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Does the smell of a substance diminish wrt time?

  1. May 18, 2008 #1

    Simfish

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    And if so, how long does it take for the smell of something (say, a perfume, food, or artificial source) to decrease? And is this length significant in the span of say, a human lifetime?

    I'd expect the smell of a substance to decrease like a function like 1/x. This hunch is based on the fact that something has to release bits of its own chemicals into the air in order to produce smells (however, it only needs to release a very little bit of itself in order to be noticed - nonetheless - with enough time, it would presumably release less and less of itself such that eventually what it releases of itself would become negligible for smell [although theoretically, interactions with the surrounding air could diminish the substance's "pure" mass with respect to very large timescales])
     
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  3. May 18, 2008 #2

    DaveC426913

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    It has much less to do with the substance than with the brain's perception.

    Sight, smell, sound etc. will cause less stimulation in the brain over time. To keep the brain stimulated, you must chnge the stimulant or remove it and reintroduce it.
     
  4. May 18, 2008 #3

    Simfish

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    Ah yes. But I'm still intrigued by the substance itself (it may be more relevant to say, animals that heavily rely on smell for example)
     
  5. May 18, 2008 #4
    Graham's law gives an approximate for the rate of diffusion of a gas in air--- rate of diffusion is inversely proportional to the square rt of its density.
     
  6. May 18, 2008 #5
    If I were you, I would look up the following things. A book on reaction kinetics (They vary depending on which reaction you wish to model). They can vary a lot, remember, this is chemistry. :)

    Secondly, you should check out Fick's laws. They are PDE governing the rate of diffusion into/onto substances.
     
  7. May 19, 2008 #6

    Simfish

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    Yeah, so the "diffusion" of the substance would involve the diffusion of solid molecules suspended in air.

    I'm more concerned with the intuition than the equations here (as you can't really fit an equation onto the diffusion of a real life substance, especially since it's hard to measure the concentration of any particular real life substance [such as a perfume] in order to use Fick's law and to see if it applies). Maybe the levels of some perfumes could be measured. But this isn't a reaction strictly speaking. It's probably due more to air movements that "blow" molecules off the substance in question. (is it)? If so, the air movements [possibly involving temperature?] would need to be taken into consideration. Of course, rate of diffusion would depend on the surface area of the substance that could provide "available molecules" to be blown into the air.

    But I'm concerned about these questions:
    (a) how general is the diminishing of smell wrt time?
    (b) are the fundamental "equations" different for different substances? The way I envision it, perfumes probably have molecules that are easily transported into the air and molecules that aren't easily transported into the air. the question is, how numerous are the molecules that are easily transported into the air, as a ratio of the molecules that aren't easily transported into the air. Is this ratio usually very small for most solid substances? What of biological substances vs. plastics vs. metallic substances? I know some animals can pretty much smell *anything*
     
  8. May 19, 2008 #7

    Mapes

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    This isn't how it works. Any object at non-zero temperature has what is called a vapor pressure; some of the molecules have sufficient energy (through random energy fluctuations) to leap off the object. This process occurs whether the object is in air or vacuum and continues until the material is completely dispersed. The rate is proportional to surface area and is very strongly (exponentially) dependent on temperature.
     
  9. May 19, 2008 #8

    Simfish

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    In a real life scenario, wouldn't "diffusion" be due to *both* vaporization and air movements?
     
  10. May 19, 2008 #9

    DaveC426913

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    These equations do model "real life". It's just a matter of how many factors you wish to consider. If you are looking for an answer to your OP, you'll want to isolate and quantify each factor to the degree of your choosing.
     
  11. May 19, 2008 #10

    Mapes

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    I want to get you using the right terminology so you'll be able to find and study the literature effectively. The whole process is often referred to as mass transport. Its components are

    1. Evaporation/sublimation/vaporization (these terms are all commonly used for a solid -> vapor phase transformation, where the molecule disconnects from the solid); and
    2. Diffusion (flux in response to a variation in material concentration) and advection (flux due to fluid motion).

    In terms of rates, 1 and 2 would be modeled as occurring in series; the molecule has to let go of the solid state before it can move through the air. For most materials, the first is by far the slowest step. For perfume, the rates are more even.
     
  12. May 19, 2008 #11

    DaveC426913

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    This is of course because perfumes are carefully designed with volatility in mind.
     
  13. May 19, 2008 #12

    Simfish

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    Thanks for the clarification. Yeah, the perfume example was just to clarify possible differences in fluid transport.

    I'm interested in animal smell (which probably can detect substances in the air as a result of advection - in which case the substance concentration would flux into the air at much lower levels). Although a search on google scholar of smell + advection doesn't produce anything desirable - yet.

    http://ask.metafilter.com/33550/How-can-a-moth-trace-the-source-of-pheromones-up-to-11-km-away

    ==
    Also - somewhat unrelated question - but does increasing the humidity of air tend to make some substances vaporize more easily and other substances vaporize less easily? What of increasing the humidity of other substances liquid at room temperatures? (say bromine). is there a fundamental difference between the humidity of those substances liquid at room temperature and ones that are gas at room temperature? (and how they affect advection and vaporization rates?)
     
  14. Jun 6, 2008 #13
    Also check the theory on Brownian Motion.

    Regards
     
  15. Jun 6, 2008 #14

    DaveC426913

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    Unless I'm mistaken Brownian motion simply deals with random jostling on very short scales - the so-called random walk. I don't think it plays any significant part in transportation of molecules over any practical distance. In fact, I read somehere that, by this method, atoms make very little progress. It could take hours or more to travel only a dozen feet (because a molecule encounters untold numbers of atoms in its way, redirecting it progress). Most diffusion happens by air convection.
     
    Last edited: Jun 6, 2008
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