Programmable self-propelling actuators enabled by a dynamic helical medium

Abstract

Rotation-translation conversion is a popular way to achieve power transmission in machinery, but it is rarely selected by nature. One unique case is that of bacteria swimming, which is based on the collective reorganization and rotation of flagella. Here, we mimic such motion using the light-driven evolution of a self-organized periodic arch pattern. The range and direction of translation are altered by separately varying the alignment period and the stimulating photon energy. Programmable self-propelling actuators are realized via a specific molecular assembly within a photoresponsive cholesteric medium. Through rationally presetting alignments, parallel transports of microspheres in customized trajectories are demonstrated, including convergence, divergence, gathering, and orbital revolution. This work extends the understanding of the rotation-translation conversion performed in an exquisitely self-organized system and may inspire future designs for functional materials and intelligent robotics.

Fig. 1. Unidirectionally aligned semi-free CLC.

(A) Helical configuration of the semi-free film (bottom) and corresponding fluctuated phase profile (top). l denotes the stripe period, and p indicates the helical pitch. (B) Schematic illustrations of the light-stimulated winding/unwinding of CLC helixes. Blue and green half-headed arrows denote the light stimulation conditions. Orange cylinders in (A) and (B) denote the directions of grating stripes. (C) Grating textures corresponding to the states shown in (B). White arrows denote the polarizer and analyzer transmission axes, respectively. Scale bar, 20 μm.

Fig. 2. Light-stimulated evolutions of photopatterned textures.

(A) Helical configuration of the photopatterned CLC. (B) Schematic of the alignment condition. (C) POM texture of the generated periodic arch pattern. The group of blue bars denotes the local tangential directions of grating stripes in a single period. The inset illustrates a magnified texture with an edge dislocation. (D) Relationship between l and the exposure time. Insets are POM images of periodic arch patterns with different l during blue light exposure. The irradiance of blue light is about 100 μW/cm². (E) Diffraction patterns during the exposure with exposure time labeled (min). (F) Light-stimulated shift of the periodic arch pattern. The exposure time is labeled in corresponding micrographs. (G) Dependency of Δx on θ. White arrows denote the polarizer and analyzer transmission axes, respectively. Scale bars, 50 μm.

Fig. 3. Microstructure-engaged rotation-translation conversion.

(A) Transport of microspheres toward −x. White arrows denote the polarizer and analyzer transmission axes, respectively. The exposure time is labeled in corresponding micrographs. Scale bar, 50 μm. (B) Dependencies of Δx (blue) and Δy (black) on θ. (C) Spatial potential analysis on light-driven transportations of microspheres.

Fig. 4. Programmable transporting actuators.

(i) Photoalignment conditions, (ii) initial states, (iii) trajectories, and (iv) end states of dispersed microspheres for motions of (A) converging, (B) diverging, (C) aggregating, and (D) orbiting. Insets in (ii) show the amplified images of the blue rectangular marked regions. The color change from white to gray indicates the alignment direction changing from 0° to 180°. The color difference of circles in (D) (ii and iv) is to distinguish microspheres. White arrows denote the polarizer and analyzer transmission axes, respectively. Scale bars, 50 μm.