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[b]Surface Tension[/b]
The [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c4]cohesive[/url] forces between liquid molecules are responsible for the phenomenon known as surface tension. The [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c2]molecules at the surface[/url]
do not have other like molecules on all sides of them and consequently
they cohere more strongly to those directly associated with them on the
surface. This forms a surface "film" which makes it more difficult to
move an object through the surface than to move it when it is
completely submersed. Surface tension is typically measured in
dynes/cm, the force in dynes required to break a film of length 1 cm.
Equivalently, it can be stated as surface energy in ergs per square
centimeter. Water at 20°C has a surface tension of 72.8 dynes/cm
compared to 22.3 for ethyl alcohol and 465 for mercury.
[img]
http://hyperphysics.phy-astr.gsu.edu/Hbase/fluids/flupic/surten.jpg[/img] <table border="1" cellpadding="2" cellspacing="2"><tr><td>[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c3]Decrease in water surface tension with heating[/url]</td></tr></table>
[/td][td:f767 width="66"][url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html]Index[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon]Fluid concepts[/url][/td][/tr][tr][td:f767 height="17"] <table><tr><td width="450">[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html] HyperPhysics[/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html#mechcon] Mechanics [/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon] Fluids [/url]</td><td align="right">[i]R Nave[/i]</td></tr></table>[/td][td]
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[table:f767 class="posting-table" border="1" cellpadding="2" cellspacing="2" height="370" width="580"][tr][td:f767 height="309" width="514"]<table><tr><td>[b]Cohesion and Surface Tension [/b]
The [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c4]cohesive forces[/url]
between molecules down into a liquid are shared with all neighboring
atoms. Those on the surface have no neighboring atoms above, and
exhibit stronger attractive forces upon their nearest neighbors on the
surface. This enhancement of the intermolecular attractive forces at
the surface is called [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c1]surface tension[/url].
</td><td>[img]
http://hyperphysics.phy-astr.gsu.edu/Hbase/fluids/imgflu/surten2.gif[/img]</td></tr></table>
[/td][td:f767 width="66"][url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html]Index[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon]Fluid concepts[/url][/td][/tr][tr][td:f767 height="17"] <table><tr><td width="450">[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html] HyperPhysics[/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html#mechcon] Mechanics [/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon] Fluids [/url]</td><td align="right">[i]R Nave[/i]</td></tr></table>[/td][td]
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[b]Surface Tension of Water [/b]
The [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c1]surface tension[/url]
of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to
break a surface film of water 1 cm long. The surface tension of water
decreases significantly with temperature as shown in the graph. The
surface tension arises from the [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/diph2o.html#c3]polar nature[/url] of the [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/diph2o.html#c2]water molecule[/url].
<table><tr><td>Hot water is a
better cleaning agent
because the lower
surface tension makes
it a better "wetting
agent" to get into
pores and fissures
rather than bridging
them with surface
tension. Soaps and
detergents further
lower the surface
tension.
</td><td>[img]
http://hyperphysics.phy-astr.gsu.edu/Hbase/fluids/imgflu/surten3.gif[/img]</td></tr></table>
<table border="1" cellpadding="2" cellspacing="2"><tr><td>[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/chemical/water.html#c1]Water[/url]</td></tr></table>
[/td][td:f767 width="66"][url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html]Index[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon]Fluid concepts[/url][/td][/tr][tr][td:f767 height="17"] <table><tr><td width="450">[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html] HyperPhysics[/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html#mechcon] Mechanics [/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon] Fluids [/url]</td><td align="right">[i]R Nave[/i]</td></tr></table>[/td][td]
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[b]Cohesion and Adhesion[/b]
Molecules liquid
state experience strong intermolecular attractive forces. When those
forces are between like molecules, they are referred to as cohesive
forces. For example, the molecules of a water droplet are held together
by cohesive forces, and the especially strong cohesive forces at the
surface constitute [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c1]surface tension[/url].
When
the attractive forces are between unlike molecules, they are said to be
adhesive forces. The adhesive forces between water molecules and the
walls of a glass tube are stronger than the cohesive forces lead to an
upward turning meniscus at the walls of the vessel and contribute to [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten2.html#c4]capillary action[/url]. The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/chemical/waal.html#c1]van der Waals forces[/url] or van der Waals bonds.
[/td][td:f767 width="66"][url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html]Index[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon]Fluid concepts[/url][/td][/tr][tr][td:f767 height="17"] <table><tr><td width="450">[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html] HyperPhysics[/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html#mechcon] Mechanics [/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon] Fluids [/url]</td><td align="right">[i]R Nave[/i]</td></tr></table>[/td][td]
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[b]Surface Tension Examples[/b]
<table border="1" cellpadding="2" cellspacing="2"><tr><td width="255">[b]Walking on water[/b]
Small insects such as the [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c6]water strider[/url] can walk on water because their weight is not enough to penetrate the surface.
</td><td width="255"> [b]Floating a needle[/b]
If carefully placed on the surface, a small needle can be made to float
on the surface of water even though it is several times as dense as
water. If the surface is agitated to break up the surface tension, then
needle will quickly sink. </td></tr><tr><td width="255"> [b]Don't touch the tent![/b]
Common tent materials are somewhat rainproof in that the surface
tension of water will bridge the pores in the finely woven material.
But if you touch the tent material with your finger, you break the
surface tension and the rain will drip through. </td><td width="255"> [b]Soaps and detergents[/b]
help the cleaning of clothes by lowering the surface tension of the
water so that it more readily soaks into pores and soiled areas. </td></tr><tr><td width="255"> [b]Clinical test for jaundice[/b]
Normal urine has a surface tension of about 66 dynes/cm but if bile is
present (a test for jaundice), it drops to about 55. In the Hay test,
powdered sulfur is sprinkled on the urine surface. It will float on
normal urine, but sink if the S.T. is lowered by the bile. </td><td width="255"> [b]Washing with cold water [/b]
The major reason for using hot water for washing is that its surface
tension is lower and it is a better wetting agent. But if the detergent
lowers the surface tension, the heating may be unneccessary. </td></tr><tr><td width="255"> [b]Surface tension disinfectants [/b]
Disinfectants are usually solutions of low surface tension. This allow
them to spread out on the cell walls of bacteria and disrupt them. One
such disinfectant, S.T.37, has a name which points to its low surface
tension compared to the 72 dynes/cm for water. </td><td width="255"> [b]Can you think of another?[/b]
</td></tr></table>
[/td][td:f767 width="66"][url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html]Index[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon]Fluid concepts[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c1]Surface tension[/url][/td][/tr][tr][td:f767 height="17"] <table><tr><td width="450">[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html] HyperPhysics[/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html#mechcon] Mechanics [/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon] Fluids [/url]</td><td align="right">[i]R Nave[/i]</td></tr></table>[/td][td]
[/td][/tr][/table]
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[b]Surface Tension and the Water Strider[/b]
[img]
http://hyperphysics.phy-astr.gsu.edu/Hbase/fluids/flupic/162wsss.jpg[/img][img]
http://hyperphysics.phy-astr.gsu.edu/Hbase/fluids/flupic/149wsss.jpg[/img]This interesting insect can freely run along the surface of a quiet
pond. It's tiny mass and the geometry of its legs allow it to be
supported by the high [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c3]surface tension of water[/url].
<table border="1" cellpadding="2" cellspacing="2"><tr><td>[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/chemical/water.html#c1]Water[/url]</td></tr></table>
[/td][td:f767 width="66"][url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html]Index[/url]
[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon]Fluid concepts[/url][/td][/tr][tr][td:f767 height="17"] <table><tr><td width="450">[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html] HyperPhysics[/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/hph.html#mechcon] Mechanics [/url]*****[url=http://hyperphysics.phy-astr.gsu.edu/Hbase/fluid.html#flucon] Fluids [/url]</td><td align="right">[i]R Nave[/i]</td></tr></table>[/td][td]
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