Application of Environment Spatial Application of Environment Spatial Information SystemInformation System

HW – Surface TensionHW – Surface Tension

Minkasheva Alena

Thermal Fluid Engineering Lab.Department of Mechanical Engineering

Kangwon National University

HOMEWORK

ContentsContents

• 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 TensionDefinition 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 IllustrationSurface Tension Illustration

Surface Interior A water molecule deep within the liquid is pulled equally from all sides

Net force = zero

A water molecule at the surface lacks neighboring water molecules (toward the interface)

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

Surface Tension EquationSurface 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:

• 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

L

F

Two forces now come into play - the energy it would take to overcome this repulsion and the force of gravity.

g

g

Two forcesTwo forces

•If the force of gravity is strong enough, it will prevail and the object will sink

•If the gravitational force is less than the surface tension then the object will float on the surface of the water

Example: Surface Tension on a NeedleExample: 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 TensionMeasuring Surface Tension

• The force is measured just as the ring breaks free from the film

• The 2L is due to the force being exerted on the inside and outside of the ring

L

F

2

Surface Tension and Surface WettingSurface 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 forcesThe liquid “beads up”

Liquids in Contact with a Solid SurfaceLiquids in Contact with a Solid Surface

• The adhesive forces (liquid-glass) 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 AngleContact Angle

Φ > 90° and cohesive forces are greater than adhesive forces

Φ < 90° and adhesive forces are greater than cohesive forces

Capillary ActionCapillary 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

11

Capillary ActionCapillary Action

The height at which the fluid is drawn above or depressed below the surface of the surrounding liquid (density ρ) is given by:

- surface tensionφ - contact angle r - tube radius

cos2

grh Water rises because

of capillary action

22

Surface Tension with TemperatureSurface Tension with Temperature

Water molecule representative

low T high T

• Weaker intermolecular forces• Increase of surface area• Lower Surface Tension

Variation of water surface tension with temperature

Examples of Surface TensionExamples 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

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

Surface Tension - Final RemarksSurface Tension - Final Remarks

THE ENDTHE END