Science 6.03.2020

INSIGHTS | PERSPECTIVES

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the 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 these

observations 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. j

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10.1126/science.aba5319

250

mM

250

250 mM mM

Active recovery

Passive recovery Bending

Deformations 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

–½

1076 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