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

Activity gradients in two- and three-dimensional active nematics