Colloidal microswimmers in liquid crystalsSelvin Robert, Linsy Jane (Avtor) Muševič, Igor (Mentor) Ravnik, Miha (Komentor) active colloidsliquid crystalelectric field3D-printingThis thesis primarily focuses on the propulsion mechanism behind the 3D-printed colloidal microswimmers in liquid crystals, which are driven by an applied alternating electric field. The first results chapter demonstrates the influence of the aspect ratio of polymer microrods on their propulsion velocities at low frequencies of 6 Hz and 11 Hz, in planar nematic liquid crystal. A higher-order relationship between the velocity of the dipolar microrods and the electric field (v ⠝ Eα; α = 3 ⠒ 5) is observed in contrast to the traditional quadratic dependency. This is partially explained by the shear-thinning behaviour of the liquid crystal. Fluctuations of the nematic director around the microrods appear above the electric field threshold for swimming, due to the vertical motion of the microrod along the electric field direction. At a lower frequency of 90 mHz, fluorescence confocal microscopy reveals the tilting motion of the microrod along the field, with sideways wobbling and propulsion along the nematic director. The temporal analysis reveals quadratic coupling of the tilt angle of the microrod to the applied electric field, and linear coupling of the centre of mass of the microrod to the field. The influence of the planar cell’s rubbing direction is demonstrated across all three frequencies, exhibiting even higher power law behaviours of α = 9 ⠒ 10 for 6 Hz and 11 Hz. In the second results chapter, the topological defects accompanying different arc lengths of the torus are investigated in the equilibrium state, in planar nematics. To analyse the defects, laser tweezers are used to thermally quench the region of the colloidal arc and observe the relaxation dynamics of the disclination lines around it. The disclination lines around the colloidal arc reveal complex winding possibilities, which are discussed in detail. There are cases where soliton-like structures connecting two point defects in a torus were observed. In toroidal arcs, pinning of a disclination line along the cross-section of the arc, accompanied by bright brushes, is observed. The final results chapter gives a brief overview of the propulsion of various geometries of printed colloids (cubes and tori) in twisted nematics, exhibiting both translational and rotational motion. We observe sideways rolling and rotational motion, with net propagation along the nematic director.20262026-09-16 08:15:10Doktorsko delo/naloga187893sl