Introduction Hands-on Homework (HOH or H2O) Kits
John Selker, Oregon State University, selkerj@engr.orst.edu
Kit Contents:
 (1) 400 mL beaker
 (1) sponge
 (10) 4.5 mL cuvettes
 (10) cuvette covers
 (1) bag 30/40 sand
 (1) vial blue dye
 (1) 1 m tubing
 (1) barbed connector
 (1) plastic hose clamp
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(2)
2 mL cylinder
capillary tubes (2 each: 3 sizes)
culture tube
size 0 stopper
25 mL cylinder
funnel
30 cm acrylic pipe
3-hole size 7 stopper
square nylon mesh
The tasks facilitated through these kits let you observe the basic physical processes that will be
discussed in the class. You will need to think carefully about what these simple experiments are
telling you. Many of these tasks will be the basis for quantitative calculations. Beyond the materials
provided, you will only need a source of tap water, a ruler, and a watch. Although used for a number
of years, PLEASE feel free to let me know how to make them work better for the next time we use
these. They should be fun, informative, and useful in developing your intuition for water movement
in unsaturated porous media. All materials must be returned when finished, including the DRY sand.
Overview of HOH’s
Study of Capillary Forces
Develop intuition for capillary pressure and water retention: Sponge
Quantitative understanding of capillary attraction and hysterisis: Micropipetes
Quantitative understanding of surface tension: Soapy water and Micropipetes
Retention of water by porous media: Homemade Bukner Funnel experiment
Flow in Porous Media
Flow in single capillary tubes: Poiseuille flow in capillaries
Darcy’s Law: Column experiment with sand
Solute Transport
Observation of advection and dispersion: Sand columns with blue dye
Suggested approach: Each person should do their experiments independently, taking all data from
their own apparatus. Some have found that two sets of hands is helpful in carrying out the capillary
rise and solute transport experiments, which is fine, but make sure that each person does the full
analysis independently to get the message of the exercise. Working on data with discussions in
groups, just as with any other homework, is a good idea when the going gets tough. As in other
homework, interpretation is key: what do the results suggest is happening? Are they reasonable?
On what basis? There need be no more than one paragraph responses to each question on the
handouts (to keep you from feeling that you must write about every detail of what you observe).
HOH Homework #1: The Sponge of Mystery – Due October 9, 2006
Goal: to start you thinking about hydrostatic forces and the retention of liquid by porous media.
1. Hydrostatics in pure water.
Wash your beaker and fill it to the 400ml mark with water or some beverage you want to drink.
a. What is the pressure at the top of the liquid surface? Why?
b. What is the pressure at the bottom of the liquid? Why? (Please be careful with the units you
use in expressing pressure.)
c. Draw a plot of liquid pressure versus height in the beaker.
d. Drink (or pour out) half of the liquid and repeat a-c.
e. Comment on how your result would have been effected by the sugar content of the drink you
selected (hint: assume the sugar changes the density).
f. Finish the drink and wash the beaker.
2. Capillary forces.
Refill your beaker with water and take out the kit sponge.
a. Using a dry sponge, hold the narrow edge of the sponge 5 mm into the water (it should just
fit in) for 3 minutes.
i. Doesn't this lifting of water upward into the sponge defy gravity? Try to describe the
physics of what made the water rise in the sponge.
ii. By pinching the sponge between your fingers going from the top of the sponge to the
bottom as a way to measure water content, make a qualitative plot of the moisture
distribution in the sponge.
b. Dip the entire sponge in the beaker, wait for the sponge to take up water, then lift the sponge
so that only the last 5 mm is in the water, just as in step 2a (now the sponge is draining
water back into the beaker), and hold this for 3 minutes.
i. Now repeat parts 2a i and 2a ii, drawing the plot of moisture content on the same figure
but with a different color pen. Try to explain why the two moisture contents aren't the
same.
ii. Do you think that if you had waited 24 hours that the two moisture profiles be the same
(assuming that we didn't allow evaporation from the sponge)?
iii.Plot what you think the water pressure is from the bottom of the glass of water to the top
of the sponge.
3. Hydrostatics and Capillarity.
Get a pan big enough to lay your sponge in. Lay the sponge flat in a pool of water and carefully
lift the sponge out while keeping it flat (a spatula might be helpful here).
a. How much water came out? How long did it take to stop dripping?
b. Turn the sponge on its edge by holding it by its longest edge. Did more water come out?
Why? How long did it take to stop dripping compared to above?
c. Now for the last step: Turn the sponge so that it is hanging by its shortest edge, by holding it
by its shortest edge. Did more water come out? Why? About how long did it take to stop
dripping compared to a. and b.?
HOH Homework #2: Capillary tubes – Due October 16, 2006
In your homework kit you will find three sizes of capillary tubes (color coded). Each has a different
internal diameter. Please follow the following directions to explore the issues of La Place's equation
and capillary hysteresis.
Procedures: (You may have to blow the water out of the tubes between tests)
0.
1.
2.
3.
4.
Clean the tubes. Fill the glass graduated reservoir tube with a septum on the bottom with
water. Now poke each of the capillary tubes into the septum and rinse through with clean
water. Pull each tube off and blow the water out of the tube with your mouth.
Hold the tubes vertically throughout this procedure. Fill your beaker with tap water. Dip the
very end of each tube (as little as possible) into the water. When it has filled to an
equilibrium height (this happens fast), pull the tube from the water and measure the height of
the capillary rise.
Hold the tubes vertically throughout this procedure. Attach the graduated reservoir to the
tube and fill it with water. Once the tube is full, remove it from the reservoir and let the tube
drain (This also happens fast). Measure the distance from the bottom of the tube to the water
level.
Repeat step 1 holding the tube at a 45 degree angle.
Add a few drops of dish washing liquid to your beaker of water. Mix the solution thoroughly
and repeat steps 1 and 2.
Table of Measurements - in cm
Tube
Procedure 1 - Wetting
Red
Orange
White
Procedure 2 - Draining
Procedure 3 - 45 deg
Procedure 4 - Soap
Wetting
Procedure 4 - Soap
Draining
Exercises
I. a. Given the results of steps 1 and 2, assuming the contact angle was zero during draining,
estimate the contact angle that must have existed between the water and the wall of the tube
zero in the initial wetting stage. Explain why a zero contact angle assumption is reasonable for
the drying stage.
b. Since we are using pure clean glass in this experiment, without surface defects or
contamination, speculate on the wetting contact angle you might expect in typical soils.
II.
Do the results from procedures 1 and 3 agree physically? Explain quantitatively (with
numbers).
III.
Compute the surface tension of the soap solution using the results of step 4. Are the values
consistent between the various capillary tubes?
HOH Homework #3: Permeability - Due November 6, 2006
In this set of experiments you will explore Darcy's Law from the perspective of a single capillary
tube, and a saturated column of sandy soil.
1. Capillary Flow (Parts: three capillary tubes and the graduated reservoir tube)
Attach the cylindrical plastic reservoir to the top of each of your three capillary tubes in turn.
Measure the time required for between 1.0 and 0.1 ml of the reservoir to pour through the capillary
tubes depending on the tube. Select a volume such that the time of the experiment is about 1 minute.
Be careful to measure the height of the water surface above the bottom of the capillary tube to obtain
the total potential across the flow path. Using the flow versus radius relationship of Poiseuille,
compute the radius of the capillary tubes. Compare your answers to those values found for the radii
in the capillary rise experiments (some people didn’t compute the radius using the capillary rise
experiment: now’s your time to do this if you didn’t on the last go around!).
3 bonus pts: run the same experiment with the tubes held between two bags of ice water. Calculate
the apparent change in viscosity between using room temperature water and water at 0 deg C.
Compare these results with published values.
2 MORE bonus pts: run the same experiment with the tubes pushed up into the reservoir so that
the total head across the system is lower. Show that the flow is linear with total head loss.
2. Flow in Saturated Sand (Parts: 30 cm acrylic tube; #7 white rubber stopper; bag of
sand; 0.75 m flexible vinyl tube; white plastic tubing connector; nylon mesh)
Plug the bottom of the plastic column with the white rubber stopper with the white nylon screen
tightly wrapped around the stopper so that the sand cannot enter the open hole in the stopper. Fill
the clear plastic pipe about 2/3’s of the way with the sand provided. Set up the sand column in a
stable situation so that you do not have to hold it upright in your hands (you could tape it to the edge
of a door, or use a rubber band around the handle to hold it). Push the white plastic barb fitting into
the bottom of the stopper and attach the flexible clear plastic tube, which has been filled with water
and with the clamp closure on the tube, to the bottom of the system so that the open end of the
flexible tube is even with the bottom of the stopper. Put the open end of the flexible tube in a beaker
full of water. Elevate the vinyl tube from the bottom of the column to the height of the top of the
column so that the column fills with water. Now clamp the flexible tube closed and drop the outflow
pipe to be even with the bottom of the column. Pour water in the top of the column until the water is
about 1 cm from the top. Now open the clamp and measure the time for the water to fall from the
top of the column to the top of the sand (stop by clamping the tube). DON’T let the water level fall
below the top of the sand or it will desaturate, never to be fully saturated again! Repeat. Now do the
same experiment, but put the outlet 15 cm below the bottom of the column, then 30 cm below the
bottom of the column. In these experiments make sure that the vinyl pipe stays full of water by
letting the water flow out of it upwards.
a.
b.
c.
Compute Ks for each test based on each of the three experiments.
Based on the value obtained, using the Poiseuille-law, estimate the average pore side in the
sand.
Based on this average pore size, how high would you expect water to rise in this sand due to
capillarity?
HOH Homework #4: Water Retention Curve Exercise - Due 11/13/06
(Most efficiently carried out by two people working together, pairs of students may share results)
1. Put both mesh screens over the stopper and push the stopper into the plastic pipe. Use a tabletop to
push the stopper in as far as you can.
2. Measure 100 mL of dry sand using the beaker. Tap the beaker repeatedly on the counter and add
sand until its volume stabilizes to 100 mL. Notice that packing strongly affects porosity. Pour the
sand in the pipe and tap the pipe with a wooden spoon or another object that will not crack the pipe
until the sand level stabilizes.
3. Slide the white crimp clamp on the clear tubing and insert the barbed fitting into one end and the
funnel end into the other with the white clamp in the middle of the tubing. If the barbed fitting is not
snug, cut a little off the end of the tubing and reconnect. Fill the tubing and funnel with water,
taking care that there are no air bubbles in the tubing or funnel stem. You may tilt the pipe slightly
while inserting the barbed fitting. Fill the funnel to the top with water. From now on make sure
the funnel never drains completely, and that it is at the same level each time you record
measurements of water added or removed.
4. Readjust the column into a vertical position. Here it is helpful to attach the column to a fixed object
(tape it to a door edge or the like). Holding the water level in the funnel even with the bottom of the
sand in the column, open the clamp to let the water enter the sand from the bottom up. Keep adding
water to keep the funnel full (noting carefully how much you need to add). After about 5 minutes,
write down how much water went in, and measure how high the water rose within the sand. Now reposition the water level in the funnel at the same height as the top of the sand, and measure the water
that enters until the water level is just at the top of the sand, making a shiny surface. When the water
is at the sand surface, close the clamp.
5. Drain just enough water from the pipe so that the sand surface is no longer shiny then re-clamp the
tube closed. Empty the graduated cylinder. Now turn the sand filled pipe horizontally. Take the
funnel off the end of the tubing and place the end of the tubing level with the top of the flexible
tubing. Undo the clamp. Lower the outlet of the flexible tubing in 5-cm increments collecting the
water that drips out in the graduated cylinder. At each increment, wait until the drips have slowed to
less than one drip every 5 seconds, and record the volume of water that was drained for that
increment. Watch and record when air enters the vinyl pipe. When air enters the pipe, that is the
end of the experiment. The experiment should end when 50-60 cm pressure is applied.
Write-up:
i. What is the water entry pressure of this sand? What is the air-entry pressure of this sand?
ii. Estimate the mesh size of the screen based on the air entry pressure.
iii. Graph the characteristic curve for this sand.
iv. Estimate the pore size at which the column is 80% saturated using the Laplace equation.
v. Using the Poisselle equation and the pore size estimated above, calculate the expected
permeability of the column. Does this match up with the permeability results from experiment
3?
HOH Homework #5: Solute Transport Experiment - Due 12/1/06
The goal of this homework is to illustrate the processes of solute advection and dispersion.
1.
Prepare your concentration standards of blue dye. Prepare 1:4 dilutions of the blue dye solution
until you have four vials filled with successively more dilute mixes with the most concentrated
vial being 1/4 the concentration of the stock solution. Here’s how I would make my first two
dilutions: Fill one small glass vial with the dye and pour it into the beaker. Pour three more
vials of water into the beaker. Mix well, and remove one vial full of the 0.25 Co dilute
solution. Add 9 (why 9? You will need to understand this if you are to get correct solution
strengths for the remaining vials) more vials of water to the beaker, mix well, and fill one more
vial with this solution to obtain your 1/16 Co solution. Proceed carefully to obtain dilutions of
1:64 and 1:256.
2.
Assemble the column as before using the large plastic tube with the screen, stopper, and
outflow pipe as in the conductivity test. Fill the tube with 100 ml of dry sand. Tap the sides of
the column to make sure the porosity is minimized.
3.
Attach the flexible tube, and put the outlet even with the top of the sand so that you can collect
the outflow. Saturate the sand as in the water retention experiment being careful to measure
how much water was required to saturate the sand: this is your system’s pore volume.
4.
Pour one vial of pure blue dye onto the sand with the water table just at the top of the sand.
Now, counting vials, continue to pour vials of water onto the system to push the dye through
the sand.
5.
As soon as the dye become apparent in the outflow, collect the outflow water in a vial and
compare it to the dilute solutions. Estimate the concentration in each effluent vial.
6.
Plot the dye concentration versus outflow volume with outflow volumes expressed in “pore
volumes.” To understand this, say the column had 50 mls of water, and the vials hold 5 ml.
Then there would be 10 vials per pore volume. Thus, each vial applied would be 1/10th of a
pore volume.
7.
Fit the standard CDE Gaussian solution to your outflow data and estimate the longitudinal
Dispersivity of your sand.
8.
Comment on the results of this experiment.