Date: Fri, 19 May 2000 20:57:48 -0700
To: balloondeco@balloonhq.com
From: Mark Balzer <mbalzer@balloonhq.com>
Subject: Re: 16" Gumball Hearts w Hi Float-V00#137

>It bothered me a bit that you say there is transmission of helium
>into the air filled insider balloons.
>From a purely scientific principle if the pressure is equal on both
>sides of a membrane then there is no motion across the membrane.

Your statement is only true for impermeable membranes, which balloons are not.


>So why are you saying that the helium moves.

Because the concentration gradient across the balloon wall drives diffusion
of each gas species through the permeable balloon wall according to "Fick's
laws of diffusion."


>Are you filling the two to different pressures?

Different pressures will have an effect, but are not necessary.  The
concentration gradient is the major driver.

The term "concentration gradient" refers to the change in the gas
concentration vs distance.

Initially, the concentration of air changes from 100% to 0% across the
extremely thin insider balloon wall, and likewise the concentration of
helium changes from 0% to 100% across the extremely thin insider balloon
wall.  Mother nature doesn't like steep changes in concentration and tries
to make everything 50-50.  Believing in equal opportunity, she not only
makes the helium move (thru the balloon wall), she makes the air move too
-- just in the opposite direction.  The helium is simply more motivated to
please mom, so it moves faster (latex is more permeable to tiny, monoatomic
helium than to the larger molecules that make up air).  Since the helium is
entering faster than the air can leave, the air filled balloon swells.


I hope this helps,

Mark

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