Dear Tim,

I am sorry, I was unclear (and shortcut the reality) when I talk about 
the barrier what I have in mind it that :

"In many crystallization experiments the high levels of saturation 
needed for nucleation are not
reached. An alternative mechanism is to achieve nucleation by 
introducing in the crystallization trial a
solid material, which is termed the nucleating agent, nucleant or seed 
[14]. Nucleation occurs on the
surface of this material, which creates a higher local concentration of 
macromolecules, lowers the
energy barrier for nucleation and bypasses the high kinetic barrier of 
spontaneous nucleation. A lower
level of supersaturation is required under such circumstances as the 
nucleation step has been bypassed."

 From : An Overview of Biological Macromolecule Crystallization
Irene Russo Krauss 1, Antonello Merlino 1,2, Alessandro Vergara 1,2 and 
Filomena Sica 1,2,*

For the growing (after the nucleus is obtain), I agree with you I was 
wrong when I illustrate that with a run after a jump over a barrier. 
It's actually a down run (small slope).

It's clearly better explain here than in my message :

"Recent analyses of protein crystallization thermodynamics have shown 
that 
G cryst is only moderately negative, i.e. within the range 10 to 
100 kJ mol1 (Vekilov, 2003). This is in contrast to the 
crystallization of inorganic salts, e.g. NaCl, where the absolute 
free-energy change can be of significantly greater magnitude. In the 
case of protein crystallization 
G cryst is small and can easily be 
shifted to positive values by concurrent solution phenomena, making 
crystallization thermodynamically
impossible. This explains why protein crystallization is so sensitive to 
even the slightest changes in conditions."

 From : Entropy and surface engineering in protein
crystallization .

Zygmunt S. Derewendaa* and
Peter G. Vekilovb


I hope this clarify my point of view. Moreover, thank you for your 
practical illustration which place the kids to the center of the 
demonstration. More interesting and fun.

Cheers,

Nicolas

Nicolas Foos
PhD
Structural Biology Group
European Synchrotron Radiation Facility (E.S.R.F)
71, avenue des Martyrs
CS 40220
38043 GRENOBLE Cedex 9
+33 (0)6 76 88 14 87
+33 (0)4 76 88 45 19

On 05/01/2017 08:56, Tim Gruene wrote:
> Dear Nicolas,
>
> are you sure this concept is correct?
>
> The entropy of molecules arranged in a crystal must be greatly larger than in
> solution. Hence the driving force for crystallisation is actually a drop in
> energy, and I am not sure there is actually a barrier.
>
> I would take the kids to the play ground and let them run around randomly.
> When I blow a wistle they should line up in an orderly manner.
>
> I am sure it will take some time before some sort of order is achieved, for
> the kids could face in various directions, or line up in blocks, or other
> shapes. Once a seed is there, i.e. once e.g. 4-5 kids have created a regular
> block, it will be much easier for the rest to line up (this is consistent with
> Patrick's explanation).
>
> Cheers,
> Tim
>
> On Wednesday 04 January 2017 05:45:50 PM Nicolas FOOS wrote:
>> Dear Evette,
>>
>> If I was is your situation (explaining nucleation and other concept). I
>> will discuss in terms of energy.
>>
>> I mean obtaining the initial nuclei is the "costly" step in terms of
>> energy. To represent that, out the classical curve of energy, I will use
>> a metaphoric representation such as jump over a barrier and run after.
>>
>> With this analogy, it's possible to explain that the first step is
>> difficult and the second more accessible. If the barrier is to high,
>> it's impossible to continue and run. If you don't have any barrier it's
>> easy to run and if you only have a small barrier is not to difficult to
>> jump over and run. But It also allow you to explain that if you
>> facilitate the apparition of the first "surface" thanks to appropriate
>> method (seeding, dust...) you can help the first step (to continue with
>> the barrier story, it like you have ladder to help, or the ability to
>> decrease the size of the barrier.
>>
>> For why the crystal and how, I will maybe use the example of orange
>> pyramid in the food store. Orange are stable together because they have
>> enough contact, because they have relatively homogeneous shape. If you
>> mixed orange with water melon it's difficult to obtain nice pyramid.
>>
>> For crystallization experiment which work, I have no Idea out of the one
>> you already mentioned.
>>
>>
>> Hope this help.
>>
>> Nicolas
>>
>> Nicolas Foos
>> PhD
>> Structural Biology Group
>> European Synchrotron Radiation Facility (E.S.R.F)
>> 71, avenue des Martyrs
>> CS 40220
>> 38043 GRENOBLE Cedex 9
>> +33 (0)6 76 88 14 87
>> +33 (0)4 76 88 45 19
>>
>> On 30/12/2016 11:06, Radisky, Evette S., Ph.D. wrote:
>>> Can anyone point to some especially useful resources to help explain
>>> to kids (pre-chemistry, ~age 10-12) how and why molecules crystallize?
>>> Maybe a good online movie or animation?  I am especially needing help
>>> with the concept of nucleation, and why nucleation is slower and then
>>> crystal growth faster once nuclei have formed.  I have been
>>> supervising some experiments growing sucrose crystals from
>>> supersaturated solutions, which have worked really well, but I am
>>> having more difficulty in explaining the underlying fundamental
>>> concepts in a way that is understandable to the kids.
>>>
>>> Thanks!
>>> Evette
>>>
>>> Evette Radisky, PhD
>>>
>>> Associate Professor of Cancer Biology
>>>
>>> Mayo Clinic Cancer Center
>>>
>>> Griffin Cancer Research Building
>>>
>>> 4500 San Pablo Road <x-apple-data-detectors://3/0>
>>>
>>> Jacksonville, FL 32224 <x-apple-data-detectors://3/0>
>>>
>>> tel: 904-953-6372 <tel:904-953-6372>
>>>
>>> fax: 904-953-0277 <tel:904-953-0277>