Systematic coarse-graining of the dynamics of entangled polymer melts: the road fromchemistry to rheology J T Padding and W J Briels -
Self-propelled particles (SPP) exhibit complex collective motions, mimicking autonomous behaviors that are often seen in the natural world, but essentially are generated by simple mutual interactions. Previous research on SPP systems focuses on collective behaviors of a uniform population. However, very little is known about the evolution of individual particles under the same global influence. Here we show self-organized rotating spiral coils in a two-dimensional (2D) active system. By using swarming bacteria Vibrio alginolyticus as an ideal experimental realization of a well-controlled 2D self-propelled system, we study the interaction between ultra-long cells and short background active cells. The self-propulsion of long cells and their interactions with neighboring short cells leads to a self-organized, stable spiral rotational state in 2D. We find four types of spiral coils with two main features: the rotating direction (clockwise or counter-clockwise) and the central structure (single or double spiral). The body length of the spiral coils falls between 32 and 296 μm and their rotational speed is within a range from 2.22 to 22.96 rad s(-1). The dynamics of these spiral coils involves folding and unfolding processes, which require local velocity changes of the long bacterium. This phenomenon can be qualitatively replicated by a Brownian dynamics simulation using a simple rule of the propulsion thrust, imitating the reorientation of bacterial flagella. Apart from the physical and biological interests in swarming cells, the formation of self-organized spiral coils could be useful for the next generation of microfabrication.
We describe a mechanochemical and percolation cascade that augments myosin's regulatory network to tune cytoskeletal forces. Actomyosin forces collectively generate cytoskeletal forces during cell oscillations and ingression, which we quantify by elastic percolation of the internally driven, cross-linked actin network. Contractile units can produce relatively large, oscillatory forces that disrupt crosslinks to reduce cytoskeletal forces. A (reverse) Hopf bifurcation switches contractile units to produce smaller, steady forces that enhance crosslinking and consequently boost cytoskeletal forces to promote ingression. We describe cell-shape changes and cell ingression in terms of intercellular force imbalances along common cell junctions.
A model with solution viscoelasticity is proposed to explain the ratchetlike stretching of DNA by a symmetric ac electric field in polymer solutions. In this model, DNA is stretched by the interaction between the fluid elasticity and the oscillatory flow induced by DNA. Predictions of the model are confirmed by DNA stretching experiments performed in various polymer solutions and the corresponding rheological measurements of the solutions. In particular, experiments have verified that a net migration of stretched DNA in polymer solutions can be induced by a zero-mean asymmetric ac electric field. This last finding cannot be explained by other existing models.
Active matter with continuous energy injection that exhibits various nonequilibrium emergent behaviors, such as swarming and motility induced phase separation, has been extensively studied in the past decades. Achieving desired patterns and phases in fabricating functional materials by assembling active matter is a rising and challenging direction. Nevertheless, the ossibility of a stably ordered structure of active matter remains elusive. Toward this goal, the interplay between the active force and the volume exclusion provides a new way to manipulate mechanical stability. Here, we demonstrate a new type of active, two dimensional (2D) pseudocrystal system consisting of arrays of active rods. The pseudocrystals with tetratic array demonstrate robust stability against the thermal noise. Increasing the active force leads to a phase transition from pseudocrystal to swarming via shear melting. In the traveling pseudocrystals, on the contrary, the synchronized movement of active rods reduces internal stresses and enhances the stability of pseudocrystals. Topological defects quickly propagate in the traveling pseudocrystals and assist the stability. The present framework provides innovative insights into potentially new designs and manipulations of active materials. PACS numbers:
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