III. Estimation of surface energies of solids from contact angle data. J. Phys. Chem. 64, King, P.M., Comparison of methods for measuring severity of water repellence of sandy soils and assessment of some factors that affect its measurement. Aust. J. Soil Res. 19, Letey, J., Osborn, J., Pelishek, R.E., Measurement of liquid solid contact angles in soil and sand. Soil Sci. 93, Marmur, A., Penetration of a small drop into a capillary. J. Colloid Interface Sci. 122 (1), Miyamoto, S., Letey, J., Determination of solid air surface tension of porous media. Soil Sci. Soc. Am. Proc. 35, Tillman, R.W., Scotter, D.R., Wallis, M.G., Clothier, B.E., Water-repellency and its measurement by using intrinsic sorptivity. Aust. J. Soil Res. 27, Tschapek, M., Criteria for determining the hydrophilicity hydrophobicity of soils. Z. Pflanzenernähr. Bodenkd. 147, Watson, C.L., Letey, J., Indices for characterizing soil-water repellency based upon contact angle surface tension relationships. Soil Sci. Soc. Am. Proc. 34, Young, T., On the cohesion of fluids. Phil. Trans. Roy. Soc. London, A84.