February 2015
Capillary Action—the Physics of Cookies and Milk
Introduction:
Why is it that when you dunk a fresh
homemade cookie into a glass of milk, some of
the milk gets soaked up into the cookie, far
beyond the point where you’ve dipped your
cookie?
The cookie, soaked with milk that seemed to
crawl up inside it, is a consequence of
capillarity or “capillary action.”
How Does Capillary Action Happen?
Capillary action results from a combination of factors (or forces); one of them is called adhesion.
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another (think of dew drops
clinging to a spider’s web), while cohesion is the term used to describe molecules sticking to other
molecules of the same compound, being mutually attractive. Essentially, cohesion and adhesion are the
"stickiness" that water molecules have for each other and for other substances, respectively. Adhesion and
cohesion are water properties that affect every water molecule
on earth and also the interaction of water molecules with
molecules of other substances.
If the concepts of cohesion and adhesion are difficult to
differentiate, remember that cohesion is what makes a water
drop a drop. In the photo of the spider’s web, adhesion is
holding the drops in place, for now that is, until the gravitational
pull is greater than the adhesive force that holds them there.
Another factor in capillary action, surface tension, is allowed by
cohesion and can be described as a contractive (see def.)
tendency of a liquid that allows it resist an external force.
(Surface tension is what causes a water droplet to appear to have
a “skin” where it meets the air, when in reality there is nothing
but water in the drop.)
So, to recap, the chief properties involved in capillary action are
adhesion, cohesion, and surface tension. But what actually
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“moves” the liquid? Well, in some situations, the
molecular attraction between the liquid molecules and the
surrounding surface (adhesion) is stronger than the
molecular attraction between the liquid molecules
(cohesion). In these situations, the liquid seems to defy
the force of gravity and move in unexpected ways. This
mechanism is how plants pull water out of the ground. It’s
also the way an artist’s brush draws water color into the
tip with a mere touch. Thus, the capillary action we
observe in our dunked cookie is due to the active, and
often unbalanced, pressures of cohesion and adhesion,
which cause the liquid to work against gravity.
Experiment:
Let’s try some experiments to test the idea of capillary action. First we will test the ability of capillary
action to transfer water from a full glass to an empty glass. Then, we will watch
as capillary action moves water up through a stalk of celery—much like the
movement of water up into a tree. Capillary action is one of the primary reasons
trees can grow to great heights.
Materials:




One sheet of paper towel
Two cups or short glasses (fill one about 2/3 full of water)
One stalk of celery with the leaves still on, but the white end cut off
A bowl or jar of water colored with food coloring; use a color that will
easily show up in the celery stalk, such as red or blue.
Why Do Water Molecules
“Stick” Together?
Water is highly cohesive—the most
cohesive of the non-metallic liquids—but
chemistry and electricity are involved at a
detailed level. The positive and negative
charges of the hydrogen and oxygen
atoms that make up water molecules
makes them attracted to each other. More
precisely, the two hydrogen atoms align
themselves along one side of the oxygen
atom, with the result being that the
oxygen side has a slight negative (-)
charge and the side with the hydrogen
atoms has a slight positive (+) charge.
Thus, when the positive side on one
water molecule comes near the negative
side of another water molecule, they
attract each other and form a bond. This
"bipolar" nature of water molecules gives
water its cohesive nature.
Procedure 1:
1.
2.
3.
Take the sheet of paper
towel and roll it
Water and blue dye are moved to the
leaves of a celery stalk.
lengthwise to form a
(Photo:
Adam
Thornton’s Science Splat.)
rope. Place the two
cups close to each other.
Place one end of the paper towel rope in the glass without
the water. Place the other end in the glass that contains the
water.
After a while (this will take some time), notice that some of
the water has moved through the rope and is now contained
in the other glass. This process will continue until the two
cups each have the same amount of liquid in them.
Capillary Action—the Physics of Cookies and Milk
February 2015
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Procedure 2:
Now let’s look at the celery stalk.
1. Place the cut end of the stalk into the bowl with the dyed water.
2. Let the stalk sit in the water over night.
The next day, notice how the colored water is visible in the stalk, as
well as in the leaves, resulting from the water being drawn up into
the celery.
Discussion:
Fun Fact
The capillarity of a liquid is said to be
high when adhesion is greater than
cohesion, or vice versa. Hence,
knowledge of the liquid is not enough
to determine capillarity, since we must
also know the chemical composition of
the tube. These two, together with the
contact area (the tube's diameter),
comprise the key variables. Water in a
thin glass tube has strong adhesive
forces due to the hydrogen bonds that
form between the water molecules and
the oxygen atoms in the tube wall. In
contrast, mercury is characterized by
stronger cohesion, and hence its
capillarity is much lower.
This same phenomenon can be observed in a test tube or very skinny
glass, where we can observe how water clings high on the sides to
form what is called a meniscus (see def.). Because the attractive
force (adhesion) between the glass and the water molecules is
stronger than the force (cohesion) attracting the water molecules
together, the water is pulled up the side of the glass. Surface tension also plays a part. Because the surface
tension of the water is trying to hold the water together, a smooth, downward curve is formed around the
edge. This concaved surface is the balance between the two forces. In short, if the diameter of the tube is
sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid)
and adhesive forces of the liquid and the container act to lift the liquid. So as we said in the beginning,
capillary action is due to the pressures of cohesion and adhesion, which cause the liquid to move against
gravity.
Cohesive and Adhesive Forces:
Given what we discovered about adhesion and cohesion, if we were to observe a more dense liquid, such
as mercury, do you think the result would be the same?
There is more cohesive force in mercury relative to the little adhesive force between the mercury and
glass (greater cohesion than adhesion). This is due to the fact that liquid mercury is non-polar. So liquid
mercury forms a convex (see def.) shape as
seen in the picture.
This same effect can be seen with a water drop
on wax paper. Because there is little adhesive
force between the water and the wax, the water
(being is highly cohesive), forms a drop,
instead of a puddle.
The concave and convex meniscus of water and
mercury. (Photo: University of Hawaii, Manoa.)
Capillary Action—the Physics of Cookies and Milk
February 2015
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Fun Fact
When reading a depth scale on the side of an
instrument filled with liquid, such as a water
level device, scientists and other professionals
take into account the meniscus to obtain an
accurate measurement. Depth must be measured
with the meniscus at eye level and at the center
of the meniscus, i.e. the top of a convex
meniscus or the bottom of a concave meniscus.
Meniscus.
(Photo: P. R. Haney, Wikipedia.)
Capillary Action Can Happen Almost
Anyplace
In wet climates, most modern construction
generally now includes a barrier called a
“capillary break” beneath the building’s slab. The
capillary break is installed before a building is
constructed. It is usually designed from a 4-in.deep layer of sand covered by geotextile (see def.)
matting, or it is designed by simply using a 4-in.
layer of ½-in.-diameter aggregate (gravel). The
barrier prevents the creep of moisture via
capillary action through and up into the
foundation, or even walls, of a building.
Conclusion:
Water is wicked through and up the foundation of this building
by way of capillary action. (Photo: Department of Energy,
Building America Solution Center.)
Water is a very “sticky” or “clumpable” substance, but given certain circumstances, the nature of water
can seem to defy gravity. However, water isn’t the only liquid that can move through the pores of another
material, as shown with a dunked cookie. The chief forces at work in creating these unique circumstances
are adhesion, cohesion, and surface tension. Understanding these concepts informs our understanding of
the action known as capillary action, which can happen almost anyplace!
Fun Fact:
Albert Einstein’s first paper
submitted to Annalen der Physik in
1900 was on capillary action.
Definitions:
Contractive. Able to contract, i.e., decrease in size, number, or
range.
Convex. A shape, outline, or surface that is curved like the
exterior of a circle or sphere—curved outward.
Capillary Action—the Physics of Cookies and Milk
February 2015
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Geotextile. Permeable fabrics that, when used in association with soil, have the ability to separate, filter,
reinforce, protect, or drain.
Meniscus. The curve in the upper surface of a liquid close to the surface of the container or another
object, caused by surface tension. A meniscus can be either convex or concave, depending on the liquid
and the surface.
References:
http://science.jrank.org/pages/1182/Capillary-Action.html
http://hyperphysics.phy-astr.gsu.edu/hbase/surten2.html
http://www.chemprofessor.com/liquids.htm
http://water.usgs.gov/edu/adhesion.html
http://www.sciencesplat.com/ValentineScience.html
https://manoa.hawaii.edu/exploringourfluidearth/chemical/properties-water/comparison-water-otherliquids/activity-comparison-water-other-liquids
https://basc.pnnl.gov/resource-guides/capillary-break-beneath-slab-polyethylene-sheeting-or-rigidinsulation-over#block-views-guide-static-blocks-block-2
Contact us:
Website: http://www.portageinc.com/community/physics.aspx
E-mail: Physics@portageinc.com Capillary Action—the Physics of Cookies and Milk
February 2015
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