Blurring the Boundaries of the Animate and Inanimate: elucidating the fundamental behavior of active matter

Active matter is a collection of objects each of which consumes energy to fuel its own dynamics.  Many examples of active matter are found in the living world at multiple length scales, ranging from the flocking of birds, and swarming of bacteria, to cytoplasmic streaming flows found in the interiors of some cells.

A decade ago, the field of active matter was mostly confined to theoretical research.  Since then, spurred by the emergence of diverse experimental model systems, the entire field has experienced a phase of rapid growth that continues to this day.  Understanding the laws that govern the collective and highly dynamical behavior of such inherently non-equilibrium systems remain a significant challenge in the quest to understand the physical bases that underlie biological function.  As well as to develop synthetic materials that can mimic the remarkable functionalities found in the living word.  However, the complexity of living matter makes the acquisition of quantitative data to test theoretical models and guide their further development very challenging.

With the support of the W. M. Keck Foundation, Zvonimir Dogic and Seth Fraden at Brandeis University developed a hierarchy of systematically tunable model systems and materials that capture the essential functionalities found in the living organisms.  Starting from biochemically well-characterized filamentous microtubules and clusters of molecular motors they developed a novel category of active gels, liquid crystals, emulsions, and vesicles.  For example, microtubule-based active liquid crystals powered by molecular motors exhibit a host of new sought-after properties that are not seen in typical liquid crystals (used in display devices).  Specifically, active liquid crystals are inherently unstable and spontaneously generate self-propelled topological singularities that stream through the sample before annihilating with each other, Video 1.  This process of creation and annihilation of singularities is accompanied by the spontaneous generation of self-fracture lines and their subsequent self-healing.

In another example, Dogic and his collaborators have encapsulated an active microtubule gel within a lipid vesicle.  Upon encapsulation, microtubules form a thin cortex of aligned filaments that covers the inner vesicle surface.  When the vesicles are deflated using osmotic stress, four streaming topological singularities inside the vesicles drive formation of four highly dynamical motile filopodia-like protrusions, Video 2.  The resulting dynamical assemblage has an appearance of a living organism but is a synthetic material comprised of four simple components. 

Video 1.  A 2D active nematic liquid crystal composted of microtubules driven by motor proteins.  The internally generated flows are characterized by extension, buckling, and internal fracture of the nematic domains, which generate topological singularities seen as protruding fingers (+1/2 defect) and Y-shaped regions (-1/2 defect).  The +1/2 and -1/2 defects are created in pairs and annihilate one another when they meet. 

Video 2.  A vesicle containing an active liquid crystal gel.  The geometry leads to four topological singularities in the gel, which result in the formation of four filopodia-like structures as the vesicle slowly deflates.

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