Most solids are crystalline in nature because the energy released during the formation of ordered structure is more than that released during the formation of disordered structure such that the crystalline state is the lower energy state. So if we take different marbles in a box and shake it then shouldn't they arrange themselves in order to get to a low energy state? But we see they arrange in a disorderly way. Why do different phenomenon occur in these two cases?
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15$\begingroup$ Object to "Most solids are crystalline in nature". My yard is made of solids, almost none of which are crystals. My driveway is solid and is not a crystal. My windows are solid, but are not a crystal. The crust of the Earth is solid and is not a crystal. $\endgroup$– Eric TowersCommented Nov 25, 2018 at 17:15
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3$\begingroup$ But most of the solid are crystalline $\endgroup$– user342326Commented Nov 25, 2018 at 17:31
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14$\begingroup$ Your marbles are crystalline. Does this mean any arrangement of them is a crystal? Steel is made of crystal domains -- does this mean steel is a crystal? (No, steel is not a crystal.) Clay is made of crystalline particles. Clay flows like a liquid when barely wetted and under pressure -- clay is not a crystal. Concrete is a solid suspension of many different components, so is an amorphous solid. Glass is not a crystal -- if you want the crystal, you find quartz. Most solids are amorphous suspensions of solids, some of which are crystals. $\endgroup$– Eric TowersCommented Nov 25, 2018 at 17:36
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10$\begingroup$ Look around you. Are you surrounded by crystals or by disordered amalgams of crystals? $\endgroup$– Eric TowersCommented Nov 25, 2018 at 19:48
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3$\begingroup$ @EricTowers Your yard is probably mostly made of rock (or sand or gravel) - unless that rock is obsidian, it is made of crystals. The marbles however are (almost certainly) not crystals - they are (usually) made of glass. $\endgroup$– Martin Bonner supports MonicaCommented Nov 26, 2018 at 14:50
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8 Answers
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Interaction between marbles is very similar to the hard sphere (HS) interaction model i.e. a pair-wise potential energy which is zero if spheres do not overlap and $+\infty$ elsewhere.
Hard spheres are one of the first systems studied via computer simulation and one of the first big surprise was that by increasing pressure, they are able to crystallize from a disordered fluid to an fcc crystal, in 3D, or to a triangular lattice in 2D. After the first pioneering studies the scenario has been confirmed many times and fully understood. Moreover, in the nineties, the experiments by the Pusey's group in UK have shown that the theoretical scenario is closely followed by colloidal systems designed to mimic as closely as possible a real system of HS (Pusey, P. N., & Van Megen, W. (1986). Phase behaviour of concentrated suspensions of nearly hard colloidal spheres. Nature, 320(6060), 340.)
It is interesting to notice that the HS crystal is stable on the base of entropic reasons. Neither attraction nor quantum mechanics are needed and the density of the coexistent solid at freezing is about 30% smaller than the close packing density (which means that in the HS crystal at the freezing point spheres collides frequently but do not touch all the time). Probably one of the most interesting things about the HS solid is that is a very nice illustration why the naïf equation entropy="spacial disorder" is wrong: the HS crystal has a higher entropy per particle than the coexisting liquid.
What can be said about marbles, taking into account HS? Although their interaction is a very good representation of the HS potential, usually they lack the dynamics underlying the behavior of a true thermodynamic system. Dissipative effects are quite strong and in a short time, without an external continuous feed of energy, the kinetic energy of marbles gets dissipated. In the very old times of the study of liquids, somebody performed experiments with a 2D system of marbles in a tray put on top of a hi fi speaker as a tool to feed kinetic energy randomly. However, without such a flux of energy, what can be observed by shaking a 2D or 3D container almost filled with marbles is that, if the system is highly disordered at the beginning, after some shaking part of the "defects" are eliminated and, at least locally, the system looks like a crystalline solid at the close packing. But this is a situation not directly related with the thermodynamic transition. It has more to do with the stability with respect to perturbations of purely mechanical equilibrium configurations. As a last comment, I would add that the dynamic behavior of marble-like particles has been and still is an active research topic in the physics of granular media.
answered Nov 25, 2018 at 9:51
GiorgioP-DoomsdayClockIsAt-90GiorgioP-DoomsdayClockIsAt-90
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4$\begingroup$ How can I interprete the fact that entropy is higher in a lattice than in the liquid? Then geometrically or statically this should be true even for interacting (repelling or attracting not just colliding) particle. Please give me a hint. @GiorgioP $\endgroup$ Commented Nov 25, 2018 at 12:42
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1$\begingroup$ @Alchimista: according to statistical mechanics, the highest is the entropy the largest the number of states in the phase space. Comparing liquid and crystal entropy is the same as comparing the number of states for systems under the constraint: i) there is no long range order (fluid), ii) there is (crystalline solid). While for non interacting systems the configurations in phase space corresponding to i) overcome those corresponding to ii), in a dense interacting system the ordered configurations ensure the maximum available space (as everybody trying to pack a suitcase knows very well). $\endgroup$ Commented Nov 25, 2018 at 13:04
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$\begingroup$ Yes but I need a suite case and i do need to pack - ??? If there is a container then it make sense but a crystal does need that $\endgroup$ Commented Nov 25, 2018 at 13:07
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1$\begingroup$ A crystal at a given density is obtained by packing atoms in a periodically repeated cell of fixed volume. That volume is the "suitcase" for atoms. $\endgroup$ Commented Nov 25, 2018 at 13:25
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1$\begingroup$ Interacting means that there are explicit forces between particles. From the point of view of the accessible phase space, in an interacting system, the accessible configuration space is strongly reduced by the presence of the harsh repulsion between particles. $\endgroup$ Commented Nov 25, 2018 at 15:32
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They do. It is easiest to do show this in two dimensions. I used to demonstrate this on an overhead projector, with lead shot in a transparent CD-case. It is probably better to use smaller spheres (more spheres) than marbles. The other classic demo is with bubble rafts, which can also demonstrate the movement of dislocations.
In three dimensions, it is difficult to see this in a jar. One only observes the regions close to the glass. But I made this video, where I had prepared a regular surface of spheres as a seed: https://play.lnu.se/media/t/0_bmg6kye7 (after 1:00 minute in the Swedish video)
And very small spheres of glass or plastic can form colloidal close-packed crystals. In nature, this has created gems, opal, when the lattice constant is of the order of the wavelength of visible light.
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1$\begingroup$ There is a video which discusses lattice formation and crystal defects in masses of ball bearings and larger spheres (in 2D and 3D) here: youtube.com/watch?v=O3RsDIWB7s0 $\endgroup$ Commented Nov 25, 2018 at 11:13
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1$\begingroup$ You are not addressing the "such that the crystalline state is the lower energy state" . the rearrangement of classical spheres is equipotential at the same gravitational level.. It is at the quantum mechanical level that energy enters the game. $\endgroup$– anna vCommented Nov 25, 2018 at 12:19
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5$\begingroup$ @annav It is because of the Earth's gravitational field that close-packed arrangements of marbles are lowest in energy. $\endgroup$– user137289Commented Nov 25, 2018 at 16:55
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"So if we take different marbles in a box and shake it then shouldn't they arrange themselves in order to get to a low energy state?"
They certainly do - they will adopt a hexagonal (2D) or close packed (3D) configuration. In a real life scenario we may not immediately see that. That is so because of friction between marbles at their point (or rather area) of contact. However, if you remove that constraint, i.e. assume absence of friction, the entire collection of marbles will adopt the ordered configurations (hexagonal or close packed) after each time you shake the container.
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You should study annealing. Vastly oversimplifying:
- If you cool the sample slowly enough, it retains enough energy long enough to explore its state space and find very low entropy, crystalline, states.
- If you cool a sample rapidly, it loses energy too rapidly to explore more than a tiny neighborhood of its state space, and produces non-crystalline states.
In your example with shaking marbles, you cool very rapidly. To simulate slow cooling, you would shake for a very long time, gradually tapering the amplitude of the shaking. Given the energy barrier to dislocation with reasonably sized marbles, you would have to taper very slowly.
Note that shaking may not be the best way to provide energy to the system to quickly find deep valleys in the energy landscape. "Dice Become Ordered When Stirred, Not Shaken" (The article also shows that too rapid stirring prevents settling to an ordered state -- the system continues exploring nearby less ordered states.)
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As HolgerFielder and Pieter said in their answers, marbles do form crystalline arrangements. Notice, though, in Holger Fielder's illustration that the arrangement is much less ordered near a boundary.
If marbles were confined in a way that did not impose hard boundary conditions, then they would almost always form perfectly crystalline arrangements. A jar imposes boundary conditions that are geometrically incompatible with a perfectly crystalline arrangement.
answered Nov 25, 2018 at 12:51
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$\begingroup$ Isn't the real answer that there is not enough time / sufficient high temperature to form large crystals? $\endgroup$ Commented Nov 25, 2018 at 15:52
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$\begingroup$ The trouble with marbles in an arbitrary jar is that, given gravity abd the hard constraint of the jar shape, the lowest energy configuration is at best close-packed only some distance from the inside surface of the jar. If you cram a close-packed bunch of marbles into the jar, there will be gaps around the edges that a marble won't quite fit into. Shake it up, though, and a few more marbles may fit in. But the close-packed arrangement is disturbed. $\endgroup$ Commented Nov 25, 2018 at 19:22
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$\begingroup$ And crystals with real long-range order would not form I think because with marbles there are only contact forces. There is no difference in packing fraction between ABC stacking (face-centered cubic) and ABAB stacking (hexagonally close packed). $\endgroup$– user137289Commented Nov 25, 2018 at 20:10
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$\begingroup$ don't boundary conditions also have an affect on actual crystals too? $\endgroup$– jk.Commented Nov 26, 2018 at 13:49
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This seems to be more a question for chemistry. The reason lays on the atomic bonds.
First at all, for high enough temperatures all solids will go to a liquid or and gaseous-like state and behave like marbles. And for temperatures near 0 Kelvin the marbles will behave more or less like a crystal.
... if we take different marbles in a box and shake it then shouldn't they arrange themselves in order to get to a low energy state.
It does. Nearly all marbles will lay in the closed-packing of equal spheres.
And why the balls do not stick together an the crystalline level? Because it is needed some activation energy. But that’s all about chemistry.
answered Nov 25, 2018 at 7:13
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4$\begingroup$ Re "...a question for chemistry": "The division between chemistry and physics becomes diffuse at the interface of the two branches ... crystallography ..." $\endgroup$ Commented Nov 25, 2018 at 15:47
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$\begingroup$ As I understand the question, there are "different marbles" in the box, not equal ones. $\endgroup$ Commented Nov 25, 2018 at 19:54
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$\begingroup$ Chemistry is just the application of physics :) $\endgroup$– UKMonkeyCommented Nov 26, 2018 at 12:32
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You are confusing two systems. The quantum mechanical of crystalline structure of solids, and the classical marbles , even if they are supposed to be perfect spheres. There is no quantization in the classical state to "lock" a marble in a position, it is free to assume any rotational position and translational on the horizontal, so it becomes a classical statistics problem. If they are packed tight they will organize themselves into a regular structure. If there is space, a single marble on a horizontal level of marbles, (gravity organizes them at levels because of potential energy) cannot "stick" to any position without the slightest impulse sending it sliding over the lower level. There are no bound states.
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2$\begingroup$ No, it is not really quantum mechanical. One can make crystals of colloid spheres. Opals are a natural example. $\endgroup$– user137289Commented Nov 25, 2018 at 9:57
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$\begingroup$ @Pieter I note the "colloid" , is not that molecular binding levels? i.e. quantum mechanics? You can also make structures with LEGO. $\endgroup$– anna vCommented Nov 25, 2018 at 12:12
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$\begingroup$ @anna v no is not that molecular level but it doesn't mean that quantum the $\endgroup$ Commented Nov 25, 2018 at 12:47
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$\begingroup$ Ory isn't at play. The fact is that I find all A reasonable as we are at two different levels. One is merely topological and statistical and one as in crystals including strong interaction. Apparently non binding hs are not at equipotential even in the same gravitational potential. I did ask @GiorgioP for a hint in how to see that. $\endgroup$ Commented Nov 25, 2018 at 12:53
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1$\begingroup$ @annav No, colloids do not involve quantum mechanics. There are repulsive Coulomb forces because the particles often have a surface charge. Together with confinement, this is probably enough to explain long-range ordering (I do not know much about colloids either). $\endgroup$– user137289Commented Nov 25, 2018 at 17:08
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The van der waals forces between marbles are very small when compared to other forces acting on the marbles such as gravity and friction. This is due to their relatively large size when compared with atoms.
However, when you go down to the size of individual atoms in a crystal lattice, the van der waals forces between them begin to become more comparable in size to other forces, which means that in some cases they can overcome these forces in order to arrange themselves in the lowest energy state, dependant on the material, temperature and pressure.
Only the atoms on the very edge of the marbles can contribute to van der waals forces between the marbles. The amount of atoms on the edge in comparison to the amount of atoms inside is extremely small. For individual atoms in a crystal lattice, the whole atom takes part in van der waals forces.
