Molecules

Active Matter

	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).

		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.

• Preparation -
  • The microtubule filaments are immersed into a special solution (GMPCPP) stabilizing its length to an average of 1.5m.
  • Another kind of potion called PEG is added to induce attraction between filaments (to form bundles). This is the depletion force (aka Entropic Force) created from molecular re-arrangement. The bundle is the basic unit in the experiment.
  • Kinesins are introduced to drive the system to non-equilibrium state. These molecular motors converts chemical energy from ATP hydrolysis into mechanical movement (sliding) along a microtubule.
  • ATP is supplied to drive the motion of the system (Figure 12-35o, diagram a).



Low Concentration - At low concentration, the microtubule bundles constantly merge, extend, and break away in a steady state (Figure 12-35o, diagrams b, c). High concentration - At high enough concentration, the microbubules form a percolating active network characterized by internally driven flows as shown in Figure 12-35o, diagram d. Velocity of the flow is measured by embedding micrometre-sized beads into the bundled active networks (BANs). Figure 12-35p shows the dependence of the velocity on ATP concentrations (colour-coded on the figure), MSD is the mean square displacement. The velocity can be estimated by computing (MSD)1/2/ timelag.

Figure 12-35p Velocity Measurement

Trajectories of the particles (see red paths, Figure 12-35p insert) reveal highly non-Brownian motion.




BANs in/on Emulsified Droplet - In an attempt to show that active matter is similar to the biological system, BANs encapsulated in aqueous droplets are emulsified in fluorinated oil. When squeezed between two surfaces, such droplets exhibited an autonomous motility as shown in Figure 12-35q diagram a, with the trajectory in blue color and speed up to ~ 1 m/s. The path is much shorter in diagram b with the ATP supply running low. Diagram c displays the streaming flows (blue arrows) of the BANs on the surface of the droplet (red arrow indicates the motion of the whole droplet). Diagram d shows the same droplet with no BANs inside.

Figure 12-35q BANs in/on Emulsified Droplets

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