Babak Sanii and Atul Parikh tell Soft Matter  about their hot communication

Can you briefly describe what you achieved in this article?
We are generally familiar with the phenomenon of a drop of water on a surface - if the surface is hydrophilic (e.g. clean glass) it spreads out, if it's hydrophobic (e.g. Teflon) the water beads up.  Here we show that lipids, the molecules that form our cells' membranes, spread on both hydrophobic and hydrophilic surfaces.  However, using ellipsometry and fluorescence microscopy we see that their spreading morphologies and kinetics are quite different.  On hydrophobic surfaces lipids spread as monolayers, on hydrophilic ones as bilayers, and monolayers spread about twice as quickly.  
When we consider the amount of energy released by the modification of the hydrophobic surface we find that, all things being equal, monolayers should be spreading much, much faster than we observe - as much as a thousand times faster than bilayers.  We reconcile this by noting a recent study that indicates that it may be experiencing an increased drag of a surprisingly similar order.

Could you explain the significance of your article to the non-specialist?
Biology uses lipids to form membranes around cells, possibly because of their ability to dynamically reconfigure.  Our article shows how lipids reconfigure to spread over both hydrophobic and hydrophilic surfaces, with dramatic differences in their spreading characteristics.  In addition to establishing a technique to form large membranes (of possible use as part of a self-healing battery), the article exposes a unique two-dimensional platform for seeing how single sheets of molecules spread.  

What has motivated you to conduct this work?
Initially, we were searching for an alternative method to form high quality supported monolayers and bilayers.  However, consistent differences between monolayer and bilayer spreading characteristics soon revealed relatively rich physics.  Our initial naïve understanding of the spreading energetics proved inconsistent, and subsequent analysis proved interesting.

Where do you see this work developing in the future?
One of the most interesting aspects of the system is that the spreading is two-dimensional.  In that context we've begun exploring collisions, mixing and inter-leaflet dynamics. One technological potential of this work is as a means to map the surface-energy of a sample, another is using the spreading as a driving force to induce component separation.
There are also biological implications to this work. Eye tear-films and lung-linings function by way of spreading lipid films.  Lipid spreading is a vital, material-driven process that will need to be understood in order to better address major disorders such as Acute Respiratory Distress Syndrome, and minor irritants such as excessively dry eyes.  

Are there any particular challenges facing future research in this area?
The most difficult challenge is that the lipid layers are molecularly thin - about 2-6 nm. While the indirect observation techniques (e.g. fluorescence, ellipsometry) are very well developed, they are unable to capture some exciting possibilities.  For example, the two-dimensional lipid sheets are likely to have out-of-plane fluctuations.  How these fluctuations are affected by spreading collisions is not easy to examine.