Speed of diffusion?

[Joe_Cool]

I was thinking about this today when I saw the end of “Deep Blue Sea” on Cinemax. The girl cuts herself and jumps into the water to attract the shark to attack her. Obviously it’s just a (really, really stupid) movie and the shark was responding mainly to her splash in the water, but you always hear about sharks’ sense of smell, and how they can smell one part per however many billions of blood in water.

So I got to wondering how fast the blood particles are diffused over a given area. When something drops into water you can see it spread rapidly, but how fast?

Does this speed depend on what’s being dissolved, or what it’s being dissolved in? In other words, would blood diffuse at different rates in gasoline or cooking oil, vs in water? Do blood, salt, and baking soda all diffuse at different rates in the same medium?

What’s the straight dope?

[xash]

*Originally posted by Joe_Cool *
**What’s the straight dope? **

aah…The Straight Dope is a neat website that answers questions like yours. then maybe sometimes it doesn’t

ohh… you said what ? ohh… i’m sure someone will come by with an answer…

::dives back into water to play with shark… 'tis better than being bitten by the straight dopers… ::

[xash]

a quick search could only yield a bit more information on the topic… no mention of actual speeds or comparisons of different solvents…but these links might be of interest…

http://web.ukonline.co.uk/webwise/spinneret/life/osmsis.htm

Diffusion is faster in gases than in liquids, and at higher temperatures, because heat speeds up the process by making particles move faster.

http://encyclopedia.com/printable/48781.html

Because of the random nature of the motion of molecules, the rate of diffusion of molecules out of any region in a substance is proportional to the concentration of molecules in that region, and the rate of diffusion into the region is proportional to the concentration of molecules in the surrounding regions. Thus, while molecules continuously flow both into and out of all regions, the net flow is from regions of higher concentration to regions of lower concentration. Generally, the greater the difference in concentration, the faster the diffusion.

…an increase in temperature represents an increase in the average molecular speed, diffusion occurs faster at higher temperatures.

[Joe_Cool]

So even though they don’t just come out and say it, your citations seem to imply that the speed depends on the medium, right?

[Squink]

Yes, the speed depends on the medium. The molecular weight, density, size and shape of the diffusing substance also affect the speed of diffusion.
The reason you don’t find tables of “typical” diffusion rates for compounds in various solvents is that there are too many variables involved to make the tables useful to anyone.

[Smeghead]

There’s a pretty simple equation for figuring it out that’s included in most college chemistry and/or physics textbooks. Sadly, I don’t remember what it is.

[Squink]

xash gave us the english language version of Fick’s Law:

the rate of diffusion into the region is proportional to the concentration of molecules in the surrounding regions

J[sub]z[/sub] = -D X d concentration/d z where J[sub]z[/sub] is flow in the z direction.
Practical calculations of the rate of diffusion are complicated because the the value of the proportionality factor D is sensitive to many different factors.