Abstract
The physics of active liquid crystals is mostly governed by the interplay between elastic forces that align their constituents, and active stresses that destabilize the order with constant nucleation of topological defects and chaotic flows. The average distance between defects, also called active length scale, depends on the competition between these forces. Here, in experiments with the microtubule/kinesin active nematic system, we show that the intrinsic active length scale loses its relevance under strong lateral confinement. Transitions are observed from chaotic to vortex lattices and defect-free unidirectional flows. Defects, which determine the active flow behaviour, are created and annihilated on the channel walls rather than in the bulk, and acquire a strong orientational order in narrow channels. Their nucleation is governed by an instability whose wavelength is effectively screened by the channel width. These results are recovered in simulations, and the comparison highlights the role of boundary conditions.
Highlights
The physics of active liquid crystals is mostly governed by the interplay between elastic forces that align their constituents, and active stresses that destabilize the order with constant nucleation of topological defects and chaotic flows
Theoretical work predicted that laterally-confined active nematics undergo an instability to spontaneous laminar flow when the channel width reaches a typical length scale that depends on the strength of the activity[30]
The active nematic was prepared using an open-cell design[32], in which 2 μL of the active aqueous microtubule-mixture was placed inside a custom-made pool of 5 mm diameter and was covered with 60 μL of 100 cSt silicon oil
Summary
The physics of active liquid crystals is mostly governed by the interplay between elastic forces that align their constituents, and active stresses that destabilize the order with constant nucleation of topological defects and chaotic flows. Defects, which determine the active flow behaviour, are created and annihilated on the channel walls rather than in the bulk, and acquire a strong orientational order in narrow channels Their nucleation is governed by an instability whose wavelength is effectively screened by the channel width. Theoretical work predicted that laterally-confined active nematics undergo an instability to spontaneous laminar flow when the channel width reaches a typical length scale that depends on the strength of the activity[30]. This prediction has been recently confirmed in experiments with spindle-shaped cells[11]. We stress the close interplay between the velocity field and the defect dynamics, and highlight the emergence of a new length scale that, contrary to the classical active length scale, does not depend on the activity level but merely on geometrical parameters
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