This is the force that holds the molecules on the top of a liquid together. The force is stronger the higher the density of the liquid. It is why you can fill a glass of water slightly higher than the rim without it spilling. Several kinds of insects are light enough to take advantage of surface tension, and are subsequently able to walk on water. In zero gravity, this force will cause free-floating liquids to attain a spherical shape.

Polar molecules possess a weak electromagnetic field which is shared with their neighbours (known as the Van Der Waals or cohesive force). At the surface there are fewer neighbours (the Van Der Waals force is also present in non-polar molecules). Molecules on the surface of a liquid will experience a net downward force simply due to the absence of neighbours above. As the surface is pulled downward, the surface area is minimised and the density in the layer increases until limited by Coulomb repulsive forces. The energy density in the surface layer is higher than in the body of the liquid (where viscous forces reign). Surface tension produces effects such as a pin, of density much higher than water, floating if placed on the surface carefully.

Another force to consider in determining how the surface of a liquid will shape itself is the adhesive force between the liquid and a solid with which it is in contact. Water adheres strongly to glass. The water molecules in contact with the glass will tend to be attracted upwards. The surface tension effect will simultaneously rearrange the shape of the surface of the water until it is again in the lowest energy configuration. This process (obviously over a very short time-scale) will continue until the angle that the surface of the water makes with the wall of the vessel reaches a certain critical angle. This turned up at the edges shape is known as the meniscus. Water does not adhere well with wax, explaining why rain falling on the waxed surface of a car will curl up into droplets while that falling on the windscreen will spread all over the surface.

Reducing the surface tension of water will increase the degree to which it will spread over a surface (i.e. its 'wetness'). Raising the temperature of water or adding cleaning agents (surfactants) such as detergent will reduce surface tension. A washing machine is a surface tension reducer! A third method is to physically create turbulence on the surface so that the membrane is broken. Water is sprayed on the spot where a diver will enter the water so that a less painful impact results. OK, since I know by now you will be dying to know how to calculate the surface tension of a liquid, I present to you-

How to determine surface tension by the capillary tube method
What you will need-

  • 1 capillary tube
  • Some liquid (avoid mercury)
  • 1 PC microscope (available for under 100 dollars)
  • 1 PC with printer (most probably you have these)
  • 1 ruler
  • 1 micrometer screw gauge
  • 1 beaker (a transparent cup would do)
  • You will need to know the density of the liquid.
If you don't have a micrometer screw gauge you could try your best with the ruler. First start up your image capture software and clamp the capillary tube into the beaker full of water. Notice the liquid rise up the tube due to a combination of surface tension and adhesive forces with the glass. Now the surface tension (which I will henceforth call Gamma) is the force per unit length along the circumference of the liquid in the tube.
Gamma= F/2*Pi*R
where R is the radius of the tube. This force is balanced out by the force due to gravity, namely the mass of the liquid that has risen multiplied by the acceleration due to gravity g (9.8 ms-2). This mass is the product of volume of liquid in the tube and the density of the liquid (water= 1000 kg m-3). So lets write that out in equation form. Furthermore, the tension force acts parallel to the surface of the liquid where it meets the glass. It is its vertical component which balances gravity. We take care of this by multipying Gamma by Cos(theta) where theta is the angle made between the liquid and the glass.
2*Pi*R*Gamma*Cos(theta)=Volume*density*g
The volume of liquid is best calculated in two parts. First, there is the liquid beneath the lowest part of the meniscus which is at a height h above the level of the main body of water. This is simply given by the volume of a cylinder Pi*R2h. Next there is the water above the lowest part of the meniscus. In a narrow tube such as this the surface of the water assumes a hemispherical shape of radius R. The volume of this hemisphere is 2/3*PI*R3, while the total cylindrical volume of this section (of height R) is PI*R3. This means the water in this section has volume 1/3*PI*R3 which is equivalent to the volume of a cylinder of height R/3. Thus the total volume of the water in the capillary tube is PI*R2(h+R/3) (it might be helpful to draw a diagram of this). Finally, plug this expression for the volume into the equation above and one has
Gamma=(1/2*Cos(theta))*density*R(h+R/3)
or if you have are using browser that supports such symbols
γ=(1/2Cos(θ))*ρR(h+R/3)
So thats the theory and the apparatus has been set up. Better get on with the experiment in that case. We need to find R, h and theta in order to obtain a value for Gamma.

  1. Measure the outer diameter of the capillary tube using the micrometer screw gauge. The accuracy of this device is a fraction of a millimeter.
  2. Take image of capillary tube using microscope. The image should include the level of the main body of liquid and the meniscus in the tube.
  3. Print out image.
  4. Measure outer diameter on capillary tube using your trusty 12".
  5. Calculate ratio of the outer diameter as it is represented on the printed page and the one measured more accurately in step 1. This is your scale factor between reality and image.
  6. From the image measure the height of the liquid in the tube. The position of the base of the liquid may be difficult to ascertain due to the pulling up of the water at the edge of the beaker. Scale down using ratio determined in previous step to find h.
  7. Measure the inner diameter on the image. Scale down and divide by two to find the radius R of the liquid in the tube
  8. Get out your long lost protractor and measure theta on the printed image. Remember this is the angle the surface of the liquid makes with the tube at the glass/liquid interface.
  9. Plug figures for R, h and theta and the known value of density into the formula for the sought after surface tension
Now you are equipped to test the claims of laundary detergent manufacturers. First find the surface tension of tap water. Next add your favourite non-Bio to test if Gamma is reduced. Try replicating conditions in washing machine by raising the temperature of water. Remember any contaminants in the liquid or on the surface of the capillary tube will severely skew the results.

Sources
Undergrad Physics
tdent clarified the physics for me.

you were only this nothing piece of life, pulled out of shape by the spiralling energy of all the everything that seemed so much more real than you, people who you used to be able to touch turning into vortices tearing at the edges of your mind, television ghosts caught like flies on a clear membrane, a stretching film of reality like a soap bubble that you'd like to put your finger through to see if it really would pop - friends and family and the blurred shop window faces of the city reflected and splayed onto the curved and infinite surface of a bedroom or a gutter

nice to see how much pressure it would take, how strong the bubbles really are, string them together like a stage magician making castles and millwheels and blowing cigarette smoke through a straw into the cavities, memories floating and moulded and squashed into one another, so that the cobalt and scarlet once-in-a-lifetime sunset becomes one with the evening you got drunk for the first time on beer and sweet martinis; your grandparents who never met sit uneasily beside each other in deep armchairs in a weird melded dream house full only of the things that you can remember; and you, a million collected images from photographs and mirrors and the eyes of others morphed into a twisted thangka deity with a thousand arms and eyes and a smile that flickers from fear to rage to joy and an alien voice chanting is this me? is this me?

you should recognize this world because it's yours - you're the starship captain staring from the silent bright bridge at the screen, at the tunnel of stars streaming into a funnel of warped space, travelling between landmarks in an empty darkness; you're the film star at the wheel of the insane racecar on your way to end your imaginary life in flames, just like all those times as a child in bed when your mother would read nursery rhymes and you wondered if it would hurt when the weasel went POP and the whole dream of everything would disappear and the streets would wind up like spaghetti around a fork and the sky would drop gently like an old handkerchief, and if you closed your eyes your wouldn't be able to tell your feet from your head, like a ball of energy, a bubble of archetypal light with something at its heart, something indestructible

your sister cried her makeup off and told you how much she'd missed you since you moved away, just because she was high and it was true and she'd never thought to say it before, and suddenly it was as if no time had ever passed between you, and you could see her again, not the smear of her image on the strange surface where your minds met, but her, the real her, because the finger had gone through and the weasel had gone POP and you were really touching the world for a few moments, and you were only sad because you knew it and she didn't, and you knew she would go away again

or when you saw your wife for the first time in Chicago and you'd never seen anything as real as her face and her movements and her expressions and the smell and feel of her skin when you finally worked up the courage to touch her; the shimmer of air from the lake behind the shopping mall; soft leather taxi interiors and walking through quiet suburbs for hours realizing that this was someone else's life; someone luckier than you had left for someplace brighter and left this girl behind for you, to teach you what was real

and maybe that's why you're here, to see the bubbles and pop them if you can, and you already know you'd give a year of life or all you owned to show someone, anyone, their reflection on the ghostly surface between you and say, this isn't you, is it? and hear them reply quietly,

no

On bad days, days when every breath
was a conscious effort and when taking the stairs
to the basement to change out the laundry
felt like descending into the bowels of a battleship at sea,
the weight of the basket braced against one thigh
giving him the strangled gait of an inexperienced sailor,
he'd imagine he was running his fingertips along the chair rails
in his family home, covered in decades of paint and plaster and trapped dust,
the roughness of it leaving him as numb as the stairs did
but selectively and by choice,
the difference between a hot tub and a rip-tide.

A sonnet

Sometimes, when the world is looking the other way,
I am gorgeous. My eyes and hair shine, I have a glow
I walk with my shoulders back, more graceful than any ballet.
I am a goddess – not that anyone would ever know.
When glances fall on me, I stumble, stutter, spill -
become clumsy and clownish, my radiance fades.
It doesn't matter. Another glorious moment will
sneak up on me, when life's dull tirades
grind me so far down that I need to be lovely again.
I keep my beauty, like treasure, in a velvet box
ready to be applied, a secret balm for secret pain –
a shield against time passing, a curse on clocks.
Remember, when you look, the surface that you see
is only your impression. It never was me.

Sur"face ten"sion. (Physics)

That property, due to molecular forces, which exists in the surface film of all liquids and tends to bring the contained volume into a form having the least superficial area. The thickness of this film, amounting to less than a thousandth of a millimeter, is considered to equal the radius of the sphere of molecular action, that is, the greatest distance at which there is cohesion between two particles. Particles lying below this film, being equally acted on from all sides, are in equilibrium as to forces of cohesion, but those in the film are on the whole attracted inward, and tension results.

 

© Webster 1913

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