Fig. 5: Controllable 3D self-propulsion locomotion in polar solvents.
a Schematic diagram demonstrated the hydrogel performing 3D self-propulsion motion driven by NIR light to pass through an obstacle. Initially, the buoyancy flow stimulated the gel to perform a 3D drifting motion under the photothermal effect. Then, programmable swimming in the liquid surface was driven by the Marangoni effect. When the light irradiated at one end of the gel, the temperature increase caused the surface tension γ1 < γ2, and Marangoni propulsion force FM was along the surface tension gradient. Meanwhile, the contraction force FS in the irradiated region perpendicular to the lamellae produced a reaction force FS’ exerted by the liquid. A torque τs generated by the resultant force of the FM and FS’ drove a rotational swimming. b Optical images and simulation of convective velocity distribution around the gel during the floating. A maximum flow velocity was 21.7 mm s−1 at 1.6 s when the initial temperature difference was 10 K. c Schematic illustration and superimposed photographs showed linear swimming of the hydrogel at α = 0°. The red arrows indicated the swimming directions. d Sequencing images showed the clockwise or anticlockwise rotational swimming of the hydrogels at α = 45° or 135° within 3 s of radiation. One end of the gel marked by red dot indicated the angular displacements of the rotation. e Sine of angular displacement (sinθ) as a function of time and sine curve fitting showed steady rotation under the light. f Summary of the floating and linear swimming speeds in different polar solvents. Data are presented as mean values  ±  SD (n = 3). g Schematic diagrams and optical images of the hydrogel at α = 135° performing 3D self-propulsion under NIR stimulation to cross the obstacles at the interface of acetonitrile and n-hexane.
