Inhomogeneous Steady States of Diffusion-Limited Coalescence, A+A<-->A
  We study the steady state of diffusion-limited coalescence, A+A<-->A, in the presence of a trap and with a background drift. In one dimension this model can be analyzed exactly through the method of inter-particle distribution functions (IPDF). Because of the irreversible trap the steady state of the system is a non-equilibrium state. An interesting phase transition, controlled by the drift away from the trap, takes place: from a non-trivial steady state, when the drift is weak, to a trivial steady state (the vacuum), as the drift increases beyond some critical point. Surprisingly, regardless of the drift strength, the computed IPDF resembles that of an homogeneous equilibrium system, without the trap. We suggest that this is due to "shielding": the particle nearest to the trap shields the remaining particles from the effects of the trap. Finally, we compare the exact solution to that of a reaction-diffusion equation, and we determine the optimal values of the appropriate rate coefficients. 
Daniel ben-Avraham (Clarkson University)