Fluids%20

1
Fluids Special Topics

• Surface Tension
• Capillary Action
• Viscous Fluid Flow
• Diffusion

2
Surface Tension

• Molecule A attracted equally in all directions
• Molecule B attracted to the bulk of the fluid
• Surface molecules being pulled on cause
  contraction
• Surface area is minimized
• Needle lying on surface supported by surface
  tension

Fig. 9.39, p. 300
Fig. 9.40, p. 300
3
Surface of Liquids

• Cohesive forces between like molecules
• Adhesive forces between unlike molecules
• Contact angle ?

Adhesive gt Cohesive
Adhesive lt Cohesive
Fig. 9.43, p. 302
4
Capillary Action

• Combines surface tension with contact angle

Fig. 9.45, p. 303
Fig. 9.46, p. 303
Note h lt 0 if ? gt 90
5
Viscous Flow

• Adhesive forces make velocity zero (v 0) at
  wall
• Highest v at center
• Flow is still streamlined
• For tubes use Poiseuilles Law for flow rate

Ideal Fluid
Viscous Fluid
Fig. 9.47, p. 304
Fig. 9.49, p. 305
6
Turbulent Flow

• For sufficiently high v, streamlined flow turns
  to turbulent flow
• Onset of turbulence depends on Reynolds number, RN

2,000
3,000
RN
Streamlined
Unstable
Turbulent
Flow
7
Diffusion

• Molecules move from region of high concentration
  to region of low concentration
• Due to their kinetic energy
• Governed by Ficks Law for diffusion rate

Fig. 9.50, p. 307