fluid mechanics-i spring 2010 lecture #02. 2 viscosity dependence coefficient of viscosity for...
Post on 13-Dec-2015
215 Views
Preview:
TRANSCRIPT
Fluid Mechanics-ISpring 2010
Lecture #02
2
Viscosity Dependence
Coefficient of Viscosity
For Liquids, No effect of pressure on dynamic or Kinematic viscosity.
For gases, viscosity varies with pressure as
( , )p T
p
3
Viscosity as a function of Temperature
4
5
Why it is? 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 droplets from rain or dew hang from branches or leaves of trees;
A soap bubble released into the air forms a spherical shape;
Water beads up into small drops on flower petals
6
Interface
Interface
High Density of Molecules
Less dense Molecules
7
Interfacial Forces
The liquid molecules tend to minimize their surface area because of surface tension.
8
Surface Tension
Surface tension is generated due to intermolecular forces at the interface between two immiscible fluids with density difference.
The interface behaves like a stretched membrane.
9
Examples of Surface Tension
Water drops formation on smooth surfaces
Insects walking on Water surface
10
Surface Energy
Amount of energy in a stretched membrane.
Energy in a stretched membrane leads to pressure jump in curved surfaces.
11
Surface Tension Coefficient ( )
Surface energy per unit area is called surface tension coefficient.
Surface Energy
Area
Force
Distance
12
Surface Tension in a Hatched Cylinder
13
Surface Tension in a Spherical Droplet
14
Surface Tension for a General Curved Surface
15
Contact Angle (Liquid-Solid Interface)
If θ<90 Wetting Liquid
If θ>90 Non-wetting Liquid
16
Example of Contact Angle
Water wets soap but does not wet wax.
The rise or fall of liquid in a narrow tube is due to the
combined effect of contact angle and surface tension. This is known as Capillary Effect.
17
Capillary Tube
Patm
PatmPatm
top related