The design of artificial microswimmers is often inspired by the strategies of natural microorganisms. Many of these creatures exploit the fact that elasticity breaks the time-reversal symmetry of motion at low Reynolds numbers, but this principle has been notably absent from model systems of active, self-propelled microswimmers. Here we introduce a class of microswimmers that spontaneously self-assembles and swims without using external forces, driven instead by surface phase transitions induced by temperature variations. The swimmers are made from alkane droplets dispersed in an aqueous surfactant solution, which start to self-propel on cooling, pushed by rapidly growing thin elastic tails. When heated, the same droplets recharge by retracting their tails, swimming for up to tens of minutes in each cycle. Thermal oscillations of approximately 5 °C induce the swimmers to harness heat from the environment and recharge multiple times. We develop a detailed elasto-hydrodynamic model of these processes and highlight the molecular mechanisms involved. The system offers a convenient platform for examining symmetry breaking in the motion of swimmers exploiting flagellar elasticity. The mild conditions and biocompatible media render these microswimmers potential probes for studying biological propulsion and interactions between artificial and biological swimmers.
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This study was funded by the European Research Council (ERC) EMATTER (no. 280078) and the Engineering and Physical Sciences Research Council Fellowship no. EP/R028915/1 to S.K.S. This project has received funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 682754 to E.L.). The study received financial support from project no. KP-06-DV-4/2019 with the Bulgarian Ministry of Education and Science, under the National Research Program ‘VIHREN’ to N.D. The work has been supported by the National Science Center of Poland SONATA grant no. 2018/31/D/ST3/02408 to M.L. The study falls under the umbrella of European network COST CA17120 Chemobrionics. We are grateful to M. Paraskova (Sofia University) for her help with part of the image analysis and for the preparation of some figures.
The authors declare no competing interests.
Peer review information Nature Physics thanks Marisol Ripoll and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Supplementary Figs. 1–11, Tables 1 and 2, experimental results and theoretical modelling description.
Extrusion of a single fibre from tetradecane swimmers in 1.5 wt% Brij 58 surfactant solution at 0.4 °C min–1 cooling rate. Extrusion rate UF ≈ 14.0 μm s–1 and swimming speed US ≈ 0.7 μm s–1.
Extrusion of two in-phase fibres from a pentadecane swimmer in 1.5 wt% Brij 58 surfactant solution at 0.25 °C min–1 cooling rate. Extrusion rate UF ≈ 2.70 μm s–1 and swimming speed US ≈ 0.23 μm s–1.
Extrusion of two out-of-phase fibres for a pentadecane swimmer in 1.5 wt% Brij 58 surfactant solution at 0.25 °C min–1 cooling rate. Extrusion rate UF ≈ 3.10 μm s–1 and swimming speed US ≈ 0.25 μm s–1.
Simulation results, presented in dimensionless scales, for an extruded filament pushed straight into a fluid at a constant speed, UF, which sets the velocity scale. The length scale is set by the buckling length of the filament, l. The dimensionless time shown at the bottom is set by the ratio l/UF. Changing the material or the dynamic parameters of the process affects the dimensional time at which the buckling occurs and the dimensional distance at which the beam becomes unstable. After a short period of straight motion, the fluid-drag-induced tension in the beam increases beyond the buckling threshold and the beam deflects. As a result, a meander-type pattern is created, which closely resembles the experimentally observed fibre deformations.
Simulation results for a filament retracted from the fluid at a constant speed, shown in a dimensionless form (as shown in the caption of Supplementary Video 4). Starting from a deformed shape of the beam, the dynamics is quite different compared with extrusion. The beam initially straightens and then it is dragged into the nozzle without buckling. The final stage of the process is a linear decrease in the extruded length. The slight tilt with respect to the horizontal is a result of the initial asymmetry of the configuration.
Fibre retraction on the heating of a pentadecane swimmer in 1.5 wt% Brij 58 surfactant solution at ~3 °C min–1 heating rate. Although each of the pre-extruded fibres is >2,500 µm long, they completely retract on heating back into the initial droplet and recharge the swimmer.
One full cycle of fibre extrusion on cooling at 0.3 °C min–1 and fibre retraction on heating at 0.2 °C min–1 for several pentadecane swimmers in 1.5 wt% Brij 58 surfactant solution (one fibre per swimmer).
Three full, consecutive cycles of fibre extrusion on cooling and retraction on heating at 0.23 °C min–1 for a pentadecane swimmer with two fibres in 1.5 wt% Brij 58 surfactant solution.
Video of pentadecane swimmers in 1.5 wt% Brij 58 solution extruding two fibres. Different colour curves show the tracking of individual droplets performed with MTrackJ plugin in the ImageJ program. For estimating the swimming speed, US, we used the tracks of the ‘white dots’ in the centre of the swimmers, numbered as 1, 5 and 9 in the video.
Video of an extruding filament (tail) on cooling a typical swimmer droplet of pentadecane in a 0.5 wt% Brij S20 solution. Cooling rate, 0.9 °C min–1; extrusion rate, 2.50 μm s–1; swimmer speed, 0.34 μm s–1. Scale bar, 20 μm. The video is played at four times speed.
Video of an extruding filament (tail) on cooling a typical swimmer droplet of pentadecane in a 0.5 wt% Brij S20 solution. Cooling rate, 2.8 °C min–1; extrusion rate, 12.40 μm s–1; swimmer speed, 0.68 μm s–1. Scale bar, 20 μm. The video is played at four times speed.
Video of a retracting filament on heating a typical swimmer droplet of pentadecane in a 0.5 wt% Brij S20 solution. Heating rate, 1.5 °C min–1; retraction rate, 4.8 µm s–1. Scale bar, 20 µm.
Data included in Figs. 1 and 3.
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Cholakova, D., Lisicki, M., Smoukov, S.K. et al. Rechargeable self-assembled droplet microswimmers driven by surface phase transitions. Nat. Phys. 17, 1050–1055 (2021). https://doi.org/10.1038/s41567-021-01291-3
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