Active materials are capable of converting free energy into mechanical work to produce autonomous motion, and exhibit striking collective dynamics that biology relies on for essential functions. Controlling those dynamics and transport in synthetic systems has been particularly challenging. Here, we introduce the concept of spatially structured activity as a means of controlling and manipulating transport in active nematic liquid crystals consisting of actin filaments and light-sensitive myosin motors. Simulations and experiments are used to demonstrate that topological defects can be generated at will and then constrained to move along specified trajectories by inducing local stresses in an otherwise passive material. These results provide a foundation for the design of autonomous and reconfigurable microfluidic systems where transport is controlled by modulating activity with light.
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All experimental image data are available on the Dryad server (https://doi.org/10.5061/dryad.7wm37pvr6). Additional data are available upon request.
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R.Z. and A.M. are grateful to The University of Chicago Research Computing Center for assistance with the calculations carried out in this work. This work is primarily supported by The University of Chicago Materials Research Science and Engineering Center, which is funded by the National Science Foundation (NSF) under award DMR-2011854. J.J.d.P. acknowledges support from NSF grant DMR-1710318. The calculations presented here were performed on the GPU cluster supported by the NSF under grant DMR-1828629. N.K. acknowledges the Yen Fellowship of the Institute for Biophysical Dynamics, The University of Chicago. Z.B. acknowledges support from NIH R01 GM114627 and a W. M. Keck Foundation grant to Z.B. and M. Prakash. M.L.G. acknowledges support from NSF Grant DMR-1905675 and NIH GM104032.
The authors declare no competing interests.
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Supplementary Video legends 1–9, Figs. 1–10, Table 1 and discussion.
Active nematic with time varying activity. Time-lapse fluorescence images of liquid crystal with conditions detailed in Supplementary Table 1. To stimulate motor gear shifting, the sample was illuminated with a 491 nm laser across the entire field of view in addition to imaging wavelengths. Stimulated frames are indicated with ‘Light on' in the upper left-hand corner.
Active nematic with spatially varying activity. Time-lapse fluorescence images of liquid crystal with conditions detailed in Supplementary Table 1. To stimulate motor gear shifting in only one portion of the sample, a 470 nm LED was targeted to only the region outlined by the red box using a digital micromirror array for the duration of the video (see Methods).
Simulations of flat interface showing that a +½ defect (highlighted in a red circle) originally nucleated in an active region (red shadowed) and straying into a passive region is eventually attracted back to the active region.
Simulations of defect generation using a rectangular pattern. Two pairs of defects are generated, followed by complex motion, annihilation and regeneration of defects.
Simulations of defect generation using a triangular pattern. A single pair of defects arises from the asymmetric activity pattern.
Simulations of a pair of ±½ defects. Annihilation for uniform activity α/α0 = 0.2 showing that the +½ defect moves along its symmetry axis and eventually annihilates the −½ defect.
Active nematic with spatially directed defect motion. Time-lapse fluorescence images of liquid crystal with conditions detailed in Supplementary Table 1. To stimulate motor gear shifting a 470 nm LED was targeted to only the quarter-annulus region outlined in red using a digital micromirror array (see Methods). The defect centre is indicated with a blue dot while the tail indicates the defect path since the start of the experiment. Replicate 1.
Simulations show that the annulus pattern can constrain and guide the +½ defect, supporting the experimental observation that activity patterns can guide the defect.
Simulations of defect pathways in a ‘H' channel using two different activity patterns, showing that the activity pattern is able to guide defect pathways through a microfluidic device.
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Zhang, R., Redford, S.A., Ruijgrok, P.V. et al. Spatiotemporal control of liquid crystal structure and dynamics through activity patterning. Nat. Mater. 20, 875–882 (2021). https://doi.org/10.1038/s41563-020-00901-4
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