LECTURE 6

Properties Of Fluids-Cont.

By Dr. Mohamed Fekry

2 nd Sem.1434

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PROPERTIES OF FLUIDS

Density (r) and Specific Volume (v)Specific Gravity (SG)Specific Weight (g)Density of ideal gasCoefficient of Compressibility (k)Coefficient of Volume Expansion (b)Viscosity (m)Surface Tension (s)& Capillary Effect (h)

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2–7 SURFACE TENSION AND CAPILLARY EFFECT

It is often observed that a drop of blood forms a hump on a horizontal glass; a drop of mercury forms a near-perfect sphere and can be rolled just like a steel ball over a smooth surface; water dripping from a leaky faucet falls as spherical droplets; a soap bubble released into the air forms a spherical shape

In these and other observances, liquid droplets behave like small spherical balloons filled with the liquid, and the surface of the liquid acts like a stretched elastic membrane under tension. The pulling force that causes this tension acts parallel to the surface and is due to the attractive forces between the molecules of the liquid. The magnitude of this force per unit

length is called surface tension ss and is

usually expressed in the unit N/m (or lbf/ft in English units).

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Molecular forces:Cohesion: Cohesion enables a liquid to resist tensile stress (inner force between liquidmolecules)Adhesion: adhesion enables it to adhere to another body (attraction force between liquids,and a solid surface).Definition: Surface tension (σ): A liquid’s ability to resist tension.

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The magnitude of the capillary rise in a circular tube can be determined from a force balance on the cylindrical liquid column of height h in the tube

The weight of the liquid column is approximately

the vertical component of the surface tension force

Fsurface = 2pRss cos f

Equating the vertical component of the surface tension force to the weight gives

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Solving for h gives the capillary rise to beCapillary rise:

This relation is also valid for nonwetting liquids (such as mercury in glass) and gives the capillary drop. In this case f > 90° and thus cos f < 0, which makes h negative. Therefore, a negative value of capillary rise corresponds to a capillary drop (Fig. 2–25).

Capillary EffectAnother interesting consequence of surface tension is the capillary effect,which is the rise or fall of a liquid in a small-diameter tube inserted into theliquid.

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