Surface Tension

In Physics , surface tension is an effect within the surface layer of a Liquid that causes that layer to behave as an elastic sheet. This effect allows Insect s (such as the Water Strider ) to walk on Water . It allows small metal objects such as needles, razor blades, or foil fragments to float on the surface of water, and causes Capillary Action .

The physical and chemical behavior of liquids cannot be understood without taking surface tension into account. It governs the shape that small masses of liquid can assume and the degree of contact a liquid can make with another substance.

When a liquid makes a contact with another liquid (oil with water, for example), the same effect is observed, but in this case is called interface tension.

THE CAUSE

Surface tension is caused by the attraction between the Molecule s of the liquid by various Intermolecular Force s. In the bulk of the liquid each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid but they are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore all of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.

Another way to think about it is that a molecule in contact with a neighbor is in a lower state of energy than if it weren't in contact with a neighbor. The interior molecules all have as many neighbors as they can possibly have. But the boundary molecules have fewer neighbors than interior molecules and are therefore in a higher state of energy. For the liquid to minimize its energy state, it must minimize its number of boundary molecules and therefore minimize its surface area.White, Harvey E. ''Modern College Physics'', van Nostrand 1948

As a result of this minimizing of surface area, the surface will assume the smoothest flattest shape it can (rigorous proof that "smooth" shapes minimize surface area relies on use of the Euler-Lagrange Equation ). Since any curvature in the surface shape results in higher area, a higher energy will also result. Consequently, the surface will push back on the disturbing object in much the same way a ball pushed uphill will push back to minimize its gravitational energy.

EFFECTS IN EVERYDAY LIFE

Some examples of the effects of surface tension seen with ordinary water:

Beading of rain water on the surface of a waxed automobile. Water adheres weakly to wax and strongly to itself, so water clusters in drops. Surface tension gives them their near-spherical shape, because a sphere has the smallest possible surface area to volume ratio

Formation of Drop s occurs when a mass of liquid is stretched. The animation shows water adhering to the faucet gaining mass until it is stretched to a point where the surface tension can no longer bind it to the faucet. It then separates and surface tension forms the drop into a sphere. If a stream of water were running from the faucet, the stream would break up into drops during its fall. This is because of gravity stretching the stream, and surface tension then pinching it into spheres.¹

Floatation of objects denser than water occurs when the object is nonwettable and its weight is small enough to be born by the forces arising from surface tension.

Surface tension has a big influence on other common phenomena, especially when certain substances, Surfactants , are used to decrease it:

Soap Bubble s have very large surface areas for very small masses. Bubbles cannot be formed from pure water because water has very high surface tension, but the use of surfactants can reduce the surface tension more than tenfold, making it very easy to increase its surface area.

Emulsion s are a type of solution where surface tension is also very important. Oil will not spontaneously mix with water, but the presence of a surfactant provides a decrease in surface tension that allows the formation of small droplets of oil in the bulk of water (or vice versa).

BASIC PHYSICS

Definition

Surface tension is represented by the symbol '' σ '', '' γ '' or '' T '' and is defined as the force along a line of unit length where the force is parallel to the surface but perpendicular to the line. One way to picture this is to imagine a flat soap film bounded on one side by a taut thread of length, ''L''. The thread will be pulled toward the interior of the film by a force equal to ''2γL'' (the factor of 2 is because the soap film has two sides hence two surfaces). Surface tension is therefore measured in Newton s per Meter (N·m^-1), although the Cgs unit of Dynes per Cm is normally used.²

An equivalent definition of surface tension, in order to treat its Thermodynamics , is Work done per unit area. As such, in order to increase the surface area of a mass of liquid an amount, ''δA'', a quantity of work, ''γδA'', is needed. Since mechanical systems try to find a state of minimum potential energy, a free droplet of liquid naturally assumes a spherical shape. This is because a sphere has the minimum surface area for a given volume. Therefore surface tension can be also measured in joules per square metre (J·m^-2), or, in the Cgs system, Ergs per Cm ².

The equivalence of measurement of energy per unit area to force per unit length can be proven by Dimensional Analysis .

Water striders

The photograph shows Water Strider s standing on the surface of a pond. It is clearly visible that their feet cause indentations in the water's surface. And it is intuitively evident that the surface with indentations has more surface area than a flat surface. If surface tension tends to minimize surface area, how is it that the water striders are increasing the surface area?

Recall that what nature really tries to minimize is potential energy. By increasing the surface area of the water, the water striders have increased the potential energy of that surface. But note also that the water striders' center of mass is lower than it would be if they were standing on a flat surface. So their potential energy is decreased. Indeed when you combine the two effects, the net potential energy is minimized. If the water striders depressed the surface any more, the increased surface energy would more than cancel the decreased energy of lowering the insects' center of mass. If they depressed the surface any less, their higher center of mass would more than cancel the reduction in surface energy.³

The photo of the water striders also illustrates the notion of surface tension being like having an elastic film over the surface of the liquid. In the surface depressions at their feet it is easy to see that the reaction of that imagined elastic film is exactly countering the weight of the insects.

Surface curvature and pressure

]]

If no force acts normal to a tensioned surface, the surface must remain flat. But if the pressure on one side of the surface differs from pressure on the other side, the pressure difference times surface area results in a normal force. In order for the surface tension forces to cancel the force due to pressure, the surface must be curved. The diagram shows how surface curvature of a tiny patch of surface leads to a net component of surface tension forces acting normal to the center of the patch. When all the forces are balanced, the resulting equation is known as the Young-Laplace Equation :

::

\Delta P\ =\ \gamma \left( rac{1}{R_x} + rac{1}{R_y} 
ight)

where:

''ΔP'' is the pressure difference.

''γ'' is surface tension.

''R_x'' and ''R_y'' are radii of curvature in each of the axes that are parallel to the surface.

Solutions to this equation determine the shape of water drops, puddles, menisci, soap bubbles, and all other shapes determined by surface tension (such as the shape of the impressions that a water strider's feet make on the surface of a pond).

The table below shows how the internal pressure of a water droplet increases with decreasing radius. For not very small drops the effect is subtle, but the pressure difference becomes enormous when the drop sizes approach the molecular size.

Liquid surface as a computer

To find the shape of the Minimal Surface bounded by some arbitrary shaped frame using strictly mathematical means can be a daunting task. Yet by fashioning the frame out of wire and dipping it in soap-solution, an approximately minimal surface (exact in the absence of gravity) will appear in the resulting soap-film within seconds. Without a single calculation, the soap-film arrives at a solution to a complex minimization equation on its own.

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{ Align

"right"

\gamma_{\mathrm{la}}\ >\ \gamma_{\mathrm{ls}}\ >\ 0

In the diagram, both the vertical and horizontal forces must cancel exactly at the contact point. The horizontal component of

\scriptstyle f_\mathrm{la}

is canceled by the adhesive force,

\scriptstyle f_\mathrm{A}

.

:

f_\mathrm{A}\ =\ f_\mathrm{la} \sin 	heta

The more telling balance of forces, though, is in the vertical direction. The vertical component of

\scriptstyle f_\mathrm{la}

must exactly cancel the force,

\scriptstyle f_\mathrm{ls}

.

:

f_\mathrm{ls}\ =\ -f_\mathrm{la} \cos 	heta

		"http://wwwinformationdelightinfo/information/entry/ethanol" class="copylinks">Ethanol
		"http://wwwinformationdelightinfo/information/entry/diethyl_ether" class="copylinks">Diethyl Ether
		"http://wwwinformationdelightinfo/information/entry/carbon_tetrachloride" class="copylinks">Carbon Tetrachloride
		"http://wwwinformationdelightinfo/information/entry/glycerol" class="copylinks">Glycerol
		"http://wwwinformationdelightinfo/information/entry/acetic_acid" class="copylinks">Acetic Acid
	Rowspan	"2" water
	Align	"center" 107°
	Align	"center" 90°
	Rowspan	"3" Methyl Iodide
	Align	"center" 29°
	Align	"center" 30°
	Align	"center" 33°
		"http://wwwinformationdelightinfo/information/entry/mercury_(element)" class="copylinks">Mercury
	Align	"center" 140°
	Colspan	"3" align="center" '''Some liquid/solid contact angles'''

METHODS OF MEASUREMENT

''Du Noüy Ring method'': The traditional method used to measure surface or interfacial tension. Wetting properties of the surface or interface have little influence on this measuring technique. Maximum pull exerted on the ring by the surface is measured.

''Wilhelmy plate method'': A universal method especially suited to check surface tension over long time intervals. A vertical plate of known perimeter is attached to a balance, and the force due to wetting is measured.⁴

''Spinning drop method'': This technique is ideal for measuring low interfacial tensions. The diameter of a drop within a heavy phase is measured while both are rotated.

''Pendant drop method'': Surface and interfacial tension can be measured by this technique, even at elevated temperatures and pressures. Geometry of a drop is analyzed optically. For details, see Drop .

''Bubble pressure method (Jaeger's method)'': A measurement technique for determining surface tension at short surface ages. Maximum pressure of each bubble is measured.

''Drop volume method'': A method for determining interfacial tension as a function of interface age. Liquid of one density is pumped into a second liquid of a different density and time between drops produced is measured.⁵

''Capillary rise method'': The end of a capillary is immersed into the solution. The height at which the solution reaches inside the capillary is related to the surface tension by the previously discussed equation.⁶

''Stalagmometric method'': A method of weighting and reading a drop of liquid.

''Sessile drop method'': A method for determining surface tension and Density by placing a drop on a substrate and measuring the Contact Angle (see Sessile Drop Technique ).⁷

EFFECTS

Liquid in a vertical tube

An old style Mercury Barometer consists of a vertical glass tube about 1 cm in diameter partially filled with mercury, and with a vacuum in the unfilled volume (see diagram to the right). Notice that the mercury level at the center of the tube is higher than at the edges, making the upper surface of the mercury dome-shaped. The center of mass of the entire column of mercury would be slightly lower if the top surface of the mercury were flat over the entire crossection of the tube. But the dome-shaped top gives slightly less surface area to the entire mass of mercury. Again the two effects combine to minimize the total potential energy. Such a surface shape is known as a convex Meniscus .

The reason we consider the surface area of the entire mass of mercury, including the part of the surface that is in contact with the glass, is because mercury does not adhere at all to glass. So the surface tension of the mercury acts over its entire surface area, including where it is in contact with the glass. If instead of glass, the tube were made out of copper, the situation would be very different. Mercury aggressively adheres to copper. So in a copper tube, the level of mercury at the center of the tube will be lower rather than higher than at the edges (that is, it would be a concave meniscus). In a situation where the liquid adheres to the walls of its container, we consider the part of the fluid's surface area that is in contact with the container to have ''negative'' surface tension. The fluid then works to maximize the contact surface area. So in this case increasing the area in contact with the container decreases rather than increases the potential energy. That decrease is enough to compensate for the increased potential energy associated with lifting the fluid near the walls of the container.

If a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong, surface tension can draw liquid up the tube in a phenomenon known as Capillary Action . The height the column is lifted to is given by:

::

h\ =\ rac {2\gamma_\mathrm{la} \cos	heta}{
ho g r}

where

$\scriptstyle h$ is the height the liquid is lifted,

$\scriptstyle \gamma_\mathrm{la}$ is the liquid-air surface tension,

$\scriptstyle
ho$ is the density of the liquid,

$\scriptstyle r$ is the radius of the capillary,

$\scriptstyle g$ is the acceleration due to gravity,

$\scriptstyle heta$ is the angle of contact described above. Note that if $\scriptstyle heta$ is greater than 90°, as with mercury in a glass container, the liquid will be depressed rather than lifted.

Puddles on a surface

Pouring mercury onto a horizontal flat sheet of glass results in a puddle that has a perceptible thickness (do not try this except under a fume hood. Mercury vapor is a toxic hazard). The puddle will spread out only to the point where it is a little under half a centimeter thick, and no thinner. Again this is due to the action of mercury's strong surface tension. The liquid mass flattens out because that brings as much of the mercury to as low a level as possible. But the surface tension, at the same time, is acting to reduce the total surface area. The result is the compromise of a puddle of a nearly fixed thickness.

The same surface tension demonstration can be done with water, but only on a surface made of a substance that the water does not adhere to. Wax is such a substance. Water poured onto a smooth, flat, horizontal wax surface, say a waxed sheet of glass, will behave similarly to the mercury poured onto glass.

The thickness of a puddle of liquid on a surface whose contact angle is 180° is given by:⁸

::

h\ =\ 2 \sqrt{rac{\gamma} {g
ho}}

where

In reality, the thicknesses of the puddles will be slightly less than what is predicted by the above formula because very few surfaces have a contact angle of 180° with any liquid. When the contact angle is less than 180°, the thickness is given by:

::

h\ =\  \sqrt{rac{2\gamma_\mathrm{la}\left( 1 - \cos 	heta 
ight)} {g
ho}}

For mercury on glass,

\scriptstyle \gamma_\mathrm{Hg}\ =\ 487\ \mathrm{rac{dyn}{cm}}

\scriptstyle 
ho_\mathrm{Hg}\ =\ 13.5\ \mathrm{rac{g}{cm^3}}

, and

\scriptstyle 	heta = 140^\circ

, which gives

\scriptstyle h_\mathrm{Hg}\ =\ 0.36\ \mathrm{cm}

. For water on paraffin at 25 °C,

\scriptstyle \gamma_\mathrm{H_2O}\ =\ 72\ \mathrm{rac{dyn}{cm}}

\scriptstyle 
ho_\mathrm{H_2O}\ = 1.0\ \mathrm{rac{g}{cm^3}}

, and

\scriptstyle 	heta = 107^\circ

which gives

\scriptstyle h_\mathrm{H_2O}\ =\ 0.49\ \mathrm{cm}

.

The formula also predicts that when the contact angle is 0°, the liquid will spread out into a micro-thin layer over the surface. Such a surface is said to be fully wettable by the liquid.

The break up of streams into drops

In day to day life we all observe that a stream of water emerging from a faucet will break up into droplets, no matter how smoothly the stream is emitted from the faucet. This is due to a phenomenon called the Plateau-Rayleigh Instability , which is entirely a consequence of the effects of surface tension.

The explanation of this instability begins with the existence of tiny perturbations in the stream. These are always present, no matter how smooth the stream is. If the perturbations are resolved into Sinusoidal components, we find that some components grow with time while others decay with time. Among those that grow with time, some grow at faster rates than others. Whether a component decays or grows, and how fast it grows is entirely a function of its wave number (a measure of how many peaks and troughs per centimeter) and the radius of the original cylindrical stream. The diagram to the right shows an exaggeration of a single component.

By assuming that all possible components exist initially in roughly equal (but minuscule) amplitudes, the size of the final drops can be predicted by determining by wave number which component grows the fastest. As time progresses, it is the component whose growth rate is maximum that will come to dominate and will eventually be the one that pinches the stream into drops.

Although a thorough understanding of how this happens requires a mathematical development (see references), the diagram can provide a conceptual understanding. Observe the two bands shown girdling the stream – one at a peak and the other at a trough of the wave. At the trough, the radius of the stream is smaller, hence according to the Young-Laplace Equation (discussed above) the pressure due to surface tension is increased. Likewise at the peak the radius of the stream is greater and, by the same reasoning, pressure due to surface tension is reduced. If this were the only effect, we would expect that the higher pressure in the trough would squeeze liquid into the lower pressure region in the peak. In this way we see how the wave grows in amplitude over time.

But the Young-Laplace Equation is influenced by two separate radius components. In this case one is the radius, already discussed, of the stream itself. The other is the radius of curvature of the wave itself. The fitted arcs in the diagram show these at a peak and at a trough. Observe that the radius of curvature at the trough is, in fact, negative, meaning that, according to Young-Laplace, it actually ''decreases'' the pressure in the trough. Likewise the radius of curvature at the peak is positive and increases the pressure in that region. The effect of these components is opposite the effects of the radius of the stream itself.

The two effects, in general, do not exactly cancel. One of them will have greater magnitude than the other, depending upon wave number and the initial radius of the stream. When the wave number is such that the radius of curvature of the wave dominates that of the radius of the stream, such components will decay over time. When the effect of the radius of the stream dominates that of the curvature of the wave, such components grow exponentially with time.

When all the math is done, it is found that unstable components (that is, components that grow over time) are only those where the product of the wave number with the initial radius is less than unity (

\scriptstyle kR_0 \ < \ 1

). The component that grows the fastest is the one whose wave number satisfies the equation:

::

kR_0 \ \simeq \ 0.697

THERMODYNAMICS

Thermodynamic definition

As stated above, the mechanical work needed to increase a surface is

\scriptstyle dW \ = \ \gamma dA

. Hence at constant temperature and pressure, surface tension equals Gibbs Free Energy per surface area:

::

\gamma = \left( rac{\partial G}{\partial A} 
ight)_{T,V,n}

where

\scriptstyle G

is Gibbs free energy and

\scriptstyle A

is the area.

Thermodynamics requires that all spontaneous changes of state are accompanied by a decrease in Gibbs free energy.

From this it is easy to understand why decreasing the surface area of a mass of liquid is always Spontaneous (

\scriptstyle \Delta G \ < \ 0

), provided it is not coupled to any other energy changes. It follows that in order to increase surface area, a certain amount of energy must be added.

Gibbs free energy is defined by the equation,⁹

\scriptstyle G \ = \ H \ - \ TS

, where

\scriptstyle H

is Enthalpy and

\scriptstyle S

is Entropy . Based upon this and the fact that surface tension is Gibbs free energy per unit area, it is possible to obtain the following expression for entropy per unit area:

::

\left( rac{\partial \gamma}{\partial T} 
ight)_{A,P}=-S^{A}

By rearranging the previous equations, Kelvin's Equation I is obtained. It states that surface enthalpy or surface energy (different from surface free energy) depends both on surface tension and its derivative with temperature at constant pressure by the relationship¹⁰

::

H^A\ =\ \gamma - T \left( rac {\partial \gamma}{\partial T} 
ight)_P

Influence of temperature

]]

Surface tension is dependent on temperature. For that reason, when a value is given for the surface tension of an interface, temperature must be explicitly stated. The general trend is that surface tension decreases with the increase of temperature, reaching a value of 0 at the Critical Temperature . For further details see Eötvös Rule . There are only empirical equations to relate surface tension and temperature:

Eötvös:¹¹

\gamma V^{2/3}=k(T_C-T)\,\!

$\scriptstyle V$ is the molar volume of that substance

$\scriptstyle T_C$ is the Critical Temperature

$\scriptstyle k$ is a constant for each substance.

For example for water ''k'' = 1.03 erg/°C (103 NJ / K ), ''V'' = 18 ml/mol and ''T''_C = 374 °C.

A variant on Eötvös is described by Ramay and '''Shields''':
::

\gamma V^{2/3} = k\left(T_C - T - 6
ight)

where the temperature offset of 6 Kelvins provides the formula with a better fit to reality at lower temperatures.

Guggenheim-Katayama:

\gamma = \gamma^o \left( 1-rac{T}{T_C} 
ight)^n

\scriptstyle \gamma^o

is a constant for each liquid and n is an empirical factor, whose value is 11/9 for organic liquids. This equation was also proposed by Van Der Waals , who further proposed that

\scriptstyle \gamma^0

could be given by the expression,

\scriptstyle K_2 T^{rac {1}{3}}_c P^{rac {2}{3}}_c

, where

\scriptstyle K_2

is a universal constant for all liquids, and

\scriptstyle P_c

is the Critical Pressure of the liquid (although later experiments found

\scriptstyle K_2

to vary to some degree from one liquid to another).

Both Guggenheim-Katayama and Eötvös take into account the fact that surface tension reaches 0 at the critical temperature, whereas Ramay and Shields fails to match reality at this endpoint.

Influence of solute concentration

Solutes can have different effects on surface tension depending on their structure:

No effect, for example Sugar

Increase of surface tension, Inorganic Salts

Decrease surface tension progressively, Alcohols

Decrease surface tension and, once a minimum is reached, no more effect: Surfactants

What complicates the effect is that a solute can exist in a different concentration at the surface of a solvent than in its bulk. This difference varies from one solute/solvent combination to another.

Gibbs Isotherm states that:

\Gamma\ =\ - rac{1}{RT} \left( rac{\partial \gamma}{\partial \ln C} 
ight)_{T,P}

$\scriptstyle \Gamma$ is known as surface concentration, it represents excess of solute per unit area of the surface over what would be present if the bulk concentration prevailed all the way to the surface. It has units of mol/m²

$\scriptstyle C$ is the concentration of the substance in the bulk solution.

$\scriptstyle R$ is the Gas Constant and $\scriptstyle T$ the Temperature

Certain assumptions are taken in its deduction, therefore Gibbs isotherm can only be applied to ideal (very dilute) solutions with two components.

Influence of particle size on vapour pressure

Starting from Clausius-Clapeyron Relation , another equation, attributed to Kelvin, is obtained. It explains why, because of surface tension, the Vapor Pressure for small droplets of liquid in suspension is greater than standard vapor pressure of that same liquid when the interface is flat. That is to say that when a liquid is forming small droplets, the equilibrium concentration of its vapor in its surroundings is greater. This arises because the pressure inside the droplet is greater than outside.

::

P_v^{fog}=P_v^o e^{rac{V 2\gamma}{RT r_k}}

$\scriptstyle P_v^o$ is the standard vapor pressure for that liquid at that temperature and pressure.

$\scriptstyle V$ is the molar volume.

$\scriptstyle R$ is the Gas Constant

r_k

is the Kelvin radius, the radius of the droplets.

The effect explains Supersaturation of vapors. In the absence of Nucleation sites, tiny droplets must form before they can evolve into larger droplets. This requires a vapor pressure many times the vapor pressure at the Phase Transition point.

This equation is also used in Catalyst chemistry to assess mesoporosity for solids.G. Ertl, H. Knözinger and J. Weitkamp; ''Handbook of heterogeneous catalysis,'' Vol. 2, page 430; Wiley-VCH; Weinheim; 1997

The effect can be viewed in terms of the average number of molecular neighbors of surface molecules (see diagram).

The table shows some calculated values of this effect for water at different drop sizes:

The effect becomes clear for very small drop sizes, as a drop of 1 nm radius has about 100 molecules inside, which is a quantity small enough to require a Quantum Mechanics analysis.

GALLERY OF EFFECTS

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Surface Tension