LECTURE 6
Properties Of Fluids-Cont.
By Dr. Mohamed Fekry
2 nd Sem.1434
PROPERTIES OF FLUIDS
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Density (r) and Specific Volume (v)
Specific Gravity (SG)
Specific Weight (g)
Density of ideal gas
Coefficient 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
liquid
molecules)
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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Capillary Effect
Another 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 the
liquid.
Solving for h gives the capillary rise
to be
Capillary 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).
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