Superomniphobicity

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.)

RIP, Bob Richardson, and the human nature of science

I was saddened to read of the passing of Bob Richardson, who shared the Nobel Prize for Physics in 1996 with his colleague Dave Lee and their former grad student (and my thesis advisor) Doug Osheroff.   I only had a handful of chances to meet with Prof. Richardson, and he was a friendly, classy person every time.  In some ways it's a shame that more people don't have the opportunity to interact with really accomplished scientists (and engineers); the chances I've been exceedingly fortunate to have over the years to meet and talk with prize winners and national academy members have been fun professionally and revelatory in terms of showing the human side of these endeavors.  These people aren't infallible or unapproachable, and in my limited experience the vast majority are neither arrogant nor lacking in social skills (I'm looking at you, Big Bang Theory).  It would be nice if TLC or Discovery or someone would tell the stories of these people in an accessible, fun way, instead of wasting precious bandwidth on ghost-hunting moonshiners who horde ice fishing hauling equipment.    

Plasmons, polarization, and intuition

This is one of those once-every-six-months self-promotion posts about a new paper from our group.  The result is sufficiently surprising, while illustrating a generalizable idea, that I think it's worth sharing.

I've written before about plasmons, the collective "normal modes" of the electronic fluid in a metal.  Like many phenomena in condensed matter physics, there are times when it is useful to think of plasmon modes in some metal structure as generic oscillators, each analogous to a mass on a spring (only the natural frequency of the plasmon has to do with the complex dielectric function of the metal, while the natural frequency of the mass on a spring is set by the mass and the spring constant). In particular, if you take two identical oscillators, nominally of the same natural frequency \( \omega_{0} \), and you couple them together, it often makes sense to describe the coupled system in terms of two "new" normal modes "built" from linear combinations of the uncoupled modes, the symmetric ( \( \omega_{\mathrm{s}}  < \omega_{0} \) ) (the individual oscillators move in phase with each other)  and antisymmetric modes ( \( \omega_{\mathrm{as}}  > \omega_{0} \) ) (the two oscillators move \( \pi \) out of phase with each other).  In quantum mechanics we see the same idea; for example, when two 1s orbitals are coupled together, it can make more sense to think instead about "hybridized" bonding ( \(\sigma\) ) and antibonding ( \(\sigma* \) ) molecular orbitals.   As my colleagues showed almost ten years ago in this highly cited paper, plasmons can hybridize, too, and hybridization can provide real insights into the plasmonic modes of complicated structures.

In my lab, we have spent quite a bit of time over the last several years playing with and looking at the local plasmon modes that live at nanoscale gaps between lithographically fabricated Au electrodes.  In many ways, these structures look a bit like two scanning tunneling microscope tips pointing at each other.  Many other groups have made similar structures, and it has been known for a long time that placing metal tips in close proximity to each other or a tip pointing down at a very nearby metal plane leads to "tip plasmon" modes that can be useful for various spectroscopies.  In the plasmon hybridization language, the local tip modes result from the hybridization of (delocalized) surface plasmon modes of the two electrodes, thanks to their very local coupling.  For those interested in these nanogap plasmon effects, by the way, I want to point out our recent review article about this, which will appear in an issue of Phys Chem Chem Phys focusing on plasmonics.

We had lingering mysteries, however, in our own particular geometry.  For example, why did we get such good reproducibility in the resonant wavelength of the modes (always near our laser line of 785 nm), and more dramatically, why did we observe our particular polarization dependence?  It's tricky to explain what I mean without a diagram, but I'll try.  "Common sense" and intuition suggest that light polarized with the electric field across the gap between the electrodes should be best at exciting modes that are localized to the gap.  That's proven to be true in many experiments (cited in the paper).  However, in our devices we find that we get the best optical response when the light is polarized with the electric field pointing along the gap (!), and that the emitted light also is polarized along the gap.

After a series of very careful experiments and calculations (collaboration with Mark Knight of the Halas group), we know the answer.  In our system, the metal wire in which the nanogap sits has a transverse plasmon mode (because of our particular choice of material and transverse dimensions) that is well matched to our laser, and optically "bright" in the sense of having a big electric dipole coupling.  Because a given nanogap is not perfectly symmetric, the higher order, multipolar modes localized to the gap (ordinarily optically "dark" because they lack a dipole coupling) get hybridized with that bright mode.  This explains our counterintuitive polarization dependence (the dipole-active transverse piece is what couples to both the incoming and outgoing far field light), and the reproducibility of the plasmon energy (it's set largely by the wire width, not the details of the gap).  Cute stuff, and it is a good example of how even well-known physics (after all, deep down this is a matter of solving Maxwell's equations) can give interesting surprises.
 

Pomona and Harvey Mudd

It's been a busy week.  On Monday I gave a colloquium at Pomona College, and visited both there and Harvey Mudd.  It's great to see the quality of physics instruction and student research at these extremely good undergraduate institutions.  Thanks to my host, optics guru Alfred Kwok, and the other faculty with whom I met, nano-CM expert and department chair David Tanenbaum; Dwight Whittaker, who taught me about the fascinating biomechanics of exploding plants (!); and Alma Zook, who described their 1 meter telescope.  At Harvey Mudd, it was fun to visit and talk plasmonics with Peter Saeta and to meet John Townsend, the author of two books used in Rice's undergrad curriculum. 

Then on Tuesday I gave a physical chemistry seminar at UCSD, hosted by Misha Galperin.  Good conversations with Francesco Paesani and John Weare about various computational challenges, and Michael Tauber taught me about pump-probe Raman spectroscopy to understand the dynamics of charge and spin in carotenoids.  Then it was on to the physicists, visiting with Max Di Ventra, Dimitri Basov, and Ivan Schuller.  Whew!  A great visit.

Finally, at the very end of the week, I came to Boston to hit one day of the AAAS meeting, where I got a chance to hear a talk by Susan Hockfield, who spoke about the essential role of government investment in basic research.  I also got to hear a talk about geoneutrinos and a lecture by Silvan Schweber about Hans Bethe.  Good stuff.

Passing the laugh test

Some scientific claims are so outlandish that they do not pass the "laugh test".  That is, for these claims to be true it would require throwing out physical principles that have been tested in excruciating detail for decades if not hundreds of years.   Perpetual motion machines based on magnets are one example.  Another is the "EM Drive".  Depressingly, the latter has reappeared, featured in the UK edition of Wired.  The idea is that one can make a microwave resonator shaped like a truncated cone, and that somehow when this resonator is pumped with lots of electromagnetic radiation, it will experience a net thrust in one direction, despite the fact that nothing (including photons) is being exhausted.  Why is this absurd on its face?  Well, put a box around the system, and it clearly violates conservation of momentum, a principle that has been tested with extreme precision for hundreds of years.  Despite double-talk about group vs. phase velocity, reference frames, and relativistic effects, the fact remains that the theory of electricity and magnetism, upon which this device allegedly relies, does not violate conservation of momentum, and neither does the quantum version.  The reason this has come up again is that a Chinese researcher claims to have experimentally verified that the effect exists.  No offense to her particular institution, but call me when people at Beijing or USTC or Tsinghua have done this.  I won't be holding my breath.