Physics Colloquium: "Shaken, Not Stirred: Granular Equilibrium"

Prof. Mark Shattuck, CCNY Levich Institute and Physics Department
"Shaken, Not Stirred: Granular Equilibrium"

Abstract: Equilibrium statistical mechanics is generally not
applicable to systems with energy input and dissipation present,
and identifying relevant tools for understanding these
far-from-equilibrium systems poses a serious challenge. Excited
granular materials or granular fluids have become a canonical
system to explore such ideas since they are inherently dissipative
due to inter-particle frictional contacts and inelastic collisions.
Granular materials also have far reaching practical importance in
a number of industries, but accumulated ad-hoc knowledge is often
the only design tool. An important feature of granular fluids is
that the driving and dissipation mechanisms can be made to balance
such that a Non-Equilibrium Steady-State (NESS) is achieved. We
present strong experimental evidence for a NESS first-order phase
transition in a vibrated two-dimensional granular fluid. The phase
transition between a gas and a crystal is characterized by a
discontinuous change in both density and temperature and exhibits
rate dependent hysteresis. We measure a "free energy"-like function
for the system and compare and contrast this type of transition with
an equilibrium first-order phase transition and a hysteretic backward
bifurcation in a nonlinear pattern forming system.