tag:blogger.com,1999:blog-13869903.post5572765205408305301..comments2024-03-28T04:15:44.459-05:00Comments on nanoscale views: Physics in the kitchen: JammingDouglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-13869903.post-33880690083172363992018-02-17T17:07:08.253-06:002018-02-17T17:07:08.253-06:00You say that a jammed system is not in thermal equ...You say that a jammed system is not in thermal equilibrium because it is not ergodic, meaning it is mechanically hindered into only exploring a fraction of all possible microstates.<br /><br />However, I could say that a ferromagnet which spontaneously breaks symmetry so as to collectively orient itself in the +z direction is also "non-ergodic", in the sense that the order parameter does not explore any -z microstates. However, we generally don't claim such a system as being out of thermal equilibrium - we instead say that the system is ergodic in a given defined subregion of phase space, namely, that region which satisfies the constraints on the orientation of magnetization that arise from the spontaneous symmetry breaking. But such scenarios are most definitely labeled as being in thermal equilibrium.<br /><br />Would it perhaps be fair to say that the distinction between a ferromagnet and a jammed state is that, in the former scenario, the constraints on the accessible phase space arise from an intrinsically thermodynamic property of the system (namely, that the spontaneously broken symmetry is the globally thermodynamically stable minimum free energy ground state), whereas in the jammed state, the constraint arises from the system being stuck in a 'metastable' local free energy minimum? <br /><br />If so, is it really fair to call a jamming transition a 'phase transition', as it would not be a true thermodynamic equilibrium phase, but rather, a temporary metastable kinetically trapped state? Furthermore, I imagine that in such a scenario, as the size of your system scaled up to the approach the thermodynamic infinite limit, the stability of any such metastable, non-global-free-energy-minimizing states, would be suppressed more and more, until in the infinite limit, only the global thermodynamic minimum is stabilized? Granted, that thermodynamic minimum may not necessarily be globally ergodic - in addition to any of the obvious spontaneously broken symmetry states I mentioned previously, you could have things like phase separation, coexistence, pattern formation, etc... However, these too are also not necessarily viewed as indicators of thermal disequilibrium, but rather, as a consequence of the fact that the equilibrium free energy is minimized by the formation of surfaces/interfaces/etc...Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-35209384505543021642018-02-16T13:49:54.413-06:002018-02-16T13:49:54.413-06:00Well, I for one am gratified that your rice is Bro...Well, I for one am gratified that your rice is Brown Rice.<br />DanMnoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-36790710357113127762018-02-16T08:07:29.050-06:002018-02-16T08:07:29.050-06:00Peter, that's universality for ya :-) With co...Peter, that's universality for ya :-) With coffee, I've noticed that it is possible to use a fingernail to deform the packed grounds, which I could not do with the rice, but that's probably because there is some critical yield stress (force per area) I exceeded that depends on the size and shape of the grains.Douglas Natelsonhttps://www.blogger.com/profile/13340091255404229559noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-61994402315181306802018-02-16T05:58:38.182-06:002018-02-16T05:58:38.182-06:00On the well-known scientific principle that "...On the well-known scientific principle that "once is never, twice is always", vacuum packed ground coffee behaves in the same way, but with grains that are much smaller and of different geometry.Peterhttps://www.blogger.com/profile/08654675777726560464noreply@blogger.com