Interfaces, Broccoli Soup, and Carcassonne

by Michael Mullaney on April 13, 2009

A snapshot of the molecular interface between water (top) and a solid (bottom)

A snapshot of the molecular interface between water (top) and a solid (bottom)

We released a new story today about an interesting interdisciplinary collaboration between professors Shekhar Garde and Pawel Keblinski. Here’s the problem they set out to solve: How can one measure – at the nanoscale – the strength at which water bonds with a nanoparticle, if it’s physically impossible to separate or peel apart the two materials?

Well, you can’t – at least not directly. But there is a nice, neat workaround. Measuring how quickly heat passes from the solid into the liquid, Garde and Keblinski found, correlates directly with how strongly the two materials are bonded. When reading the story, and talking with the researchers, it becomes abundantly clear that interfaces are the star of the show. So let’s talk interfaces.

An interface is a common boundary shared by two different bulk materials. Think of beaker full of water. There are three bulk materials involved: water, the beaker, and the air above the beaker. But wherever the water meets the beaker, or the water meets the air, or the air meets the beaker, there’s an interface.

The situation gets exponentially more complex at the cellular level. Here is a fantastic metaphor that Garde used when we met to talk (interface?) about the news story:

“If you peek into a biological cell, it will look crowded like a thick broccoli soup, with lots and lots of proteins which are sitting in water, and salt, and other molecules.  If you go through each and every location in the cell and ask, ‘Is this is bulk, or is this is an interface?’ – you’ll find that inside of a cell is made up of millions of interfaces … Interfaces are really where the action is. They’re the next frontier. And given that at least two things come together at an interface, it makes them fundamentally more difficult to understand and study than bulk materials.  And that is precisely why they are exciting for us to study.”

Broccoli soup

Broccoli soup - food for thought

Another key discovery of this research project is that the interaction of two bulk materials can be dramatically altered by “painting” an atomic-scale layer of molecules at the interface. Depending on the “paint” used, Garde and Pawel could change how fast heat flowed from the solid to the water, and in turn alter how strongly the two materials were bonded. The finding only reinforces that the interface is the star of the show.

In our meeting, Keblinski also used a vivid, fascinating metaphor to help illustrate the unique situation of interfaces. He likened an interface to a wall that isolates a city or a region. Think of the famous fortified French town of Carcassonne, with its huge, sprawling walls, towers, and ramparts. The physical wall itself had nothing to do with the bulk property – the commerce, music, population, culture, etc. – of historical Carcassonne, or its neighboring cities. But the wall did greatly impact how Carcassonne could interact with its neighbors.

Carcassonne

So interfaces, like a wall, separate two distinct systems. But interfaces and walls also have their own unique properties, independent of the bulk properties of the two neighboring systems, which can greatly impact the interaction of those two systems.

For scientific types interested to see the journal article, it can be found at: http://link.aps.org/doi/10.1103/PhysRevLett.102.156101