David participates in the MoLE conference, 25-29 July, Donostia-San Sebastián

David and Onofrio during the poster session at MoLE conference in Donostia-San Sebastián. (Photos provided by D. Bronte Ciriza)
Between the 25th and the 29th of July 2022 David participated in the MoLE conference in Donostia-San Sebastián. The unexpected demise of Professor Juan José Sáenz, on March 22, 2020, has left his beloved family and friends in shock all over the globe. Following his spirit, MoLE conference 2022 was devoted to honouring his memory the way he would have liked: Appreciated colleagues and friends presenting and discussing their most recent advances, in both electronics and photonics. During this conference David presented a poster on “Elongated active particles in speckle fields“, where he had the chance to discuss the topic with a broad community of interested scientists.

Activity gradients in two- and three-dimensional active nematics

Spatial variations of active stress induce active torques, which aligns self-propelled defects along the gradient direction. Additionally there are also passive torques acting on defects induce by the elastic energy associated with deformations of the director field. The magnitude of active and passive torques acting on defects depends on defect type: While +1/2 defects (2D) and +1/2 disclination lines (3D) are dominated by active torque, twist-type disclination are subject to passive torques which aligns them in a way which minimizes the elastic energy of the system. (Illustration by Liam Ruske)
Activity gradients in two- and three-dimensional active nematics
Liam Ruske & Julia Yeomans
Soft Matter 18 5654-5661 (2022)
arxiv: 2206.06499
DOI: 10.1039/D2SM00228K


We numerically investigate how spatial variations of extensile or contractile active stress affect bulk active nematic systems in two and three dimensions. In the absence of defects, activity gradients drive flows which re-orient the nematic director field and thus act as an effective anchoring force. At high activity, defects are created and the system transitions into active turbulence, a chaotic flow state characterized by strong vorticity. We find that in two-dimensional (2D) systems active torques robustly align +1/2 defects parallel to activity gradients, with defect heads pointing towards contractile regions. In three-dimensional (3D) active nematics disclination lines preferentially lie in the plane perpendicular to activity gradients due to active torques acting on line segments. The average orientation of the defect structures in the plane perpendicular to the line tangent depends on the defect type, where wedge-like +1/2 defects align parallel to activity gradients, while twist defects are aligned anti-parallel. Understanding the response of active nematic fluids to activity gradients is an important step towards applying physical theories to biology, where spatial variations of active stress impact morphogenetic processes in developing embryos and affect flows and deformations in growing cell aggregates, such as tumours.

Liam Ruske presented a poster at the Active and Intelligent Living Matter conference, 26 June – 1 July 2022, Erice, Italy

The beatiful view from the conference centre on Mount Cofano. (Photo by L. Ruske)
Between the 26th of June and the 1st of July 2022 Liam participated in the Active and Intelligent Living Matter conference on Sicily, where he presented a poster summarizing several of his research projects about active continuum theories and their application to biological systems.