Gollub and Pine reply: We appreciate the intriguing note from J. C. Phillips pointing out a possible connection between the reversible regime in oscillating Couette flow containing particles, as we discussed, and the window of reversibility and lack of aging seen in alloy glasses for certain compositions. In very recent work to be published soon on the fluid case, Laurent Corté and colleagues at New York University have noted that the particles in the reversible suspension self-organize until further structural evolution ceases—that is, the suspension also becomes “non-aging.” (Further information may be obtained from the authors.) Whether the underlying physics of these two reversible states is actually similar remains to be seen, but we agree with Phillips that it is worth considering.
We regret not having referenced John Heller's demonstration of reversible low Reynolds number flow, of which we, and others, were unaware.
