LPS, amphi moyen


20 Jan 2023


11h00 - 12h00

Alfredo Sciortino – From nematic defects to polar order: polar flow of gliding filaments steered by nematic defects

PostDoc at CytoMorphoLab, CEA Grenoble / Hopital
Saint-Louis Paris

Much like traditional passive materials, active systems can be
affected by the presence of imperfections in their microscopic order,
called defects, that influence their macroscopic properties. This
suggests the possibility to steer collective patterns by introducing
and controlling defects in an active system. For instance, nematic
materials, composed of aligned, elongated building blocks, can contain
defects in the local order (nematic defects) that induce long-range
distortions and that have also been shown to play a role in a myriad
of active biological systems. While this kind of defects is mostly
associated with systems having nematic symmetry, here we show that
their presence can also lead to the formation of active polar
patterns. By using an experimental setup in which cytoskeletal
filaments propelled by molecular motors actively glide on a fluid
lipid membrane, we show that the alignment between individual
filaments resulting from these conditions leads to the formation of
collective polar structures, despite a microscopic nematic symmetry.
We also find that the transient formation of +1/2 defects plays a
pivotal role in sorting the filaments’ polarity [1]. We can then take
advantage of our ability to control nematic material and their defects
to shape the morphology of patterns. First, we extend the system to
filaments gliding inside a giant vesicle [2], i.e. in a spherical
geometry, and find that confinement enhances the global polarity,
leading the formation of highly polar vortices and bands. However, at
the same time, topological constraints on the alignment of filaments
due to the spherical geometry can affect and hinder their ability to
assemble effectively, leading to “topologically jammed” states. We
finally study the influence of passive defects on active filaments by
having microtubules actively glide inside a passive nematic material
composed of actin filaments [3]. Again, we observe the formation of
polar patterns whose emergence is now a direct effect of the presence
of spontaneously-forming passive defects, that rectify the active
motion and break the orientational symmetry, leading to globally polar
flow despite a nematic environment. These results suggest strategies
to control active patterns by tuning defects in the environment, for
example by changing its topology or by controlling the position of
individual point defects.

[1] Sciortino, Bausch, PNAS, 2021
[2] Sciortino and Hsu, et al., Nat Commun, 2022
[3] Sciortino et al., Nat Mater, (in print)