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Theoretical physicists working at a blackboard collaboration pod in the Beecroft building.
Credit: Jack Hobhouse

Amin Doostmohammadi

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Sub department

  • Rudolf Peierls Centre for Theoretical Physics
a.dstmhmdi@gmail.com
Telephone: 01865 (2)73334
Rudolf Peierls Centre for Theoretical Physics
  • About
  • Publications

Defect-mediated morphologies in growing cell colonies

Physical Review Letters American Physical Society 117:4 (2016) 048102

Authors:

Amin Doostmohammadi, Sumesh P Thampi, Julia Yeomans

Abstract:

Morphological trends in growing colonies of living cells are at the core of physiological and evolutionary processes. Using active gel equations, which include cell division, we show that shape changes during the growth can be regulated by the dynamics of topological defects in the orientation of cells. The friction between the dividing cells and underlying substrate drives anisotropic colony shapes toward more isotropic morphologies, by mediating the number density and velocity of topological defects. We show that the defects interact with the interface at a specific interaction range, set by the vorticity length scale of flows within the colony, and that the cells predominantly reorient parallel to the interface due to division-induced active stresses.
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Active micromachines: Microfluidics powered by mesoscale turbulence

Science Advances American Association for the Advancement of Science (2016)

Authors:

Julia Yeomans, Amin Doostmohammadi, Tyler N Shendruk, Sumesh P Thampi, Ramin Golestanian

Abstract:

Dense active matter, from bacterial suspensions and microtubule bundles driven by motor proteins to cellular monolayers and synthetic Janus particles, is characterised by mesoscale turbulence, the emergence of chaotic flow structures. By immersing an ordered array of symmetric rotors in an active fluid, we introduce a microfluidic system that exploits spontaneous symmetry breaking in mesoscale turbulence to generate work. The lattice of rotors self-organises into a spin-state where neighbouring discs continuously rotate in permanent alternating directions due to combined hydrodynamic and elastic effects. Our virtual prototype demonstrates a new research direction for the design of micromachines powered by the nematohydrodynamic properties of active turbulence.
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Stabilisation of active matter by flow vortex-lattices and defect ordering

Nature Communications Springer Nature 7 (2016) 10557

Authors:

Amin Doostmohammadi, Michael Adamer, Sumesh Thampi, Julia Yeomans

Abstract:

Active systems, from bacterial suspensions to cellular monolayers, are continuously driven out of equilibrium by local injection of energy from their constituent elements and exhibit turbulent-like and chaotic patterns. Here we demonstrate both theoretically and through numerical simulations, that the crossover between wet active systems, whose behaviour is dominated by hydrodynamics, and dry active matter where any flow is screened, can be achieved by using friction as a control parameter. Moreover, we discover unexpected vortex ordering at this wet–dry crossover. We show that the self organization of vortices into lattices is accompanied by the spatial ordering of topological defects leading to active crystal-like structures. The emergence of vortex lattices, which leads to the positional ordering of topological defects, suggests potential applications in the design and control of active materials.
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Upstream swimming in microbiological flows

Physical Review Letters American Physical Society 116:2 (2016) 028104

Authors:

Julia Yeomans, Arnold JTM Mathijssen, Tyler N Shendruk, Amin Doostmohammadi

Abstract:

Interactions between microorganisms and their complex flowing environments are essential in many biological systems. We develop a model for microswimmer dynamics in non-Newtonian Poiseuille flows. We predict that swimmers in shear-thickening (-thinning) fluids migrate upstream more (less) quickly than in Newtonian fluids and demonstrate that viscoelastic normal stress differences reorient swimmers causing them to migrate upstream at the centreline, in contrast to well-known boundary accumulation in quiescent Newtonian fluids. Based on these observations, we suggest a sorting mechanism to select microbes by swimming speed.
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Celebrating Soft Matter's 10th Anniversary: Cell division: a source of active stress in cellular monolayers.

Soft matter Royal Society of Chemistry 11:37 (2015) 7328-7336

Authors:

Amin Doostmohammadi, Sumesh P Thampi, Thuan B Saw, Chwee T Lim, Benoit Ladoux, Julia Yeomans

Abstract:

We introduce the notion of cell division-induced activity and show that the cell division generates extensile forces and drives dynamical patterns in cell assemblies. Extending the hydrodynamic models of lyotropic active nematics we describe turbulent-like velocity fields that are generated by the cell division in a confluent monolayer of cells. We show that the experimentally measured flow field of dividing Madin-Darby Canine Kidney (MDCK) cells is reproduced by our modeling approach. Division-induced activity acts together with intrinsic activity of the cells in extensile and contractile cell assemblies to change the flow and director patterns and the density of topological defects. Finally we model the evolution of the boundary of a cellular colony and compare the fingering instabilities induced by cell division to experimental observations on the expansion of MDCK cell cultures.
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