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• Andersen, Benjamin H.; Renaud, Julian; Rønning, Jonas; Angheluta, Luiza & Doostmohammadi, Amin (2023). Symmetry-restoring crossover from defect-free to defect-laden turbulence in polar active matter. Physical Review Fluids. 8(6). doi: 10.1103/PhysRevFluids.8.063101. Full text in Research Archive Show summary

  Coherent flows of self-propelled particles are characterized by vortices and jets that sustain chaotic flows, referred to as active turbulence. Here, we reveal a crossover between defect-free active turbulence and active turbulence laden with topological defects. Interestingly, we show that concurrent to the crossover from defect-free to defect-laden active turbulence is the restoration of the previously broken SO(2) symmetry signaled by the fast decay of the two-point correlations. By stability analyses of the topological charge density field, we provide theoretical insights on the criterion for the crossover to the defect-laden active turbulent state. Despite the distinct symmetry features between these two active turbulence regimes, the flow fluctuations exhibit universal statistical scaling behaviors at large scales, while the spectrum of polarity fluctuations decays exponentially at small length scales compared to the active energy injection length. These findings reveal a dynamical crossover between distinct spatiotemporal organization patterns in polar active matter.

• Rønning, Jonas; Renaud, Julian; Doostmohammadi, Amin & Angheluta, Luiza (2023). Spontaneous flows and dynamics of full-integer topological defects in polar active matter. Soft Matter. ISSN 1744-683X. doi: 10.1039/d3sm00316g. Full text in Research Archive Show summary

  Polar active matter of self-propelled particles sustain spontaneous flows through the full-integer topological defects. We study theoretically the incompressible flow profiles around ±1 defects induced by polar and dipolar active forces. We show that dipolar forces induce vortical flows around the +1 defect, while the flow around the −1 defect has an 8-fold rotational symmetry. The vortical flow changes its chirality near the +1 defect core in the absence of the friction with a substrate. We show analytically that the flow induced by polar active forces is vortical near the +1 defect and is 4-fold symmetric near the −1 defect, while it becomes uniform in the far-field. For a pair of oppositely charged defects, this polar flow contributes to a mutual interaction force that depends only on the orientation of the defect pair relative to the background polarization, and that enhances defect pair annihilation. This is in contradiction with the effect of dipolar active forces which decay inversely proportional with the defect separation distance. As such, our analyses reveals a long-ranged mechanism for the pairwise interaction between topological defects in polar active matter.

• Skogvoll, Vidar; Rønning, Jonas; Salvalaglio, Marco & Angheluta, Luiza (2023). A unified field theory of topological defects and non-linear local excitations. npj Computational Materials. 9(1), p. 1–13. doi: 10.1038/s41524-023-01077-6. Full text in Research Archive Show summary

  Abstract Topological defects and smooth excitations determine the properties of systems showing collective order. We introduce a generic non-singular field theory that comprehensively describes defects and excitations in systems with O ( n ) broken rotational symmetry. Within this formalism, we explore fast events, such as defect nucleation/annihilation and dynamical phase transitions where the interplay between topological defects and non-linear excitations is particularly important. To highlight its versatility, we apply this formalism in the context of Bose-Einstein condensates, active nematics, and crystal lattices.

• Rønning, Jonas & Angheluta, Luiza (2023). Precursory patterns to vortex nucleation in stirred Bose-Einstein condensates. Physical Review Research (PRResearch). 5(2). doi: 10.1103/PhysRevResearch.5.023108. Full text in Research Archive Show summary

  Within the Gross-Pitaevskii theory, we study precursory pattern formations to the nucleation of vortex dipoles in a two-dimensional Bose-Einstein condensate stirred by a Gaussian potential. We introduce a smooth superfluid vorticity field and its conservative current, which capture very well the gradual process of vortex nucleation as a mechanism of topological singularities acquiring smooth cores. This is characterized by the localization of the superfluid vorticity into core regions which harbor the phase slips. For more impenetrable obstacles, we find that there are additional phase slips that do not acquire cores, thus remaining pinned as ghost vortices to the potential. We show that the vortex kinematics is slaved to the superfluid vorticity current, which determines not only the onset of nucleation but also the shedding dynamics.

• Rønning, Jonas; Marchetti, M. Cristina & Angheluta, Luiza (2023). Defect self-propulsion in active nematic films with spatially varying activity. Royal Society Open Science. 10(2). doi: 10.1098/rsos.221229. Full text in Research Archive Show summary

  We study the dynamics of topological defects in active nematic films with spatially varying activity and consider two set-ups: (i) a constant activity gradient and (ii) a sharp jump in activity. A constant gradient of extensile (contractile) activity endows the comet-like +1/2 defect with a finite vorticity that drives the defect to align its nose in the direction of decreasing (increasing) gradient. A constant gradient does not, however, affect the known self-propulsion of the +1/2 defect and has no effect on the −1/2 that remains a non-motile particle. A sharp jump in activity acts like a wall that traps the defects, affecting the translational and rotational motion of both charges. The +1/2 defect slows down as it approaches the interface and the net vorticity tends to reorient the defect polarization so that it becomes perpendicular to the interface. The −1/2 defect acquires a self-propulsion towards the activity interface, while the vorticity-induced active torque tends to align the defect to a preferred orientation. This effective attraction of the negative defects to the wall is consistent with the observation of an accumulation of negative topological charge at both active/passive interfaces and physical boundaries.

• Moen, Emily Qing Zang; Olsen, Kristian Stølevik; Rønning, Jonas & Angheluta, Luiza (2022). Trapping of active Brownian and run-and-tumble particles: A first-passage time approach. Physical Review Research (PRResearch). 4(4), p. 1–8. doi: 10.1103/PhysRevResearch.4.043012. Full text in Research Archive Show summary

  We use a first-passage time approach to study the statistics of the trapping times induced by persistent motion of active particles colliding with flat boundaries. The angular first-passage time distribution and mean first-passage time are calculated exactly for active Brownian and run-and-tumble particles, and the results of the two prototypes of active particles are contrasted. Theoretical predictions are in excellent agreement with Langevin simulations of active particle dynamics. Our results shed further light on how active particles with different dynamics may have equivalent statistical properties in the bulk yet behave differently near boundaries or obstacles.

• Rønning, Jonas; Marchetti, Cristina M.; Bowick, Mark J. & Angheluta, Luiza (2022). Flow around topological defects in active nematic films. Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences. ISSN 1364-5021. 478(2257). doi: 10.1098/rspa.2021.0879. Full text in Research Archive Show summary

  We study the active flow around isolated defects and the self-propulsion velocity of +1/2 defects in an active nematic film with both viscous dissipation (with viscosity η) and frictional damping Γ with a substrate. The interplay between these two dissipation mechanisms is controlled by the hydrodynamic dissipation length ℓd=η/Γ−−−√ that screens the flows. For an isolated defect, in the absence of screening from other defects, the size of the shear vorticity around the defect is controlled by the system size R. In the presence of friction that leads to a finite value of ℓd, the vorticity field decays to zero on the lengthscales larger than ℓd. We show that the self-propulsion velocity of +1/2 defects grows with R in small systems where R<ℓd, while in the infinite system limit or when R≫ℓd, it approaches a constant value determined by ℓd.

• Rønning, Jonas; Skaugen, Audun; Hernández-García, Emilio; López, Cristóbal & Angheluta, Luiza (2020). Classical analogies for the force acting on an impurity in a Bose-Einstein condensate. New Journal of Physics. 22. doi: 10.1088/1367-2630/ab95de. Full text in Research Archive Show summary

  We study the hydrodynamic forces acting on a small impurity moving in a two-dimensional Bose–Einstein condensate at non-zero temperature. The condensate is modelled by the damped-Gross Pitaevskii (dGPE) equation and the impurity by a Gaussian repulsive potential coupled to the condensate. For weak coupling, we obtain analytical expressions for the forces acting on the impurity, and compare them with those computed through direct numerical simulations of the dGPE and with the corresponding expressions for classical forces. For non-steady flows, there is a time-dependent force dominated by inertial effects and which has a correspondence in the Maxey–Riley theory for particles in classical fluids. In the steady-state regime, the force is dominated by a self-induced drag. Unlike at zero temperature, where the drag force vanishes below a critical velocity, at low temperatures the impurity experiences a net drag even at small velocities, as a consequence of the energy dissipation through interactions of the condensate with the thermal cloud. This dissipative force due to thermal drag is similar to the classical Stokes' drag. There is still a critical velocity above which steady-state drag is dominated by acoustic excitations and behaves non-monotonically with impurity's speed.

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• Angheluta, Luiza & Rønning, Jonas (2022). Dynamics of topological defects in active nematic films. Full text in Research Archive

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