I spend quite a bit of my time talking to different groups about nanotechnology, including its potential and its challenges. And as a result, I’m constantly on the prowl for new ways of illustrating why nanotechnology is important. In particular, I’ve been keeping my eyes peeled for a quick and dirty (and fun) demonstration to show that size matters.

Which is why I finally cracked this weekend and started messing around with packs of Mentos Mints and bottles of Coke.

One of the important ideas underpinning nanotechnology is that, as stuff gets smaller, things change. It may be that the smaller stuff can get to new places or be used in new ways. It may just be that the stuff is able to do more of its “stuff” when it’s smaller. Or it may be that the original stuff starts behaving like completely different stuff when it gets really small.

Whatever, when it comes to nanotechnology, size matters.

But how do you convince someone of this when they can’t see or experience what is happening at the nanoscale? After all, we are all endowed with brains that have evolved to handle things we can see and touch—not stuff that is invisible to the naked eye.

One approach is to use analogies between stuff that can be seen and touched, and nanoscale materials that cannot be experienced so readily. Along these lines, I’ve been wondering for some time whether the notorious reaction between Mentos and Coke could be exploited in some way to demonstrate aspects of nanotechnology.

Dropping Mentos into a bottle of coke causes a rapid release of carbon dioxide from the liquid, and a frothy geyser to erupt from the container. (For those of you who have no idea of what I’m talking about, just check out the videos at Eepybird.com). If it’s particle surface that drives the reaction between the Mentos and the Coke, grinding the candy up into smaller bits before adding it to should lead to more vigorous “eruption.”

The result—if it works—lots of fun, and a great illustration of one way in which size matters.

Having nothing better to do this weekend, I drafted my kids and an unwitting friend of my daughters into testing the idea. The concept—crush a couple of Mentos into medium and small bits, add to a 2 liter bottle of Coke, and watch what happens.

Saved for posterity, here’s the video of the great event:

Unfortunately, the finely crushed Mentos didn’t create as stunningly superior a geyser as I had hoped. Lesson number one—there’s often a yawning chasm between hypothesis and reality. However, there was a clear size-effect: The medium sized chunks of candy gave the highest geyser, while the finest chunks led to the longest reaction.

Clearly size mattered—just not in the way that might have been predicted…

Despite the disappointing performance of the fine stuff in this instance, the experience has convinced me there’s considerable mileage in using Mentos to explore some of the ideas underpinning nanotechnology. The experiment clearly demonstrates to those involved that making something into smaller pieces changes how it behaves—that’s a pretty important concept.

But that’s just the beginning. Mentos are a great example of a particle with a core-shell structure—the outside of each Mento is different to the inside. Many engineered nanoparticles have a similar structure, so we’re on good analogy ground here.

It’s likely that the Mentos’ outer shell has something to do with the vigor of the reaction with Coke—as New Scientist reported last year, surface roughness and chemistry probably play a role in making the whole Mentos-Coke thing work. So just crushing the candy up wouldn’t necessarily make the reaction go that much faster, as you’re not adding any more of the outer coating to the mix.

However, what if that outer coating was removed? I haven’t tried this, but it would be a cool experiment to wash (or suck perpahs) the outer coating off the Mentos, and see how it affects their geyser-forming properties. You could even go one step further, and see how crushing the denuded Mentos into increasingly finer particles changed things.

This could have the makings of a fun experiment for exploring the importance of size and surfaces—and all with a pack of mints and a bottle of Coke. How much simpler could things get?

Of course, the down-side is that someone needs to clear the mess up afterwards!