INSIGHTS | PERSPECTIVES
sciencemag.org SCIENCEGRAPHIC: A. KITTERMAN/SCIENCEthe more typical small molecules
that from nematic phases.
These slender biological col-
loids provide a prototypical nem-
atic phase at room temperature
( 7 ). The authors built on earlier
works from Dogic and co-work-
ers and dispersed a small frac-
tion of cross-linked microtubules
and molecular motors extracted
from eukaryotic cells into this
liquid crystal to create an ac-
tive material ( 8 ). When fueled
with adenosine triphosphate,
the molecular motors elongated
the microtubule bundles, which
in turn generated active stresses
that drove bending instabilities
and bulk flows throughout the
entire sample.
The authors then solved the
second formidable challenge
of observing the inner chaotic
structure of this chimera of
nematic molecules and motors.
Elucidating the spatiotemporal
structure of a 3D active nematic
must overcome two conflicting
constraints. One is imaging the
nematic orientation from mi-
crometer to millimeter scales,
which is a time-consuming task,
and the other is computing the
spatial-orientation maps with
high temporal resolution. The
optical transparency of the ac-
tive material made it possible
to resolve this technical conflict
by taking advantage of multiv-
iew light-sheet microscopy ( 9 ).
This high-speed, high-resolution
evolution of selective-plane illumination mi-
croscopy provided the quantitative measure-
ments of the local director field over the en-
tire volume of a 3D active material. The po-
tential of this state-of-the-art experiment can
be immediately grasped from the authors’
supplementary movies, which also illustrate
the much faster flows in active nematics.
As the molecular motors drove activity, a
living network of disclinations emerged and
constantly rewired its connectivity (see the
figure, bottom left, which reveals the regions
of space where the active-nematic director is
singular or discontinuous). These living net-
works of singularities do not reflect built-in
heterogeneities of the nematic structure. Du-
clos et al. instead found that the fundamental
excitations of 3D active nematics are discli-
nation loops that can nucleate, shrink, open,
and merge to form spatially extended struc-
tures out of smooth and homogeneous re-
gions of space (see the figure, bottom right).
A careful analysis of the nematic orientation
around the singular loops revealed that two
ring-shape excitations chiefly rule the emer-
gent distortions in the liquid crystal.
Both families of disclination loops are
topologically neutral but are at the core of
geometrically distinct textures. Wedge-twist
loops connect sections of spaces hosting the
familiar +½ and –½ defects found in 2D
nematics (see the figure, bottom right). This
continuous connection is made possible by
a smooth 3D rotation of the nematic units
along the loop. At mid-distance between the
two planar wedges, the variations of the di-
rector around the centerline of the disclina-
tion line forms the equivalent of a Möbius
strip, a one-sided shape that can be visual-
ized by taping together the ends of a paper
band after applying a half-turn twist ( 10 ).
The second and most abundant ring-
shape singularities are pure-twist loops
along which Möbius conformations encircle
the entire defect line. Comparisons with nu-
merical resolutions further confirmed theseobservations and established
that the two elementary ring
singularities originate from
activity-induced flows, so that
active hydrodynamic theories
delivered compelling accuracy.
However, 3D active turbulence
cannot be reduced to interact-
ing-particle models. Quantita-
tive connections between defect
topology, loop geometry, and
flows are only emerging and re-
main to be elucidated.
In a field dominated by nu-
merical simulations and theory,
Duclos et al. offer a formidable
experimental platform with
which to gain a deep insight in
active matter. One can envision
the synthesis of active smectics
(that have a layered structure)
or chiral nematics that have no
2D counterpart. Another ap-
pealing prospect would be ma-
nipulation of disclination net-
works around solid inclusions.
Particle-laden liquid crystals are
not bound to hosting only neu-
tral topological excitations. For
example, isolated point charges
exist in three dimensions, such
as hedgehogs, in which the di-
rector orientations resemble
hedgehog spikes. Activity could
be put to work to dynamically
twist, braid, and knot disclina-
tion lines or to couple them to
point defects not yet observed
in the experiments of Duclos et
al. ( 6 , 11 ). The number of oppor-
tunities delivered by the combi-
nation of a versatile active material and a
state-of-the art observation tool seems virtu-
ally unlimited. Last, the production of actual
volumes of synthetic active nematics is a
major milestone on the route toward practi-
cal application of smart active materials. jREFERENCES AND NOTES- M. C. Marchetti et al., Rev. Mod. Phys. 85 , 1143 (2013).
- J. Zhang, E. Luijten, B. A. Grzybowski, S. Granick, Chem.
Soc. Rev. 46 , 5551 (2017). - A. Doostmohammadi, J. Ignés-Mullol, J. M. Yeomans, F.
Sagués, Nat. Commun. 9 , 3246 (2018). - G. Duclos et al., Science 367 , 1120 (2020).
- L. Giomi, Phys. Rev. X 5 , 031003 (2015).
- G. P. Alexander, B. Gin-ge Chen, E. A. Matsumoto, R. D.
Kamien, Rev. Mod. Phys. 84 , 499 (2012). - Z. Dogic, S. Fraden, Curr. Opin. Colloid Interface Sci. 11 ,
47 (2006). - T. Sanchez, D. T. N. Chen, S. J. DeCamp, M. Heymann, Z.
Dogic, Nature 491 , 431 (2012). - U. Krzic, S. Gunther, T. E. Saunders, S. J. Streichan, L.
Hufnagel, Nat. Methods 9 , 730 (2012). - Y. Bouligand, J. Phys. (Paris) 35 , 215 (1974).
- U. Tkalec, M. Ravnik, S. Čopar, S. Žumer, I. Muševič, Sci-
ence 333 , 62 (2011).
10.1126/science.aba5319250
mM250
250 mM mMActive recoveryPassive recovery BendingDeformations in thin flms
Bending a nematic LC (middle) distorts the molecular alignment. In a passive
nematic (left), the orientation along the director simply relaxes back. In an active LC
(right), distortions are exponentially amplifed and create pairs of +½ and –½
topological defects.Deformations in 3D LCs
Duclos et al. used high-resolution
microscopy to image the director feld.
They located the extended defects
(the linear structures) formed by
internally driven bending instabilties.Defect connections
The wedge-twist loop connects
+½ and –½ defects like a Möbius
strip. This smooth evolution can
only occur in 3D materials and
not in thin flms.–½ +½+½–½1076 6 MARCH 2020 • VOL 367 ISSUE 6482
Topological defects in active liquid crystals
In active matter, internal molecular motors can drive fluid or particle motion and
amplify responses to external stimuli such as bending forces. Most examples have
been thin films, but Duclos et al. now report three-dimensional (3D) active matter,
a viral particle liquid crystal (LC) driven by embedded microtubule networks.Published by AAAS