Prepared by

Engr. Joseph R. Ortenero

Mapua Institute of Technology at Laguna

Malayan Colleges Laguna

Fluid Properties

Surface Tension

represents the stretching work that

needs to be done to increase the

surface area of a liquid by a unit

amount.

The surface tension of a substance

can be changed considerably by

impurities.

Therefore, certain chemicals, called

surfactants, can be added to a liquid to

decrease its surface tension.

Surfactants are

compounds that lower the

surface tension of a liquid,

allowing easier spreading,

and lowering of the

interfacial tension

between two liquids, or

between a liquid and a

solid.

Then a force balance on the

movable wire gives F= 2bσ,

and thus the surface tension

can be expressed as

Note that surface tension acts

along the circumference and

the pressure acts on the area

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.

Such narrow tubes or

confined flow channels are

called capillaries.

The strength of the capillary effect is

quantified by the contact (or wetting)

angle φ, defined as the angle that the

tangent to the liquid surface makes

with the solid surface at the point of

contact.

CAPILLARY RISE

Viscosity

The shear force acting on a Newtonian fluid layer (or, by Newton’s third law, the force acting on the plate) is

where again A is the contact area between the plate and the fluid.

Then the force F required

to move the upper plate at a

constant velocity of V while

the lower plate remains

stationary is

Sample Problem

The viscosity of a fluid is to be measured by a viscometer

constructed of two 75-cm-long concentric cylinders. The

outer diameter of the inner cylinder is 15 cm, and the gap

between the two cylinders is 0.12 cm. The inner cylinder is

rotated at 200 rpm, and the torque is measured to be 0.8 N-

m. Determine the viscosity of the fluid.

Buoyancy

ARCHIMEDES PRINCIPLE

The buoyant force acting on a body immersed in a fluid is

equal to the weight of the fluid displaced by the body, and it

acts upward through the centroid of the displaced volume

For floating bodies, the weight

of the entire body must be

equal to the buoyant force,

which is the weight of the fluid

whose volume is equal to the

volume of the submerged

portion of the floating body.

Sample Problem

WEIGHT LOSS OF AN OBJECT IN SEAWATER

A crane is used to lower weights into the sea (density 1025

kg/m3) for an underwater construction project. Determine

the tension in the rope of the crane due to a rectangular 0.4

m x 0.4-m x 3-m concrete block (density 2300 kg/m3)

when it is (a) suspended in the air and (b) completely

immersed in water.

End Of Lecture