Beyond the extensile/contractile dichotomy in active nematics
Objectives
Active nematics is one of the most studied situations in active matter physics, both in theory and experiments. The emblematic in-vitro system of microtubules and kinesins (L1) occupies much of the literature and is the quintessential example of extensile activity, in which moving microtubules push out fluid along their elongation axis [1]. Other, more biologically relevant, active nematic systems have been observed, such as dense suspensions of swarming bacteria (L3) [2] or layers of confluent cells (L3) [3]. This last case of two-dimensional (2d) tissues formed by cells reaching confluence is of particular interest since they are often precursors of 3d morphogenesis (L3), via the topological defects that may appear spontaneously or be induced in the 2d nematic order field [4]. However, whether the activity is extensile, contractile – which pull fluid in along their axis – or more complex can be unclear in these systems. Standard active nematic theories frequently invoke a simple contractile/extensile dichotomy, usually chosen to be implied by the individual units composing the system: many biological cells are termed contractile because of the forces they exert on the substrate they lie upon, setting the sign of the active stress term. Recent work, however, has pointed to the importance of terms generically ignored for explaining why this dichotomy is unable to account for the behaviour of active nematic systems around topological defects [5,6].
Activities of the Doctoral Candidate
This project will pursue this approach and explore in detail the mechanisms at play in various cases of interest, including systems where the local order is not nematic but polar (T4). Collaboration with experimentalists working on cells (Masaki Sano, Shanghai Jiao Tong University) and in vitro systems (Isabella Guido, University of Surrey) will give access to recent data. By analysing these experiments and advancing numerical simulations (T4) [7,8], we will propose models for understanding biologically relevant active nematics beyond the idealised extensile/contractile dichotomy.
Facilities Provided
TBC.
Employment Contract
TBC.
Period of Doctorate and Funding
TBC.
References
1] Doostmohammadi, A, et al. (2018) Nat Comm 9:3246 [2] Li, H, et al. (2018) Proc Natl Acad Sci 116:777 [3] Kawaguchi, K, et al. (2017) Nature 545:327 [4] Guillamat, P, et al. (2022) Nat Mater 21:588 [5] Zhao, Z, et al (2024) bioRxiv 2024.08.28.610106 [6] Zhao, Z, et al. (2024) arXiv:2408.15431 [7] Kozhukhov, T, et al. (2024) Commun Phys 7:251 [8] Li, H, et al. (2024) Phys Rev X 14:041006