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Soil Science Society of America Journal 64:564-567 (2000)
© 2000 Soil Science Society of America

DIVISION S-1-SOIL PHYSICS

Modified sessile drop method for assessing initial soil–water contact angle of sandy soil

J. Bachmanna, R. Hortonb, R.R. van der Ploega and S. Wochea

a Inst. of Soil Sci., Univ. of Hannover, Herrenhaeuser Str. 2, 30419 Hannover, Fed. Rep. Germany
b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1020 USA

bachmann{at}mbox.ifbk.uni-hannover.de


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Existing methods for determining the soil–water contact angle as a measure of water repellency are either indirect, cumbersome, or time-consuming. Our objective was to develop a method that is simpler than existing procedures and that still yields accurate results. The proposed method represents a modified sessile drop method for measuring the initial contact angle of powdered or granular material. The measurements are made by placing a layer of uniform soil particles onto adhesive tape, adding droplets of deionized water, and reading off the contact angle at the three-phase boundary line with a goniometer-fitted microscope immediately after placing the drops on the soil sample. Sieved soil fractions <63 µm, 63 to 100 µm, and 100 to 200 µm were used to ensure particle layer uniformity. The method was tested on 10 samples from different depths of a sandy soil profile. The contact angles measured on these soil fractions were compared with the water drop penetration time (WDPT) test and the capillary-rise method. The contact angles of the fractions <63 µm and 63 to 100 µm compared reasonably well with those measured with the capillary-rise method and their rank order agreed in general with that of the WDPT test. We conclude that the new method appears to be promising for the simple, rapid, and reproducible determination of the contact angle of sandy soils. The sessile drop method can be used in a wider range of water repellency conditions compared with either the capillary-rise or the WDPT method.

Abbreviations: WDPT, water drop penetration time


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
WETTABILITY OF SOILS has been investigated because of its influence on water movement, particularly on infiltration. An important phenomenon affecting infiltration or water movement in soils is the hydrophobization of originally wettable mineral particles by coatings of organic substances of different origin (Wallis and Horne, 1992). The difficulties of assessing surface properties of soil particles are considerably larger than those for plain and smooth surfaces. As a consequence, a direct measurement of the contact angle is not possible and the nature of the soil particle surfaces has led to the use of indirect methods. The most common methods include the water drop penetration time (WDPT) test, the ring infiltrometer test, or the molarity test for a droplet of a water–ethanol mixture (Wallis and Horne, 1992). These methods are easily applied, but they have to be considered as indirect and thus lead to relative values. Some procedures that apply basic physical principles are the capillary-rise method, based on Washburn's equation (Letey et al., 1962; Siebold et al., 1997); the measurement of the breakthrough pressure in combination with different water–ethanol mixtures (Carillo et al., 1999); the mathematically extended capillary-rise method (Marmur and Cohen, 1997); or the Wilhelmy plate method (Spelt and Vargha-Butler, 1996).

A way to directly assess wetting properties of a solid surface is to measure the contact angle of water droplets placed on the surface (Adamson, 1990). To measure precise contact angles, a homogeneous and absolutely plain surface is needed (Drelich, 1997; Hazlett, 1992). Since powdered or granular material has a porous surface, exact measurements for soil material are, strictly speaking, not possible. However, one can try to establish a quasi-plain surface from closely packed, uniformly shaped particles. The objectives of the present study were to prepare such surfaces and to adapt the so-called sessile drop method for measuring initial contact angles of dry soil. The contact angles measured with the modified sessile drop method were compared with the results from two independent, and generally accepted, techniques: the capillary-rise method and the WDPT test. Another objective of this study was to determine the sessile drop–contact angles measured on different particle-size fractions and to compare these data with two standard methods for assessing soil water repellency.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Ten samples (S1–S10) from different depths of a single soil profile were used in this study (Table 1) . The sampling site was a dense pine forest ({approx}55 yr old), about 30 km north of Hannover in northern Germany. The soil was a Gleyic Podzol (FAO-System) or Aquod (U.S. Soil Taxonomy), developed in fine to medium-fine sand. Ten disturbed samples of {approx}1 kg were taken from depths varying from 10 to 110 cm. To avoid extreme alteration of the surfaces due to a high drying temperature (Dekker, 1998), the samples were dried for 24 hr at 40°C and then stored air-dry at 20°C.


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  		Table 1 Some physical and chemical properties of the soil samples (<2000 µm) used for the present study

 
Sessile Drop Method
To ensure a uniform and smooth surface, narrowly sieved particle-size fractions (<63, 63–100, and 100–200 µm) were used. Contact angle measurements were performed on a thin layer of the sieved soil fractions, which were fixed on an adhesive tape. This method may be considered to be a modification of the contact angle measurement of a water drop placed directly on the surface of a flat soil surface (Bond, 1968) or on peat pellets (Valat et al., 1991). The sessile drop–contact angle of the unsieved soil was approximately assessed using the entire fraction <630 µm. The coarse sand fraction was excluded to avoid large heterogeneities. The percentage of the coarse sand fraction varied between 1 and 11% by weight. The major reason for using sieved fractions was to obtain uniform surfaces, which should decrease heterogeneity. Following King (1981), we assumed that careful sieving does not affect the repellency of the sieved subsample.

A smooth microscope glass slide was covered with a double-sided adhesive tape (TESA, type 55733, Beiersdorf, Hamburg, Germany). The dry soil was sprinkled on a 2- by 3-cm area. Particles were pressed to the tape with a 100-g weight for 3 to 5 s. The slide was then shaken carefully to remove surplus grains. This preparation technique was repeated twice for each sample. Contact angles were measured at room temperature (20°C) within 10 min after sample preparation using a microscope fitted with a goniometer scale (Fig. 1) . Relative humidity during the measurements varied between 80 and 85%. Using a pipette, five drops of deionized water (drop volume was 1.7 mm3) were placed at the same time on the soil sample. Within 1 min, 10 contact angle readings were measured by adjusting the goniometer as a tangent at the point of the three-phase contact (Fig. 1). Preliminary measurements on smooth surfaces showed that the precision of these contact angle readings (10 readings for each sample) was 1 to 2° in the 20 to 140° range. It was assumed that the chosen drop size and soil particle distribution led to an optimized system that avoids gravity effects (Caroll, 1992) and large local heterogeneities. It was further assumed that a narrowly sieved soil fraction reduces the standard deviation of the contact angle measurements.



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  		Fig. 1 Schematic representation of the device for contact angle measurements

 
Capillary-Rise Method
To evaluate the newly developed sessile drop method, contact angle values obtained with the capillary-rise method were taken for comparison. This method is based on the capillary rise of a liquid in a dry column of packed powder or granular material. For details of the method, see Siebold et al. (1997). The contact angle {theta} was calculated with the Washburn equation

(1)
where h is the height of the rising liquid front in the column (cm), r is the effective radius of the uniform pores (cm) that represent the idealized pore system of the granular material, {gamma}L is the liquid surface tension (g cm s-2), {eta} is the viscosity of the liquid (g cm-1 s-1), and t is the time (s). From Eq. [1] an expression for the weight increase of the soil column during the capillary-rise process can be derived (Siebold et al., 1997). This expression can be written as

(2)
where w is the weight increase of the column (g), {rho} is the liquid density (g cm-3), and c is a geometry factor. If a liquid with a non-zero contact angle like water is used, two unknown variables have to be determined in Eq. [2]. Therefore, the factor c must be evaluated independently in a second run using the same soil with identical packing density and using a reference liquid that wets the soil particles completely . We used Ethanol as reference liquid.

The lower end of a vertical plexiglas tube (i.d. = 0.9 cm, length = 40 cm) was sealed with a rubber disk. The disk had a hole in its center (diam. = 0.3 cm), which was covered with filter paper. The tube was packed with dry soil that passed a 2-mm sieve. The column was dipped 3 mm into a beaker with water and the weight loss of the beaker (w in Eq. [2]) was recorded with an electronic balance, with an accuracy of 0.01 g. At the start of a capillary-rise experiment, the weight loss of the beaker was recorded every 5 s during the first min, then every 1 min for the next 10 min. Capillary-rise experiments were carried out at room temperature (21°C ± 1.0°C). The calculation of the contact angle was performed following the procedure of Siebold et al. (1997).

Water Drop Penetration Time Test
The WDPT test consists of simply placing a water drop on the soil surface and recording the time required for the water to infiltrate. For the WDPT test, {approx}100 g of soil (<2 mm) was placed on a dish and the surface was smoothed by hand. Three water drops, each containing 0.05 mL were randomly placed on the air-dry sample. The median value of the infiltration time of the three drops was considered as being representative. For details see Dekker and Ritsema (1994) or Dekker (1998).

Effects of Adhesive Tape on Sessile Drop–Contact Angle
To test the impact of adhesive tape on the sessile drop–contact angle, experiments were conducted with soil particle mixtures of both wettable and hydrophobized particles. Wettable sand and silt particles, respectively, were hydrophobized with Dimethyldichlorosilane (Shaw, 1975) and diluted with untreated particles to obtain a variation of the soil particle wetting properties. From earlier work it was known that applying 7.5 mL silane per kg sand and 50 mL silane per kg silt resulted in contact angles around 90°. Untreated (wettable) sand and silt particles, respectively, were added to the treated materials to produce samples with 100, 50, and 10% of treated particle surfaces. The sand had a mean particle diameter between 200 to 630 µm and the silt between 20 to 63 µm. These particle-size fractions represent approximately the smallest and largest of the four particle-size fractions that were used for the contact angle measurements of soil samples S1–S10.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Sessile Drop–Contact Angles for Different Particle-Size Fractions
Table 2 shows contact angle values that were determined with the sessile drop method for the four particle-size fractions that were examined. Also shown is the standard deviation calculated from 10 replicate measurements. It can be seen that with few exceptions, the measured contact angles decreased with increasing particle size. Whereas the values for the fractions <63 µm and 63 to 100 µm are generally close together, those for the particle-size fractions 100 to 200 µm and <630 µm are clearly different. Also, for the 100- to 200-µm fractions and the <630-µm fractions, the measured contact angle values seem to show more variability, as indicated by larger values of the standard deviation. A decreasing apparent contact angle with an increasing soil particle-size might be the combined effect of grain-size dependent wettability and capillarity. This phenomenon needs further attention.


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  		Table 2 Comparison of contact angles measured with the sessile drop method for different soil particle-size fractions

 
In Table 3 we compared contact angle values that were determined with the sessile drop method and with the capillary-rise method. Also shown are data obtained with the WDPT test. The values of the sessile drop method are for the 63- to 100-µm particle-size fraction. The sessile drop method yielded contact angles for Samples S1, S3, and S4 that were larger than 90°. For these three samples the capillary-rise method failed to yield results (no water uptake), from which a contact angle >90° can be concluded. The capillary-rise measurement of Sample S2 was probably erroneous. A WDPT value smaller than 5 s and contact angles slightly smaller than 90° (all fractions) indicate water uptake, whereas no movement of the wetting front could be observed. For the other samples, contact angles <90° were determined both with the sessile drop and the capillary-rise method. Except for Sample S6, there was reasonably close agreement between the values obtained with the different methods.


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  		Table 3 Comparison of contact angles measured with the sessile drop method (soil particle-size fraction 63–100 µm) with contact angles measured with the capillary-rise method, and water drop penetration times (WDPT)

 
From Table 3 we can also see that the data from the sessile drop method and the WDPT test generally show the same rank order. For Samples S1, S3, and S4 with sessile drop–contact angles >90°, the WDPT is 3300 s or more; for Samples S2, S7, and S8 with contact angles between 80 and 90°, the WDPT ranged from 4 to 70 s; and for contact angles <80° the penetration time was <1 s (S5, S6, S9, and possibly S10). According to Dekker (1998), WDPTs >3600 s imply extreme water repellency, and penetration times of 600 to 3600 s imply severe water repellency. Wettable or slightly repellent soils are characterized according to Dekker (1998) by penetration times <60 s, and instantaneous infiltration is observed for a penetration time <1 s. In view of the contact angles that we measured (Table 3), it appears that a rather narrow transition range of contact angles (80–90°) decides whether a sandy soil is either strongly water repellent or readily wettable. Our observations agree with the work of King (1981), who stated that the simple, qualitative WDPT test is only sensitive in a narrow contact angle interval at 90°.

Effects of Adhesive Tape on Sessile Drop–Contact Angle
The results of the experiment with the coated particles of the sand fraction showed clearly that an increase in the percentage of wettable particles resulted in contact angles decreasing from 97° (with 100% hydrophobic particles) to 67° (with 50%), to 22° (with 10%), and to 0° (with 0% hydrophobic particles). For silt-sized particles, the corresponding values were 78° (with 100% hydrophobic particles), 46° (with 50%), 0° (with 10%), and 0° (with 0% hydrophobic particles). Because the contact angles readily decreased as the fraction of wettable soil particles increased, we concluded that the adhesive tape (with a contact angle of 95.7° ± 4.6°) had insignificant impact on the soil–water contact angle.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
The experimental steps for sample preparation and measurement of the modified sessile drop method seem to provide a reproducible method for the easy characterization of the initial contact angle of sandy soils. Compared with the capillary-rise method, the range of contact angles that can be measured is extended to values larger than 90°, even if pure water is used as the testing liquid. Compared with the WDPT test, our modified sessile drop method also has an advantage. The WDPT test appears to be sensitive only in a narrow range of contact angles between 80 and 90°. Although this range appears to be decisive as to whether a soil is water repellent or wettable, the WDPT technique appears to be insensitive in the range of contact angles <80° or >90°. The modified sessile drop method, however, can be used in both ranges, where the WDPT method fails to provide precise information.

Although the modified sessile drop method appears to be promising, some critical remarks are appropriate. The effect of the particle-size fraction on the measured contact angle is not fully understood and needs further attention. Further work should focus on the problem of increasing horizontal capillarity effects caused by increasing wettability. Also, the proposed method is restricted to measurements at low soil water contents, and the assessment of the water content–dependent wettability (see de Jonge et al., 1999) is not possible. Additionally, work is needed to study the time dependence of the sessile drop–contact angle and the effect of sample preparation (drying temperature, time between sampling and measurement, air humidity during the measurement) on the results. Nevertheless, the new method could become a valuable additional technique for the study of soil water repellency.


   ACKNOWLEDGMENTS
 
The authors thank Walter Schäfer, Institute of Soil Technology, Geological Survey of Lower Saxony, Bremen, for expert technical assistance during the contact angle measurements.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Journal Paper No. J-18480 of the Iowa Agriculture and Home Economics Experiment Stn., Ames, Iowa, Project No. 3262, and supported in part by Hatch Act and State of Iowa.

Received for publication January 21, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 




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