Download Surface tension

HOMEWORK
Application of Environment Spatial Information System
HW – Surface Tension
Minkasheva Alena
Thermal Fluid Engineering Lab.
Department of Mechanical Engineering
Kangwon National University
Contents
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Definition of Surface Tension
Surface Tension Illustration
Surface Tension Equation
Two forces
Example: Surface Tension on a Needle
Measuring Surface Tension
Surface Tension and Surface Wetting
Liquids in Contact with a Solid Surface
Contact Angle
Capillary Action
Surface Tension with Temperature
Examples of Surface Tension
Surface Tension - Final Remarks
Definition of Surface Tension
Surface tension is ability of the surface of a liquid to act like a
thin, flexible film
Surface tension is the amount of work required to extend a liquid
surface
Droplets take on a spherical shape since a sphere has the smallest
surface area for a given volume
Surface Tension Illustration
Surface
A water molecule
at the surface lacks
neighboring water
molecules
(toward the
interface)
Interior
A water
molecule deep
within the
liquid is pulled
equally from all
sides
Net force is not zero!
This unequal attraction causes the water
at the air-water interface to act as a cohesive
surface and the liquid to shrink to the
smallest possible surface area
Net force = zero
Surface Tension Equation
• The surface tension is defined as the ratio of the magnitude of the
surface tension force to the length along which the force acts:
F
 
L
• represented by the symbol

or σ or T
• SI units are N/m
• Can express as J/m2 - in terms of energy, any equilibrium
configuration of an object is one in which the energy is a
minimum
Two forces
Two forces now come into play - the energy it would take to
overcome this repulsion and the force of gravity.
•If the force of gravity is strong
enough, it will prevail and
the object will sink

g
•If the gravitational force is less
than the surface tension then
the object will float on the
surface of the water

g
Example: Surface Tension on a Needle
• Surface tension allows the
needle to float, even though
the density of the steel in
the needle is much higher
than the density of the water
• The needle actually rests in
a small depression in the
liquid surface
• The vertical components of
the force balance the weight
Measuring Surface Tension
• The force is measured just
as the ring breaks free from
the film
F
 
2L
• The 2L is due to the force
being exerted on the inside
and outside of the ring
Surface Tension and Surface Wetting
The force of gravity that provides the energy required to spread the
drops via collapse of the drop, two other forces play important roles:
1. Adhesive forces are intermolecular forces between unlike molecules
2. Cohesive forces are intermolecular forces between like molecules
The shape of the surface depends upon the relative size of the
cohesive and adhesive forces
The liquid spreads, because
adhesive forces are comparable in
strength to cohesive forces
The liquid “beads up”
Liquids in Contact with a Solid Surface
• The adhesive forces (liquidglass) are greater than the
cohesive forces (liquid-liquid)
• The liquid clings to the walls
of the container
• The liquid “wets” the surface
• Cohesive forces (liquid-glass)
are greater than the adhesive
forces
• The liquid curves downward
• The liquid does not “wet” the
surface
Contact Angle
Φ > 90° and cohesive
forces are greater than
adhesive forces
Φ < 90° and adhesive
forces are greater than
cohesive forces
Capillary Action
• Capillary action is the result of
surface tension and adhesive forces
• The liquid rises in the tube when
adhesive forces are greater than
cohesive forces
• At the point of contact between the
liquid and the solid, the upward
forces are as shown in the diagram
• Here, the cohesive forces are
greater than the adhesive forces
• The level of the fluid in the tube
will be below the surface of the
surrounding fluid
1
Capillary Action
2
The height at which the fluid is drawn above or depressed below
the surface of the surrounding liquid (density ρ) is given by:
2
h
cos 
gr
 - surface tension
φ - contact angle
r - tube radius
Water rises because
of capillary action
Surface Tension with Temperature
low T
high T
• Weaker intermolecular forces
• Increase of surface area
• Lower Surface Tension
Water molecule representative
Variation of water surface
tension with temperature
Examples of Surface Tension
• Walking on water: small insects can walk on water because their
weight is not enough to penetrate the surface.
• Floating a needle: even though several times as dense as water, a
needle carefully placed on the surface of water can float. If agitated to
break up the surface tension, then needle will quickly sink.
• Soaps and detergents: help the cleaning of clothes by lowering water's
surface tension to more readily soak into soiled areas.
• Washing with hot water: hot water's surface tension is lower and it is a
better “wetting agent”.
• Tent materials: can be rainproofed so that the surface tension of water
will bridge the pores in the finely woven material. Touching the
material will break the surface film & rain will drip through.
• Clinical test: normal urine has a surface tension, g, of about 66
dynes/cm but if bile is present, it drops to about 55. In the test,
powdered sulfur is sprinkled on the urine surface; it floats on normal
urine, but sinks if g is lowered by the bile.
• Capillary action
Surface Tension - Final Remarks
Systems always search to minimize energy ⇒ minimize
surface/interface with highest energy
Surface tension defined as:
• the force along a line of unit length perpendicular to the surface
• the energy required to increase the surface area of a liquid by a
unit area
represented by the symbol
 or σ
or T
measured in Newtons per meter (N/m) or J/m2
THE END