Surface Tension Elearning

7/21/2019 Surface Tension Elearning

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Surface Tension

Compiled By: Bertha Yonata

From several sources


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Objectives

Students are able to explain

The surface tension phenomena

The methods to measure surface tension

Factors affecting surface tension


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Video 1 (why the water still in the

turning glass?)
http://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flv


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Discussion

Explain why does water still in the turning

glass


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Definition

The surface tension of a liquid resultsfrom an imbalance of intermolecularattractive forces, the cohesive forcesbetween molecules:

A molecule in the bulk liquid

experiences cohesive forces with othermolecules in all directions.

A molecule at the surface of a liquidexperiences only net inward cohesive

forces.

Surface tension is measured as the energy required toincrease the surface area of a liquid by a unit of area.


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Video 2 (cohesion, adhesion, surface

tension)
http://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flv


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A microscopic view of water illustrates the difference between

molecules at the surface of a liquid and water molecules within a

liquid.

The molecules at the surface of thissample of liquid water are notsurrounded by other watermolecules. The molecules inside thesample are surrounded by othermolecules. The average lifetime of amolecule at the surface of a liquid is

10-6s

The unbalanced attraction of molecules at

the surface of a liquid tends to pull themolecules back into the bulk liquid leavingthe minimum number of molecules on thesurface. It required energy to increase thesurface area of a liquid because a largersurface area contains more molecules inthe unbalanced situation


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As many molecules aspossible will leave theliquid surface willtherefore tend tocontract spontaneously.For this reason, dropletsof liquid and bubbles of

gas tend to attain asperical shape


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Video 3 (36 drops of water on penny)
http://localhost/var/www/apps/conversion/tmp/scratch_9/36%20Drops%20of%20Water%20on%20a%20penny.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/36%20Drops%20of%20Water%20on%20a%20penny.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/36%20Drops%20of%20Water%20on%20a%20penny.flv


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Discussion

What is the difference between surface and

interface?


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Surface vs Interface

There is no fundamental distinction between

the terms surface and interface

Surface: the boundary between two phases

one of which is gaseous phase

Interface: the boundary between two non-

gaseous phases


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Surface tension and interfacial tension against water for

liquid at 20C

Liquid 0 i

Water 72.8 -

Benzene 28.9 35.0

Acetic acid 27.6 -Acetone 23.7 -

CCl4 26.8 45.1

Ethanol 22.3 -n-octanol 27.5 8.5

n-octane 21.8 50.8

Mercury 485 375


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Additivity of Intermolecular Forces at Interface

The short-range intermolecular forces which areresponsible for surface/interfacial tension include Vander Waals forces and hydrogen bonding (in water) andmetal bonding (in mercury)

The relatively high values of the surface tensions ofwater and mercury reflect the contributions of hydrogenbonding and metal bonding respectively

The surface tension of water

For mercury:

hW

dWW

m

Hg

d

HgHg


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Oil-Water Dispersion

The interfacial tension for oil-water dispersion

is given by:

2/1).(2)( dOd

W

h

W

d

W

d

OOW


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Curved Interfaces

(The Kelvin Equation)

As a consequence of surface tension,

there is a balancing pressure difference

across any curved surface

The pressure being greater on the

concave side

Young-Laplace equation

For spherical surface:

21

11

rrp

r

p 2


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Contd

If the radius of droplet increase from r to r +

dr, the surface area increase from 4r2to

4(r+dr)2and the surface free energy will be

8...r.dr

Assuming ideal gaseous behaviour:

drrp

p

TRdn r

....8ln...0


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Since

so

A liquid which wets the wall of capillary will have aconcave liquid-vapour interface and lower vapour

pressure in the capillary than in the bulk phase. So

Kelvin equation will be written:

Mdrrdn

....4 2

r

V

r

M

p

pRT mr

22ln

0

Kelvin equation

r

V

p

pRT mr

cos2ln

0


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Measurement of Surface and Interfacial

Tensions

1. Capillary Rise Method

This method is only used when the contactangle is zero, owing to the uncertainty in

measuring contact angles correctly (=liquidvapour)

For zero contact angle:

cos2

grh

2

grh


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Capillary Action

The tendency of liquids

to rise up in narrow

tubes - capillary action. Due to the

phenomenon of surface

tension.


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The Complication of Contact Angles

The balance of forces

that results in a contact

angle, c. The contact angle gives

information on the

wettability of a

surface.


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Capillary Rise

The pressure exerted by

a column of liquid is

balanced by the

hydrostatic pressure.

This gives us one of the

best ways to measure

the surface tension of

pure liquids andsolutions. r2

gh

ghr2


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2. Wilhelmy Plate Methods

A thin mica plate or microscope slide is suspended from

the arm of a balance and dips into the liquid.

Detachmentthe container holding the liquid is

gradually lowered and the pull on the balance at the

point of detachment is noted

Staticfor measuring changes in surface tension. Thechange in the force required to maintain the palte at

constant immersion as the surface tension alters is

measured

)(2.det yxWW X =length of the plate

Y= breadth of the plate

W=weigh (at zero contact angle)


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The Wilhelmy Plate Method

a) detachment

b) static


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3. Ring Method

The force required to detach a ring from asurface or interface is measured either by

suspending the ring from the arm of a balance

or by using torsion-wire arrangement (du

Noy tensiometer)

R

F

4F= the pull of the ringR=the mean radius of the ring

=correction factor


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The Du Noy Ring Method

Measure the force required to pull the ring fromthe surface of the liquid or an interface bysuspending the ring from one arm of a sensitive

balance

Water

F

R


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The Correction Factor

The correction factor takes into account of the

small droplets that are pulled up by the ring

when it detaches from the surface


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4. Drop Weight/Drop Volume Method

A stream of liquid (e.g., H2O) falls

slowly from the tip of a glass tube as

drops


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Drop Weight Method

The drop weight is found by Counting the number of drops for a specified

liquid volume passing through the tip;

Weighing a counted number of drops

Vg= mg = 2 rg

A correction factor - F

r/v1/3


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5. Sessile Drop Method

The surface tension of a liquid may be

obtained from the shape and size of a

sessile drop resting on a horizontalsurface

e

Surface

Sessile Drop

h


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Sessile Drop Method (Contd)

Three techniques for obtaining the surfacetension from the image of the sessile drop

Measure the height of the top of a large sessile

drop above its maximum diameter. Estimate the shape factor of the drop from the

coordinates of the drop profile.

Fit the drop profile to ones that are generated

theoretically.


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Drop Profiles

The sessile drop method may also be used to

obtain the value of the equilibrium contact

angle.

Contact angle,e < 90

e


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The Maximum Bubble Pressure

Method

The maximum pressure required to force abubble through a tube is related to the

surface tension of the liquid.gas stream

b

l


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The Bubble Pressure Technique

The maximum bubble pressure is related to the

surface tension of the liquid as follows

P = g l + 2/ b = the density difference between the liquid and

the vapour

b = radius of curvature at the apex of the bubble

l = hydrostatic height to the bottom of the bubble

g = 9.807 m / s2


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The Differential Maximum Bubble

Pressure Method

Two probes of different diameters.

A differential pressure is generated, P.

b2

gas stream

b1

z2z1

t


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The Differential Bubble Pressure

Equations

The maximum bubble pressure is related to thesurface tension of the liquid as follows

P = g z11+ 2/ b1 - g z22+ 2/ b2 1= the density difference between the liquid

and the vapour of the first bubble 2= the density difference between the liquid

and the vapour of the second bubble

z1= the distance from the tip to the bottom, of

the first bubble z2= the distance from the tip to the bottom, of

the second bubble


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Methods of Measuring Surface Tension

Method Pure Liquids Solutions

Wilhelmy

Plate

quick and

easy to

operate

Good, suitable

when ageing

occurs

Du Nuy Ring Satisfactory n/a

Sessile Drop Very Good Good when

surface

ageing occurs

Drop Weight Suitable Poor when

surface

ageing occurs

Capillary

Height

Very Good n/a if

Bubble

pressure

Very Good Good when

a ein occurs


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Molecular Contributions to an Oil-

water Interfacial Tension

= Oil = water

Oil Phase

Water Phase

oil

water

oilx d

water)1/2

oilx

dwater)1/2


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Surface Tension Examples

Walking onwater: Smallinsectssuch as the water stridercan walkon water because their weight isnot enough to penetrate thesurface.

Floating a needle: If carefullyplaced on the surface, a smallneedle can be made to float on thesurface of water even though it isseveral times as dense as water. Ifthe surface is agitated to break upthe surface tension, then needlewill quickly sink
http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html


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Contd

Don't touch the tent!: Common tent materials aresomewhat rainproof in that the surface tension ofwater will bridge the pores in the finely wovenmaterial. But if you touch the tent material with your

finger, you break the surface tension and the rain willdrip through.

Washing with cold water: The major reason for usinghot water for washing is that its surface tension is

lower and it is a better wetting agent. But if thedetergent lowers the surface tension, the heatingmay be unneccessary


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A spreading drop e < 90

e

Solid Surfaces/Different Contact

Angles

Examine the following two surfaces.


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A drop with a contact angle


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The Derivation of Youngs Equation

la

sa

lse

change in the liquid-solid

interfacial area = dA

dA

e

change in the solid-air

interfacial area = - dA

change in the liquid-air

interfacial area = dA Cose


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Youngs Equation

For a liquid (as a drop or at at the surface of

a capillary) making a contact angle cwith

the solid surface

claslsa Cos

=Cosla

slsa

c


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Adhesional Wetting

The ability of the liquid to wet the solid will

be dependent on its ability to stick to the

solidliquid droplets

Solid Surface

la

droplets adhering

to solid surface

sl


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from the Young Equation

WG

AA

A

s a la s l

sa sl laCos e

W C o s A l a e

( )1

Note: the solid is completely

wetted ife= 0; it is partially

wetted for finite values ofe.


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Immersional Wetting

Immerse a solid substance in a pure

liquid or solution

area of the solid-air interface decreases interfacial contact between solid and

liquid is increased

solid particle

Water

sa

immersed

solid particle sl


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Work required to immerse the solid in theliquid

Examine the difference ion the solid-airsurfacetension and the solid-liquid interfacialtension

slsaI

IA

GW


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Applying youngs equation

elaI

I CosA

GW

If sa> sl, spontaneous wetting while

if sa< sl, work must be done to wet

the surface

Obj i


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Objectives:

students are able to

Describe the action of surfactant

Explain the effect of detergent in dirt removing

Explain the effect of temperature in surface

tension and interfacial tension

Vid 4 (th ff t f d t t/
http://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flv


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Video 4 (the affect of detergent/soap

adding to surface tension)
http://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flv


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How surfactant work
http://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flv


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53

What are surfactants?

Emulsifier, Dispersant, Wetting agent

Common examples:cleaning dirty clothes and kitchenware,writing on paper with a pen,greasing of cooking surfaces

Use of surfactants in industry:emulsion polymerisation, paper coatingfood, pharmaceuticals, cosmetics


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The Function of Detergency

The function of detergency or cleaning is a

complex combination of all functions. The

surface to be cleaned and the soil to be

removed must initially be wet and the soilssuspended, solubilised, dissolved or separated

in some way so that the soil will not just re-

deposit on the surface in question

Explain these pictures


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Explain these pictures

Surface Tension Elearning

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