Table of Contents

• Prequiz...
• Just Plain Movement
• 1: Measuring movement
• 2: Directed vs. undirected movement
• 3: Measuring movement using flux
• Introducing Gradients
• 4: Gradients and diffusion
• 5: Watching diffusion happen (applet)
• 6: Visualising the Gradient
• Fick's Laws, Part I
• 7: Fick's First Law
• 8: The gradient in Fick's First Law
• 9: A familiar equation for Fick's First Law
• 10: Graphing Fick's First Law
• 11: How does flux depend on distance?
• 12: A fragrant example
• 13: General transport equations and Fick's First Law
• Review
• 14: Summary

A fragrant example

What this means is that as distance increases, flux falls precipitously!

We'll give an example. Let’s say you walk into a room wearing your latest favourite perfume / aftershave. The gradient between you and the rest of the room is steep, because no one else in the room is wearing your fragrance. A mosquito buzzing right next to your ear gets a nose full almost immediately – diffusion (flux) occurs very quickly over this short distance. However, diffusion to even a few centimetres away occurs much more slowly. More importantly, diffusion to the object of your desire, a person standing a metre or more away, occurs at a glacial pace (assuming you and the other person are standing still and not moving closer to each other).

Luckily, you don't have to wait around for diffusion to occur. What actually happens, the reason why a person all the way across the room can smell you within seconds, is that there are also air currents carrying the perfume.