Wednesday, February 27, 2013


Thanks to developments in surface science, surface chemistry, and nanoscience, we now understand far more about the microscopic origins of friction than we ever have before.  (When I teach about this, I point out that our depth of knowledge about the detailed physics of friction really didn't advance much between 1600 and 1950.)  One way that this increased basic knowledge is paying dividends is in the design of surface coatings to control interactions between fluids and solid surfaces.  For example:  When sophomore mechanical engineers learn basic fluid mechanics, they are taught about the "no-slip condition", an assumption that turns out to be pretty good in many many macroscopic situations.  The no-slip condition says that when a fluid flows past a solid boundary, the tangential velocity of the fluid goes to zero at the boundary, and only approaches the "bulk" flow velocity some distance away from the surface.  The region where the local flow velocity is suppressed relative to the bulk far-from-wall speed is the boundary layer.  The underlying physics here is that interactions between the fluid molecules and the wall actually stop the fluid molecules adjacent to the wall, and internal interactions between fluid molecules (the origins of viscosity) tug on neighboring layers of molecules and slow those down.  

In fact, we now know that it's possible to tweak the interactions between that layer of fluid and the solid surface, in ways that make the no-slip condition a poor assumption.  We can do this directly through chemistry.  The example you all know is the use of "hydrophobic" coatings (e.g., wax on a car; fluoropolymers like teflon on a non-stick pan).  With the right kind of chemical bonds at the surface, if the fluid molecules interact attractively much stronger with each other than with the surface, the fluid will "bead up".  Water molecules can hydrogen-bond with each other, while attractive interactions with saturated hydrocarbons are much weaker.  Water beads on wax for the same reason that water and oil do not mix.

We can also leverage the surface tension of the fluid (again related directly to the attractive interactions between fluid molecules, compared with surrounding air).  If the surface morphology of the interface is really bumpy on a length scale sharper than the ability of a liquid interface to curve, it is possible to trap air at the interface and have the liquid be resting mostly on air and just a little on the tips of the surface bumps.  This is what happens when you see water running down a lotus leaf.  (Remember "nano-pants"?)

Now that the science behind these phenomena is better understood, people are trying hard to make designer coatings with remarkably extreme versions of these properties.  Something that really "repels" water or other polar liquids is said to be superhydrophobic.  Something that really "repels" oils and waxy, non-polar liquids is said to be superoleophobic.  The ultimate limit is something that manages to have very low surface affinity for both classes of liquids - a superomniphobic interface, achievable through a combination of surface chemistry and morphology control.  Lately there have been claims of achieving this, with some dramatic videos.  There's this one from Michigan, with this video, for example.  However, that coating apparently requires electrospinning to put down.  This demonstration of a two-component spray-on coating is truly amazing to watch.  The big open question here is how robust is the coating.  If it gets degraded by, e.g., exposure to sunlight, or modest abrasion, that would limit its utility.  (It may be chemically nasty as well, given the protective equipment worn by the person applying it, but that may just be showing good sense.)


Anonymous said... said...

Dear Prof,
Does this kind of surface science have application in anti-biofilm surfaces? I am unsure how cells attach to surfaces, but your post brought this to mind