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Emergent motion in confined active nematics

Rodrigo C. V. Coelho CBPF – Centro Brasileiro de Pesquisas Físicas Active nematics are nonequilibrium liquid crystals composed of self-driven elongated units that continuously inject energy at the microscopic scale, leading to spontaneous flows, active turbulence, and the emergence of topological defects. When confined or coupled to boundaries, these systems display complex hydrodynamic responses. In this seminar, I will discuss how active nematics interact with solid and deformable boundaries, focusing on two complementary situations. First, I will present numerical and experimental results showing how patterned rigid obstacles can be used to control chaotic active flows. In particular, regular arrays of anisotropic elements, such as triangular pillars, can act as active nematic pumps, generating sustained directional transport and efficient mixing in microfluidic environments. I will then focus on the dynamics of deformable capsules suspended in active nematic fluids. Using lattice Boltzmann simulations, I will show how the interplay between active stresses, elasticity, and geometry gives rise to spontaneous rotation and self-propulsion. Symmetric circular capsules can exhibit persistent rotation when confined topological defects organize into stable rotating patterns, while asymmetric shapes such as boomerangs develop directed motion driven by unbalanced active forces. Moreover, capsule flexibility is shown to suppress coherent motion by dissipating activity into shape deformations.Together, these results illustrate how confinement and topology can be exploited to design functional active materials, enabling controlled transport, mixing, and autonomous motion in soft and biological systems.