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Soil Science, 84, 5, pp. 419-422, 1958-03-01
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The Mechanism by which water moves through a porous material
subjected to a temperature gradient: 2. Salt tracer and streaming
potential to detect flow in the liquid phase
Kuzmak, J. M.; Sereda, P. J.
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Ser
TH1
N21r2
no.53
c.2
BLDGNATIONAL
RESEARCH
COUNCIL
C A N A D A
DIVISION O F BUILDING RESEARCH
THE MECHANISM BY WHICH WATER MOVES THROUGH A
POROUS MATERIAL SUBJECTED TO A TEMPERATURE
GRADIENT:
2.
SALT TRACER AND STREAMING
POTENTIAL TO DETECT FLOW
I N THE LIQUID PHASE
BY
J. M. KUZMAK AND P. J. SEREDA
R E P R I N T E D FROM S O I L S C I E N C E
V O L . 84. N O . 5. N O V E M B E R 1957. P. 419
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DIVISION O F BUILDING RESEARCH
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MARCH 1958
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Reprinted from SOIL SCIENCE
Val. 84, No. 5, November, 1957
Printed in U . S . A .
T H E MECHANISM BY WHICH WATEI3 MOVES THROUGH A 1'0- ROUS MATERIAL SUBJECTED TO A TEMPERATURE GRADIENT: 2. SALT TRACER AND STREAMING POTENTIAL TO DETECT FLOW I N T H E LIQUID PHASE
J. WI. I<UZiLIAI< AND P. J. SEREDA
hrational Research Council, L'anadal
Rcceived for publicntion Jnnunry 2, 1 9 3
A temperature gradient across a partially saturated porous material produces a movement of water from the hot to the cold side. The controversy regarding the mechanism b y which this water moves has been referred t o in a previous paper (2).
If there is flow in the liquid phase toward the cold side, the liquid may be ex- pected to carry along dissolved salt, and this flow may be expected to give rise to a streaming potential. This paper gives the results of experiments to detect, flow in the liquid phase through a bed of sand by using a salt tracer and by measuring streaming potentials.
MATERIALS AND APPARilTUS
The apparatus used was described in the preceding paper (2). Other materials used were: ( a ) sand, passing a 48-mesh screen, retained on 115; (b) sand, passing a 250-mesh screen, retained on 325; (c) calcium chloride of commercial grade; (d) platinum wire electrodes 0.082 cm. in diameter and 3.8 cm. in length; (e) potentiometer (Rubicon); and (f) distilled water.
The sand which passed a 48-mesh screen and was retained on 115 was cleaned in aqua regia and then washed in distilled water. Washing was continued until successive analyses indicated no chloride ion. The said was then placed in t h e apparatus (2). I t was held between the porous plates by a rubber ring and formed a cylinder 6.4 cm. in diameter and 1.3 cm. in length. A solution of calcium chloride (0.043 g./ml.) was placed in one side of the apparatus and distilled watel. in the other. To speed the adsorption equilibrium of the salt ions on the sand, the sand was saturated with the calcium chloride solution when i t was placed in the apparatus.
The amount of water (or solution) in the sand was regulated in the usual manner by applying a controlled positive air pressure in the chamber containing the sand. I n applying the temperature gradient, as already described (2), the side with the salt solution was made the hot side. Flow from the hot t o the cold side was observed and measured.
The amount of calcium chloride passing through the sand and into the distilled water in a given time was calculated from a determination of the chloride ion Contribution from the Division of Building Research; published with the approval of
the Director.
419
KUZMAK AND SERE1)A
content of the distilled water by the Mohr method. The rate and total volume of movement of water were noted, so that i t was possible t o calculate the quantity of salt that should have appeared in the distilled water if the total flom had taken place in the liquid phase.
Next, a constant temperature of 27°C. mias placed across the bed of sand. The rate of transfer of salt under these conditions was determined.
T o observe the transfer of salt when the water moves due to suction, a suction gradient a t a constant temperature of 27°C. was also placed across t h e bed of sand. A given air pressure applied in the chamber containing the sand produces the same pressure difference across each of the porous plates when the pressure of the water on the other side of the plates is atmospheric. By increasing the pressure on the other side of one of the plates, the pressure difference across that plate is lowered, and hence the suction is lowered. The two plates then exert different suctions and so create a suction gradient across the sample. In the present investigation, t h e suction gradients nrcre applied to draw t h e water toward the side with the distilled water.
RESULTS AND DISCUSSION
Two pressures, one of 1050 the other of 20 cm. of water, mere used t o partially unsaturate the sand during the temperature gradient experiments in which the flow rate of water was 0.2 ml./hr. For both pressures the experimentally measured rate of transfer of salt (0.0001 g./hr.) was much smaller than the calculated rate (0.01), or the rate a t mhich the salt should have passed through the bed of sand if the observed flow of water had occurred entirely in the liquid phase.
The experimental rate of transfer of salt a t constant temperature (0.0001 g./hr.) was the same as that in the experiment with the temperature gradient. The salt passes through the bed of sand a t constant temperature by diffusing through the liquid films and lenses of the pores. During this diffusion process, no flom of water from the fornlcrly hot t o cold side was observed in the measuring system. Rather, there mas osmotic flow in the opposite direction, t h a t is, into the salt solution.
The diffusion described occurs even when the temperature gradient i s applied. Therefore the flow due to diffusion must be subtracted from the flow observed in the temperature gradient experiment. The difference obtained was zero, mhich indicates that there is no transfer of salt by water moving in the liquid phase during the experiment with the temperature gradient. It is concluded, therefore, that the water passed through the bed of sand in the vapor phase.
As a further check, the increase in concentration of salt solution was calculated for the observed transfer of a given volume of water from the hot t o the cold side. The concentration was also determined experimentally. The agreement between experimental and calculated values was within 2 per cent.
I n view of the small amount of salt transferred during the temperature gra- dient experiment, i t seemed desirable t o check the amount transferred during flow caused by suction gradients (table 1). The suction of the plate on the salt solution side mas kept constant a t 10 cm. of water while the suction of the other
WATER MOVEMENT THROUGH SAND 42 3.
TABLE 1
Il'ransjer of salt through partially unsaturated sand when w a t e ~ moves due to a suclion gradient
R a t e of Salt Transfer Suction of the Flow R a t e of
o a t e
I
Mlater1
Calculated
1
p e r e t ao n . o j water
I
ml./hr.I
g./hr.I
g . / h r .I
%I I -- I - .-
* Temperature = constant a t 27OC.
t
Concentration of calcium chloride solution = 0.048 g./ml. 0.12454 and
lot
1
0,15 597 andlot
800 and 10t 0.19 1000 and 10t
1
0.21plate was increased from 454 to 1000 cm. of water. The temperature was constant a t 27°C. The concentration of the calcium chloride solution used was 0.048 g./ml. The ratio, expressed as per cent (table I), of the experimental rate of transfer of salt to the calculated rate shows a scatter around the value 100 per cent. The 100 per cent transfer of salt indicates that the movement of water due to suction gradients takes place in the liquid phase.
0.0058 0.0056 97
0.0072 0.0066 92
0.0091 0.0093
0.0101 0.0106
1
105T h e streaming potential experiment
These experiments were carried out in an attempt to detect flow in the liquid phase by measuring the streaming potential. To obtain more surface area per unit volume, sand passing a 250-mesh screen and retained on 325 was used. The sand was held between the porous plates of the apparatus as described for the salt tracer experiment. Platinum wire electrodes were placed a t the ends of the bed of sand. The voltage was measured with a Rubicon potentiometer. Distilled water was used.
To observe the effect of temperature on the voltage between the electrodes when there was no flow through the bed of sand, the sand was saturated before the temperature gradient was applied. The steady menisci in the pipettes indi- cated no flow of water. The voltage was then measured and found to vary er- ratically between 0.01 and 0.1 volt. Also, the polarity was not consistent with the direction of the temperature gradient.
The sand was then partially unsaturated by applying various pressures, and the usual flow from the hot to the cold side was observed. Since the voltage was measured and found to vary over the same range as it did in the previous experi- ment where there was no flow, no flow in the liquid phase was detected.
CONCLUSIONS
The results of these experiments indicate that there is no flow in the liquid phase when water moves, due to a temperature gradient, from the hot to the cold side of a partially unsaturated porous material. The flow, therefore, apparently occurs in the vapor phase. On the other hand, when water moves due to a suction
422 KUZMAK AND SEREDA
gradient, the flow occurs entirely in the liquid phase. These conclusions are in agreement with those of Gurr, Marshall, and Hutton (1) who used the salt mi- gration method to study moisture movement in soils.
This investigation presupposes flow in a continuous film. If the flow in the liquid phase is not coi~tinuous but involves multiple evaporation condensation steps in series as suggested by Smith (3), then it cannot be detected either by salt tracer or by the measurement of streaming potentials in the manner at- tempted in the present investigation. Experiments devised to investigate this particulax mechanism are in the planning stage.
REFERENCES
( I ) GUHR, C. G . , MARSHALL, T. J., AND HUTTON, J. T . 1952 Movement of water in soil
due t o a temperature gradient. S o i l Sci. 74: 335345.
(2) ICUZMAK, J . M., AND SEREDA, P. J . T h e mechanism by which water moves through
a porous material subjected t o a temperature gradient: I. Soil S c i . 84. 291-299.
(3) SMITH, W. 0. 1943 Thermal transfer of moisture i n soils. Trans. Am. Geophys. U n i o n
