Abstract
We numerically investigate the morphology and disclination line dynamics of active nematic droplets in three dimensions. Although our model only incorporates the simplest possible form of achiral active stress, active nematic droplets display an unprecedented range of complex morphologies. For extensile activity finger-like protrusions grow at points where disclination lines intersect the droplet surface. For contractile activity, however, the activity field drives cup-shaped droplet invagination, run-and-tumble motion or the formation of surface wrinkles. This diversity of behaviour is explained in terms of an interplay between active anchoring, active flows and the dynamics of the motile dislocation lines. We discuss our findings in the light of biological processes such as morphogenesis, collective cancer invasion and the shape control of biomembranes, suggesting that some biological systems may share the same underlying mechanisms as active nematic droplets.
Highlights
Active particles use energy from their surroundings to do work
In 2D active nematics, the flow induced by active stresses leads to an alignment of the director at an interface with an isotropic phase [23]
The averaging is performed over T 1⁄4 250 000 time steps, which is much longer than the average time it takes for a disclination line to move across the droplet τSP ≈ 15 000
Summary
Active particles use energy from their surroundings to do work. Examples range from eukaryotic cells, bacterial suspensions, and motor proteins to active colloids and shaken granular rods [1,2]. Imaging of a three-dimensional active nematic and, in particular, the associated motile disclination loops and lines [12] This imaging is achieved by dispersing forcegenerating microtubule bundles in a passive colloidal liquid crystal based on filamentous viruses. The theories of active materials are increasingly being used to describe biological systems in two dimensions, with examples including biofilm initiation [17], topological defects in cell monolayers [8,9,10], and epithelial expansion [18]. This use suggests that in three dimensions there may be relevance to the collective motion of groups of cells, in morphogenesis, or to the growth and spread of tumors. The last section of the paper summarizes the key results and points out possible connections to biological systems
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