Curling Liquid Crystal Microswimmers: A Cascade of Spontaneous Symmetry Breaking

Abstract

We report curling self-propulsion in aqueous emulsions of common mesogenic compounds. Nematic liquid crystal droplets self-propel in a surfactant solution with concentrations above the critical micelle concentration while undergoing micellar solubilization [1]. We analyzed trajectories both in a Hele-Shaw geometry and in a 3D setup at variable buoyancy. The coupling between the nematic director field and the convective flow inside the droplet leads to a second symmetry breaking which gives rise to curling motion in 2D. This is demonstrated through a reversible transition to nonhelical persistent swimming by heating to the isotropic phase. Furthermore, auto-chemotaxis can spontaneously break the inversion symmetry, leading to helical trajectories. Artificial self-propelled systems have gained attention as small scale model systems for simulating biological equivalents, either for single particles or large scale collective behaviour. In this framework, many different schemes for experimental self-propelling swimmers have been developed, mimicking various features of biological systems. The experiments can be categorized either as surfers needing direct contact with an interface [2-5], or swimmers which self-propel in the bulk. Prime examples for the latter case are Janus particles [6][7][8][9][10] and active emulsions of droplets in surfactant solutions [1,[11][12][13][14][15][16][17][18][19]. So far, all swimmer systems with spherical or polar symmetry show active Brownian behavior, with trajectories that are ballistic over short distance and diffusive over long timescales [6,7,13,18]. In contrast, helical or circular trajectories have been reported in bioflagellates close to surfaces [20][21][22], as well as in bacterial and artificial swimmers with at least twofold structural asymmetry, both in experiments and simulations [9,10,[23][24][25][26].In this paper, we report curling motion in two, as well as helical trajectories in three dimensions, for active emulsions. Our system that is spherically symmetric at rest, consisting of nematic liquid crystal droplets in an aqueous surfactant solution [1,17]. Rotational torques are generated by the interplay of surface flow and nematic order, such that the curling motion can be switched off by heating to the isotropic state and the droplet reverts to persistent swimming. Since fundamental parameters and forces can be tuned reproducibly, such emulsions are well suited for comparison to current numerical studies [27].We study droplets of the calamitic liquid crystal 4-pentyl-4 -cyanobiphenyl (5CB), which is nematic at room temperature, in an aqueous solution of the ionic surfactant tetradecyltrimethylammonium bromide (TTAB). If the TTAB concentration exceeds the critical micelle concentration, c TTAB > CMC ≈ 0.13 wt%, the droplets slowly dissolve by micellar solubilization, with the droplet radius decreasing linearly with time [17].For TTAB concentrations above c TTAB ≈ 5 wt%, the droplets self propel with typical speeds between 5 and 25 µm s −1 [1]. Shrinking rate and sp...

Chemotaxis and autochemotaxis of self-propelling droplet swimmers

“…The observation time is on the order of hours. Droplet speed and trajectory persistence are well controllable via temperature and surfactant concentration (44). The droplet propulsion is initiated at surfactant concentrations above 4 wt% TTAB.…”

“…Nematic droplet swimmers of pure 5CB exhibit a strong curling instability (44) in their propulsion, which is absent in isotropic droplets. For this study, we use only isotropic droplets, either by keeping the ambient temperature above the nematicisotropic transition at T NI = 35 • C or by substituting a mixture of 5CB and 1-Bromopentadecane (BPD) with a volume ratio of 10:1.…”

Asymmetric nanoparticle may go “active” at room temperature

“…This unexpected dynamic behavior has a similarity to that of active matters [49][50][51][52]. However, different from the self-propelled active matter which consumes energy to provide driving force [52][53][54][55], here the non-zero resultant force solely results from the thermal fluctuations.…”

Biohybrid Microtube Swimmers Driven by Single Captured Bacteria

“…C ( t ) values over 5 s for eight biohybrid swimmers with different LOPs (0–4.7 µm) are shown in Figure F. For a ballistic, straight trajectory the microtube orientation would remain undisturbed with C ( t ) = 1 . In the case of microtube oscillations (rocking motion), there is a decay in C ( t ) over time, where greater oscillations elicit a more rapid decrease of C ( t ) compared to tubes with minimal angular changes.…”