Colloidal microswimmers in liquid crystals

Selvin Robert, Linsy Jane (author), Muševič, Igor (mentor), Ravnik, Miha (comentor)

This 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.

• Language: English
• Keywords: active colloids, liquid crystal, electric field, 3D-printing
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2026
• PID: 20.500.12556/RUL-187893
• COBISS.SI-ID: 291378435
• Publication date in RUL: 16.09.2026

Secondary language

• Language: Slovenian
• Title: Koloidni mikroplavalci v tekočih kristalih

Abstract

Ta disertacija se primarno osredotoča na pogonski mehanizem 3D-tiskanih koloidnih mikroplavalcev v tekočih kristalih, ki jih poganja zunanje izmenično električno polje. Prvo poglavje prikazuje vpliv razmerja stranic polimernih mikropaličic na njihove pogonske hitrosti pri nizkih frekvencah 6 Hz in 11 Hz v planarnem nematskem tekočem kristalu. V nasprotju s tradicionalno kvadratno odvisnostjo je opažena odvisnost višjega reda med hitrostjo dipolarnih mikropaličic in električnim poljem (v ⠝ Eα; α = 3 ⠒ 5). To je delno pojasnjeno s strižno redčitvenim obnašanjem tekočega kristala. Fluktuacije nematskega direktorja okoli mikropaličic se pojavijo nad pragom električnega polja za plavanje zaradi vertikalnega gibanja mikropaličice vzdolž smeri električnega polja. Pri nižji frekvenci 90 mHz fluorescenčna konfokalna mikroskopija razkrije nagibno gibanje mikropaličice vzdolž polja, s stranskim nihanjem in pogonom vzdolž nematskega direktorja. Časovna analiza pokaže kvadratno sklopitev nagibnega kota mikropaličice z uporabljenim električnim poljem ter linearno sklopitev masnega središča mikropaličice s poljem. Vpliv smeri drgnjenja planarne celice je prikazan pri vseh treh frekvencah, pri čemer se pri 6 Hz in 11 Hz pojavijo še višje potenčne odvisnosti z α = 9 ⠒ 10. V drugem poglavju so v ravnovesnem stanju v planarnih nematikih raziskani topološki defekti, ki spremljajo različne ločne dolžine torusa. Za analizo defektov so uporabljene laserske pincete, s katerimi se območje koloidnega loka termično prekali, nato pa se opazuje relaksacijska dinamika disklinacijskih linij okoli njega. Disklinacijske linije okoli koloidnega loka razkrivajo kompleksne možnosti navijanja, ki so podrobno obravnavane. V nekaterih primerih so bile opažene solitonom podobne strukture, ki povezujejo dva točkasta defekta v torusu. Pri toroidalnih lokih je opaženo pripenjanje disklinacijske linije vzdolž preseka loka, ki ga spremljajo svetli čopiči. Zadnje poglavje poda vpogled v pogon različnih geometrij tiskanih koloidov, kot so kocke in torusi, v zasukanih nematikih, kjer se pojavita tako translacijsko kot rotacijsko gibanje. Opaženi sta bočno kotaljenje in rotacijsko gibanje, z neto gibanjem vzdolž nematskega direktorja.

• Keywords: aktivni koloidi, tekoči kristali, električno polje, 3D-tiskanje