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 > Cohesive
Adhesive < Cohesive
Fig. 9.43, p. 302

4. Combines surface tension with contact angle
Capillary Action
Combines surface tension with contact angle
Fig. 9.45, p. 303
Fig. 9.46, p. 303
Note: h < 0 if  > 90°

5. Viscous Flow Adhesive forces make velocity zero (v = 0) at wall
Highest v at center
Flow is still streamlined
For tubes use Poiseuille’s 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
RN:
2,000
3,000
Flow:
Streamlined
Unstable
Turbulent

7. Diffusion
Molecules move from region of high concentration to region of low concentration
Due to their kinetic energy
Governed by Fick’s Law for diffusion rate
Fig. 9.50, p. 307