SURFACE TENSION RL NO:- 223 SHAH POOJAN .T. RL NO:- 225 PATEL KISHAN .A. RL NO:- 229 THAKKAR SHIV.S. RL NO:- 166 RAMI DHAVAL This presentation describes the basics of the thermodynamics of surfaces. The branch of science studying surface phenomena is called “colloid science”. For additional reading, please, refer to chapter 12 of “Physical Pharmacy” by A. Martin (Lea & Fibiger, Philadelphia, London, 1993).

Definition of surface tension: The interior molecules are equally attracted by all neighboring molecules with the same force, so that the resulting force is zero. The molecules at the surface are attracted by molecules from inside, whereas no force from outside the surface. Hence the resulting force is towards the interior of the liquid. Let us compare forces acting on molecules in the bulk of a phase (liquid, solid) and its interface with the vapor. We know that attraction (“cohesive”) forces act between all molecules of the phase. (Otherwise the molecules would not stay together and the phase would not exist.) In the bulk a molecule experiences attraction forces from the “neighbors” in all directions. The molecule at the interface does not experience same attraction form the side of the vapor; the net attraction forces are directed inward. That means that some work against the attractive forces must be done in order to transfer the molecule from the bulk to the interface. In other words, the interface molecules have excessive free energy compared to the bulk molecules. The surface tension then causes drops of liquid to be round since the surface of the liquid is being minimized that way.

Everyday phenomenon of surface tension:

Surface tension is very common in everyday life Surface tension is very common in everyday life. Surface tension dominates world below millimeter. Make use of it is attractive, however, current utilizations in MEMS are limited.

Formation of Surface Separation of liquid requires Formation of the interface is equivalent to “moving molecules” from the bulk to the surface. This process always requires a work to be performed over the system. The work is needed for “breaking” the cohesive forces existing between the molecules in the bulk. Formation of a surface is exemplified by separation of a liquid along the interface. Separation of liquid requires work against cohesion forces

Surface? - Unfavorable! There is a non-compensated molecular potential on the surface Therefore formation of surface is always accompanied by an increase in the free energy (G > 0) Therefore formation of surface is always unfavorable Since work is always needed to form the interface this process is always “thermodynamically unfavorable”.

Surface Tension Surface tension ( or ) - increase in free energy as a result of formation of 1 unit (cm2, m2) of surface Formation of surface of square   results in increase in the free energy G: The surface tension is defined as the amount of work needed to form a unit of the surface area. Since work of the interface formation is always positive the surface tension is positive too.

Surface Tension is a Force! L Force: 2L units: [N/m] Soap film The surface area is similar to a force in mechanics. The experiment with a wireframe apparatus demonstrates the dynamic character of the surface surface. Since the increase in the surface area of the film is unfavorable a force (e.g. hanging mass) must be applied to stretch the foam. The force opposing the stretching applied to the moving bar (by the foam) is proportional to the surface tension. Surface tension in a vector directed the same way as the opposing force. Force: mg [N, Newton] Surface tension is a vector, i.e. has direction

Measurement of Surface Tension “Ring tearing” technique External force Surface tension x fT Force Ring tearing x One way to measure surface tension at the interface surface-vapor is to use a ring tearing technique. When the ring is is carefully removed from the solution the liquid is pulled out of the plane of the interface due to the wetting phenomena. The weight of the pulled out liquid at the moment of the ring detachment is measured using a dynamometer. This weight (the force necessary to detach the ring) is proportional to the surface tension. Other methods for determining the surface tension include use of the capillary rise principle or measuring the maximal pressure of an air bubble using a thin capillary.

CAPILLARITY

DEFINITION…. Ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity Capillary action is sometimes called capillarity, capillary motion, or wicking

EXAMPLES…. Drawing up of liquids between the hairs of a paint-brush Drainage of constantly produced tear fluid from the eye Observed in thin layer chromatography draws ink to the tips of fountain pen nibs moving groundwater from wet areas of the soil to dry areas

TYPES….. Capillarities are of two types Capillarity rise Capillarity fall

Capillary rise method For zero contact angle, Where g is gravity = 9,8 m/s2

For the rise of a liquid up a narrow capillary N.B. In practice, the capillary rise method is only used when the contact angle is zero, owing to the uncertainty in measuring contact angles correctly

CAPILLARITY RISE Tendency of liquids to rise in tubes of small diameter in opposition to, external forces like gravity

CAPILLARITY FALL Tendency of liquids to be depressed in tubes of small diameter in opposition to, external forces like gravity

APPLICATIONS… Lubricating oil spread easily on all parts because of their low surface tension. Cotton dresses are preferred in summer because cotton dresses have fine pores which act as capillaries for sweat. Dirt get removed when detergents are added while washing clothes because surface tension of water is reduced.

A fabric can be waterproof, by adding suitable waterproofing materials to the fabric. This addition increases the angle of contact, thereby making the fabric waterproof.

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