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Active Matter

Active matter is composed of large number of nearly identical objects moving in unison, i.e., in coherent whole. The formation of such system requires the infusion of energy and is in non-equilibrium state. As shown in Figure 12-35m, a few individuals in large separation would not organize into such pattern; the formation needs a large number and high density. Active matter study is a relatively new interdisciplinary field.
Active Matter in Flocking The familiar examples are schools of fish, and flocks of birds, but the study also involves microscopic objects acting as if it is living - having a life of its own. The example below illustrates its special characteristics by 21st Century experiment.

Figure 12-35m Active Matter in Flocking [view large image]

The molecular tracks and molecular motors are important components within the cells for transporting waste and products to the membrane and managing the celluar shape. The molecular motor looks eerily like a little people with two feet and with or without a head (the cargo) walking on a rail (Figure 12-35n).

Molecular Tracks and Motors Microtubules The actin and myosin were first isolated for studying in late 2000s. At high enough concentration and with the supply of ATP (the molecular fuel) then without further human intervention, the actin filaments would form pulsating clusters, swirls and bands automatically. This is the kind of phase transitions predicted by a theory of velocity alignment through some kind of interaction between neighbors such as the flocking in Figure 12-35m. An updated version in 2012 used microtubules and kinesins provides more detail as summarized in the followings (Figure 12-35o).

Figure 12-35n Molecular Tracks and Motors

Figure 12-35o Microtubules as Active Matter [view large image]

See original paper : "Spontaneous Motion in Hierarchically Assembled Active Matter" in Nature, 2012.
Noticeably, this experiment has not come up with a value for the critical concentration when the flocking starts to emerge; and it does not identify what is the "agent" causing such phenomena. Essentially, it just demonstrates that the same flocking behavior does occur in microscopic and inanimate objects.

Anyway, this is one step forward to understand how inanimate matter can turn into living thing. A lot of researches are focused on the classification of the molecules involved in cellular processes. Such kind of works is similar to the assembly of the "Periodic Table" without understanding the underlying principle. Active matter study will help to advance the search for a theory on the "Origin of Life" (see "The Physics of Life" in Nature, 2016 for a review of the subject).

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