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Inchemistry andphysics,metastability is an intermediateenergetic state within adynamical system other than the system'sstate of least energy.A ball resting in a hollow on a slope is a simple example of metastability. If the ball is only slightly pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope.Bowling pins show similar metastability by either merely wobbling for a moment or tipping over completely. A common example of metastability in science isisomerisation. Higher energy isomers are long lived because they are prevented from rearranging to their preferred ground state by (possibly large) barriers in thepotential energy.
During a metastable state of finite lifetime, all state-describing parameters reach and hold stationary values. In isolation:
The metastability concept originated in the physics offirst-order phase transitions. It then acquired new meaning in the study of aggregatedsubatomic particles (in atomic nuclei or in atoms) or in molecules, macromolecules or clusters of atoms and molecules. Later, it was borrowed for the study of decision-making and information transmission systems.
Metastability is common in physics and chemistry – from anatom (many-body assembly) to statistical ensembles ofmolecules (viscous fluids,amorphous solids,liquid crystals,minerals, etc.) at molecular levels or as a whole (seeMetastable states of matter andgrain piles below). The abundance of states is more prevalent as the systems grow larger and/or if the forces of their mutual interaction are spatially less uniform or more diverse.
Indynamic systems (withfeedback) like electronic circuits, signal trafficking, decisional, neural and immune systems, thetime-invariance of the active or reactive patterns with respect to the external influences defines stability and metastability (seebrain metastability below). In these systems, the equivalent ofthermal fluctuations in molecular systems is the "white noise" that affects signal propagation and the decision-making.
Non-equilibrium thermodynamics is a branch of physics that studies the dynamics of statistical ensembles of molecules via unstable states. Being "stuck" in a thermodynamic trough without being at the lowest energy state is known as having kinetic stability or being kinetically persistent. The particular motion orkinetics of the atoms involved has resulted in getting stuck, despite there being preferable (lower-energy) alternatives.
Metastablestates of matter (also referred asmetastates) range from melting solids (or freezing liquids), boiling liquids (or condensing gases) andsublimating solids tosupercooled liquids orsuperheated liquid-gas mixtures. Extremely pure, supercooled water stays liquid below 0 °C and remains so until applied vibrations or condensing seed doping initiatescrystallization centers. This is a common situation for the droplets of atmospheric clouds.
Metastable phases are common in condensed matter and crystallography. This is the case foranatase, a metastable polymorph oftitanium dioxide, which despite commonly being the first phase to form in many synthesis processes due to its lowersurface energy, is always metastable, withrutile being the most stable phase at all temperatures and pressures.[1]As another example,diamond is a stable phase only at very high pressures, but is a metastable form of carbon atstandard temperature and pressure. It can be converted tographite (plus leftover kinetic energy), but only after overcoming anactivation energy – an intervening hill.Martensite is a metastable phase used to control the hardness of most steel. Metastablepolymorphs ofsilica are commonly observed. In some cases, such as in theallotropes of solidboron, acquiring a sample of the stable phase is difficult.[2]
The bonds between the building blocks ofpolymers such asDNA,RNA, andproteins are also metastable.[citation needed]Adenosine triphosphate (ATP) is a highly metastable molecule, colloquially described as being "full of energy" that can be used in many ways in biology.[3]
Generally speaking,emulsions/colloidal systems andglasses are metastable. The metastability of silica glass, for example, is characterised by lifetimes on the order of 1098 years[4] (as compared with the lifetime of the universe, which is thought to be around1.3787×1010 years).[5]
Sandpiles are one system which can exhibit metastability if a steep slope or tunnel is present.Sand grains form a pile due tofriction. It is possible for an entire large sand pile to reach a point where it is stable, but the addition of a single grain causes large parts of it to collapse.
Theavalanche is a well-known problem with large piles of snow and ice crystals on steep slopes. In dry conditions, snow slopes act similarly to sandpiles. An entire mountainside of snow can suddenly slide due to the presence of a skier, or even a loud noise or vibration.
Aggregated systems ofsubatomic particles described byquantum mechanics (quarks insidenucleons, nucleons insideatomic nuclei,electrons insideatoms,molecules, oratomic clusters) are found to have many distinguishable states. Of these, one (or a smalldegenerate set) is indefinitely stable: theground state orglobal minimum.
All other states besides the ground state (or those degenerate with it) have higher energies.[6] Of all these other states, themetastable states are the ones havinglifetimes lasting at least 102 to 103 times longer than the shortest lived states of the set.[7]
Ametastable state is then long-lived (locallystable with respect to configurations of 'neighbouring' energies) but not eternal (as the globalminimum is). Being excited – of an energy above the ground state – it will eventually decay to a more stable state, releasing energy. Indeed, aboveabsolute zero, all states of a system have a non-zero probability to decay; that is, to spontaneously fall into another state (usually lower in energy). One mechanism for this to happen is throughtunnelling.
Some energetic states of anatomic nucleus (having distinct spatial mass, charge, spin,isospin distributions) are much longer-lived than others (nuclear isomers of the sameisotope), e.g.technetium-99m.[8] The isotopetantalum-180m, although being a metastable excited state, is long-lived enough that it has never been observed to decay, with a half-life calculated to be least4.5×1016 years,[9][10] over 3 million times the currentage of the universe.
Some atomic energy levels are metastable.Rydberg atoms are an example of metastable excited atomic states. Transitions from metastable excited levels are typically those forbidden by electric dipoleselection rules. This means that any transitions from this level are relatively unlikely to occur. In a sense, an electron that happens to find itself in a metastable configuration is trapped there. Since transitions from a metastable state are not impossible (merely less likely), the electron will eventually decay to a less energetic state, typically by an electric quadrupole transition, or often by non-radiative de-excitation (e.g., collisional de-excitation).
This slow-decay property of a metastable state is apparent inphosphorescence, the kind ofphotoluminescence seen in glow-in-the-dark toys that can be charged by first being exposed to bright light. Whereas spontaneous emission in atoms has a typical timescale on the order of 10−8 seconds, the decay of metastable states can typically take milliseconds to minutes, and so light emitted in phosphorescence is usually both weak and long-lasting.
In chemical systems, a system of atoms or molecules involving a change inchemical bond can be in a metastable state, which lasts for a relatively long period of time. Molecular vibrations andthermal motion make chemical species at the energetic equivalent of the top of a round hill very short-lived. Metastable states that persist for many seconds (or years) are found in energeticvalleys which are not the lowest possible valley (point 1 in illustration). A common type of metastability isisomerism.
The stability or metastability of a given chemical system depends on its environment, particularlytemperature andpressure. The difference between producing a stable vs. metastable entity can have important consequences. For instances, having the wrong crystalpolymorph can result in failure of a drug while in storage between manufacture and administration.[11] The map of which state is the most stable as a function of pressure, temperature and/or composition is known as aphase diagram. In regions where a particular state is not the most stable, it may still be metastable.Reaction intermediates are relatively short-lived, and are usually thermodynamically unstable rather than metastable. TheIUPAC recommends referring to these astransient rather than metastable.[12]
Metastability is also used to refer to specific situations in mass spectrometry[13] and spectrochemistry.[14]
A digital circuit is supposed to be found in a small number of stable digital states within a certain amount of time after an input change. However, if an input changes at the wrong moment a digital circuit which employs feedback (even a simple circuit such as aflip-flop) canenter a metastable state and take an unbounded length of time to finally settle into a fully stable digital state.
Metastability in the brain is a phenomenon studied incomputational neuroscience to elucidate how the human brain recognizes patterns. Here, the term metastability is used rather loosely. There is no lower-energy state, but there are semi-transient signals in the brain that persist for a while and are different than the usual equilibrium state.
Gilbert Simondon invokes a notion of metastability for his understanding of systems that rather than resolve their tensions and potentials for transformation into a single final state rather, 'conserves the tensions in the equilibrium of metastability instead of nullifying them in the equilibrium of stability' as a critique ofcybernetic notions ofhomeostasis.[15]
This is a highly stable molecule. About 11,500 calories of free energy are liberated when it is hydrolized to phosphate and adenosine-diphosphate (ADP).
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