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Lawrence Berkeley National Laboratory, 1 Cyclotron Rd. MS 70-108, Berkeley, CA 94720
jmwan{at}lbl.gov
Abbreviations: AWS, air–water–solid • RH, relative humidity
Recent studies by Crist et al. (2004)(2005) attempted to provide pore-scale insights into mechanisms responsible for controlling colloid transport in unsaturated porous media. However, their key observation of colloids being trapped at air–water–solid (AWS) contact lines relied on images obtained along surfaces that were open to the atmosphere and thus subject to evaporation. Our analyses using their procedure show that because of exposure to the atmosphere, evaporation-driven artifacts account for most of their observed colloid trapping at AWS contact lines. In the following analysis of their work, we show how evaporation resulted in colloid deposition at AWS contact lines. Our comments were originally submitted in response to Crist et al. (2004) before the publication of Crist et al. (2005). The evaporation problem that we noted previously, in response to the first paper, was not rectified in Crist et al. (2005). For brevity, we will restrict our present comments to Crist et al. (2004), while noting that the visualization results presented in Crist et al. (2005) also suffer from the same artifacts because the obtained images were of evaporating surfaces.
As described in Crist el al. (2004), in their experimental setup "the front panel was removed to avoid light reflections that obscured the view and, thus, exposed one side of the sand column to air". Here, we show that removal of the front panel results in creating a sequence of three critical artifacts: (i) significant evaporation; (ii) drying of thin films on grain surfaces, causing formation of AWS contact lines; and (iii) advection of colloids to AWS contact lines, where they are deposited. As explained below, these artifacts so drastically disturbed their system that the magnitude of their observed colloid entrapment is not likely to occur anywhere except within the most superficial few centimeters of soils.
Before explaining these artifacts, we note that while the trapping of colloids at AWS contact lines, as reported in Crist et al. (2004)( 2005), is largely an artifact of evaporation, colloid filtration within perimeters of pendular rings is indeed a key prediction of the film straining model (Wan and Tokunaga, 1997). In that model, colloid filtration is predicted to be more efficient below a critical water saturation when capillary connections between pendular rings become separated, connected only by thin water films. In that paper, we stated, "Retardation of ideal, nonsorbing colloids can occur at two locations: trapped within individual pendular rings due to exclusion from entry into surrounding thin films and within films" (Wan and Tokunaga, 1997). Thus, while Crist et al. (2004)(2005) implied that the film straining model applies only to retardation of colloid transport within thin films, colloid retention within perimeters of pendular rings is in fact a critical feature of our model.
Significance of Evaporation
To determine the significance of evaporation, we constructed a flow chamber having the dimensions presented in Crist et al. (2004) and repeated their wetting and drainage procedures, with and without covering the sand pack. Our tests were conducted on acid-washed Unimin sand of the same grain size (0.43–0.60 mm), at room temperature (21.5–23.5°C), under relative humidity (RH) values ranging from 24 to 40%, and without lighting or heating from an illuminating device other than ordinary fluorescent lights along the laboratory ceiling. Our tests on covered sand packs were conducted in the manner described in Crist et al. (2004), except that the cover plate remained on until the time at which moisture-content sampling was performed. The air phase in the upper portion of the covered flow chamber was vented to atmospheric pressure via a pair of syringe needles. As in Crist et al. (2004), outflow was undetectable in all of our tests beyond a few minutes after setting the sand at a 35° inclined angle. However, in open systems such as that used in Crist et al. (2004), hydrostatic conditions cannot be assumed simply because of undetectable drainage. Various moisture profiles obtained in our tests, as well as the profile at 2 h drainage reported in Crist et al. (2004)(estimated from Fig. 2 of their paper) are shown in Fig. 1 . The moisture profiles obtained in our open system at 2 h of drainage are in rough agreement with Crist et al. (2004), although they obtained drier sands through most of their profiles (40–170 mm from the top) and slightly wetter sands within the upper section (0–40 mm). Their generally lower water contents could be the result of additional heating from their lighting system (Crist, 2002, p. 62). In contrast to the moisture profiles obtained from open flow chambers, those obtained on the covered sands remained nearly saturated, even in the upper sections. Water contents in the upper region (down to the 70-mm depth along the 35° incline) of our open system were clearly much drier than in our covered system. The water loss from the flow chamber during the 2-h period (shaded area between curves in Fig. 1) normalized to the open sand area is equivalent to an evaporation rate of 83 ± 7 µm h–1. Pan evaporation rates of 82 to 120 µm h–1 were measured in the laboratory adjacent to the flow chamber during these experiments.
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Air–Water–Solid Contact Lines Are Artifacts Caused by Evaporation
The term AWS interfaces appears throughout Crist et al. (2004) and is central to their results. However, because dry air–solid interfaces need to be present for AWS contact lines to exist, the significance of such boundaries in most vadose zone environments is questionable. Below depths of a few centimeters, RH values in soils are typically higher than 95%, so thin water films coat surfaces of mineral grains lacking hydrophobic organic coatings. The development of AWS interfaces in the experiments of Crist et al. (2004) are problematic because the sands they used (and ours as well) were thoroughly acid washed, resulting in strongly hydrophilic (water-wetting) grain surfaces. In the absence of evaporation, no air–solid surfaces or AWS contact lines should exist in their experiments, not even at the lowest volumetric water content of 0.16 (water saturation of 43%). As illustrated in Fig. 2a , in partially saturated systems without evaporation water films coat sections of grain surfaces that are not directly covered by pendular rings or saturated pores. No AWS contact lines appear in partially saturated, hydrophilic porous media. However, in open systems, the top surfaces of the uppermost grains are left dry by evaporation, leaving AWS contact lines at boundaries between wet and dry surfaces, as illustrated in Fig. 2b. Note that in plan view, these AWS contact lines form rings around dry upper surfaces of grains. Because only this uppermost layer of sand grains was observable in Crist et al. (2004), their images were nearly all affected by artificially generated AWS contact lines. The few images that might not have contained AWS contact lines would have been associated with their highest water saturation locations, for example, the 16- and 18-cm depths in Fig. 5 of Crist et al. (2004).
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Having established the importance of evaporation and the generation of AWS contact lines in the Crist et al. (2004) experiments, we now explain why these artifacts severely altered colloid distributions within the exposed viewing surface. The simplest system in which colloid transport to AWS contact lines can be understood is that of evaporating droplets in contact with flat solid surfaces. Upon evaporation of droplets of solutions and suspensions, a residual ring of precipitated solutes and colloids is deposited immediately adjacent to the original AWS contact line. This depositing is a ubiquitous phenomenon observed in many familiar situations, such as ring stains left behind by dried drops of saline water, coffee, and dilute paints. An excellent review of this phenomenon, including theoretical analysis and experimental tests, is presented in Deegan et al. (2000). In brief, a component of advection toward a pinned (stationary) AWS contact line is needed to replenish the perimeter of the evaporating wetted area and keep this region of extremely thin water coverage from drying (Fig. 2). Net advection towards the perimeter causes accumulation of solutes and colloids at and near contact lines. Many of the microphotographs presented in Crist et al. (2004) illustrated the results of this process. No other mechanism could explain the strong advection of colloids to contact lines, especially after inflow and outflow ceased.
Advection toward AWS contact lines is most clearly observed through microscopy of evaporating droplets of colloidal suspensions (Fig. 3)
. For this demonstration, a suspension of hydrophilic latex microspheres was prepared in a manner similar to that of Crist et al. (2004) (
108 particles mL–1, in 0.1 mM CaCl2, pH 5.6). In our test, 1.0-µm Fluoresbrite carboxylate YG microspheres (PolySciences, Inc., Warrington, PA) was used. Approximately 1-µL droplets of these suspensions were transferred onto clean glass microscope slides. Droplets were either covered with an inverted cap containing water-saturated filter paper to maintain nearly 100% RH in the headspace or left open to evaporate in the laboratory atmosphere (31% RH). Images of colloid distributions within these droplets were obtained with a Zeiss Axioskop 20 (Carl Zeiss, Oberkochen, Germany) at 100x and 400x magnification (fluorescence illuminated through a 450–490 nm excitation filter). The capped, humidified droplets were uncapped after various equilibration times (20–40 min) and quickly placed under the microscope. Upon initial viewing, no accumulation of colloids at droplet perimeters (AWS contact lines) was detectable in these capped, humidified systems. However, colloids were observed to quickly move to stationary AWS lines and deposit in monolayer rings, and later into multilayer rings. As shown in Fig. 3, colloid enrichment along the contact line was observed within <1 min of evaporation.
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This work was carried out under U.S. Dep. of Energy Contract no. DE-AC03-76SF00098, with funding provided by the U.S. Dep. of Energy, Basic Energy Sciences (BES), Geosciences Research Program.
REFERENCES
This article has been cited by other articles:
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  P. Sharma, H. M. Abdou, and M. Flury Effect of the Lower Boundary Condition and Flotation on Colloid Mobilization in Unsaturated Sandy Sediments Vadose Zone J., August 1, 2008; 7(3): 930 - 940. [Abstract] [Full Text] [PDF]
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S. A. Bradford and S. Torkzaban Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface-, Collector-, and Pore-Scale Processes and Models Vadose Zone J., May 27, 2008; 7(2): 667 - 681. [Abstract] [Full Text] [PDF]
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T. S. Steenhuis, J. F. McCarthy, J. T. Crist, Y. Zevi, P. C. Baveye, J. A. Throop, R. L. Fehrman, A. Dathe, and B. K. Richards Reply to "Comments on 'Pore-Scale Visualization of Colloid Transport and Retention in Partly Saturated Porous Media'" Vadose Zone J., October 10, 2005; 4(4): 957 - 958. [Full Text] [PDF]
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