Soft matter is hard!

This great article by Randall Munroe from the NY Times this week brings up, in its first illustration (reproduced here), a fact that surprises me on some level every time I really stop to think about it:  The physics of "soft matter", in this case the static and dynamic properties of sand, is actually very difficult, and much remains poorly understood.  

"Soft" condensed matter typically refers to problems involving solid, liquids, or mixed phases in which quantum mechanics is comparatively unimportant - if you were to try to write down equations modeling these systems, those equations would basically be some flavor of classical mechanics ("h-bar = 0", as some would say).  (If you want to see a couple of nice talks about this field, check out this series and this KITP talk.)  This encompasses the physics of classical fluids, polymers, and mixed-phase systems like ensembles of hard particles plus gas (sand!), suspensions of colloidal particles (milk, cornstarch in water), other emergent situations like the mechanical properties of crumping paper.  (Soft matter also is sometimes said to encompass "active matter", as in living systems, but it's difficult even without that category.)

Often, soft matter problems sound simple.  Take a broomhandle, stick it a few inches into dry sand, and try to drag the handle sideways.  How much force does it take to move the handle at a certain speed?  This problem only involves classical mechanics.  Clearly the dominant forces that are relevant are gravity acting on the sand grains, friction between the grains, and the "normal force" that is the hard-core repulsion preventing sand grains from passing through each other or through the broom handle.  Maybe we need to worry about the interactions between the sand grains and the air in the spaces between grains.  Still, all of this sounds like something that should have been solved by a French mathematician in the 18th or 19th centuries - one of those people with a special function or polynomial named after them.  And yet, these problems are simultaneously extremely important for industrial purposes, and very difficult.

A key issue is that many soft matter systems are hindered - energy scales required to reshuffle their constitutents (e.g., move grains of sand around and past each other) can be larger than what's available from thermal fluctuations.  So, configurations get locked in, kinetically hung up or stuck.  This can mean that the past history of the system can be very important, in the sense that the system can get funneled into some particular configuration and then be unable to escape, even if that configuration isn't something "nice", like one that globally minimizes energy.  

A message that I think is underappreciated:  Emergent dynamic properties, not obvious at all from the building blocks and their simple rules, can happen in such soft matter systems (e.g., oscillons and creepy non-Newtonian fluids), and are not just the provenance of exotic quantum materials.  Collective responses from many interacting degrees of freedom - this is what condensed matter physics is all about.