Creating order by mechanical deformation in dense active matter

Date:
September 27, 2021
Source:
University of Go"ttingen
Summary:
Living or biological systems cannot be easily understood using the
standard laws of physics, such as thermodynamics, as scientists
would for gases, liquids or solids. Living systems are active,
demonstrating fascinating properties such as adapting to their
environment or repairing themselves. Exploring the questions posed
by living systems using computer simulations, researchers have now
discovered a novel type of ordering effect generated and sustained
by a simple mechanical deformation, specifically steady shear.



FULL STORY ========================================================================== Living or biological systems cannot be easily understood using the
standard laws of physics, such as thermodynamics, as scientists would
for gases, liquids or solids. Living systems are active, demonstrating fascinating properties such as adapting to their environment or repairing themselves. Exploring the questions posed by living systems using
computer simulations, researchers at the University of Go"ttingen have
now discovered a novel type of ordering effect generated and sustained by
a simple mechanical deformation, specifically steady shear. The results
were published in PNAS.


========================================================================== Understanding living systems, such as tissues formed by cells, poses
a significant challenge because of their unique properties, such as
adaptation, self-repair and self-propulsion. Nonetheless, they can be
studied using models that treat them as just an unusual, "active" form
of physical matter. This can reveal extraordinary dynamical or mechanical properties. One of the puzzles is how active materials behave under shear
(the deformation produced by moving the top and bottom layers sideways
in opposite directions, like sliding microscope cover plates against
each other). Researchers at the Institute for Theoretical Physics,
University of Go"ttingen explored this question and discovered a novel
type of ordering effect that is generated and sustained by steady shear deformation. The researchers used a computer model of self-propelling
particles where each particle is driven by a propulsion force that
changes direction slowly and randomly. They found that while the flow
of the particles looks similar to that in ordinary liquids, there is a
hidden order revealed by looking at the force directions: these tend to
point towards the nearest (top or bottom) plate, while particles with
sideways forces aggregate in the middle of the system.

"We were exploring the response of a model active material under steady driving, where the system is sandwiched between two walls, one stationary
and the other moving to generate shear deformation. What we saw was that
at a sufficiently strong driving force, an interesting ordering effect emerges," comments Dr Rituparno Mandal, Institute for Theoretical Physics
at the University of Go"ttingen. "We now also understand the ordering
effect using a simple analytical theory and the predictions from this
theory match surprisingly well with the simulation." Senior author
Professor Peter Sollich, also from the Institute for Theoretical
Physics, Universiy of Go"ttingen, explains, "Often an external force
or driving force destroys ordering. But here the driving by shear
flow is key in providing mobility to the particles that make up the
active material, and they actually need this mobility to achieve the
observed order. The results will open up exciting possibilities for
researchers investigating the mechanical responses of living matter." ========================================================================== Story Source: Materials provided by University_of_Go"ttingen. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Rituparno Mandal, Peter Sollich. Shear-induced orientational
ordering in
an active glass former. Proceedings of the National Academy of
Sciences, 2021; 118 (39): e2101964118 DOI: 10.1073/pnas.2101964118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210927172909.htm

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