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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Source data are provided with this paper. The data that support the findings of this study are available from the corresponding authors upon reasonable request.
The code used in this study is available from the corresponding authors upon reasonable request.
Elgeti, J., Winkler, R. G. & Gompper, G. Physics of microswimmers—single particle motion and collective behavior: a review. Rep. Prog. Phys. 78, 056601 (2015).
Lauga, E. & Powers, T. R. The hydrodynamics of swimming microorganisms. Rep. Prog. Phys. 72, 096601 (2009).
Balzani, V., Credi, A., Raymo, F. M. & Stoddart, J. F. Artificial molecular machines. Angew. Chem. Int. Ed. 39, 3348–3391 (2000).
Schwarz-Linek, J. et al. Phase separation and rotor self-assembly in active particle suspensions. Proc. Natl Acad. Sci. USA 109, 4052–4057 (2012).
Paxton, W. F. et al. Catalytic nanomotors: autonomous movement of striped nanorods. J. Am. Chem. Soc. 126, 13424–13431 (2004).
Michelin, S. & Lauga, E. Efficiency optimization and symmetry-breaking in a model of ciliary locomotion. Phys. Fluids 22, 111901 (2013).
Kassem, S. et al. Artificial molecular motors. Chem. Soc. Rev. 46, 2592–2621 (2017).
Fischer, P. Vision statement: interactive materials—drivers of future robotic systems. Adv. Mater. 32, 1–4 (2020).
Khaldi, A., Elliott, J. A. & Smoukov, S. K. Electro-mechanical actuator with muscle memory. J. Mater. Chem. C 2, 8029–8034 (2014).
Marshall, J. E., Gallagher, S., Terentjev, E. M. & Smoukov, S. K. Anisotropic colloidal micromuscles from liquid crystal elastomers. J. Am. Chem. Soc. 136, 474–479 (2014).
Khaldi, A., Plesse, C., Vidal, F. & Smoukov, S. K. Smarter actuator design with complementary and synergetic functions. Adv. Mater. 27, 4418–4422 (2015).
Wang, T. et al. Electroactive polymers for sensing. Interface Focus 6, 20160026 (2016).
Lesov, I. et al. Bottom-up synthesis of polymeric micro- and nanoparticles with regular anisotropic shapes. Macromolecules 51, 7456–7462 (2018).
Sanchez, S., Soler, L. & Katuri, J. Chemically powered micro- and nanomotors. Angew. Chem. Int. Ed. 54, 1414–1444 (2015).
Soh, S., Bishop, K. J. M. & Grzybowski, B. A. Dynamic self-assembly in ensembles of camphor boats. J. Phys. Chem. B 112, 10848–10853 (2008).
Jiang, H. R., Yoshinaga, N. & Sano, M. Active motion of a Janus particle by self-thermophoresis in a defocused laser beam. Phys. Rev. Lett. 105, 1–4 (2010).
Ren, L. et al. 3D steerable, acoustically powered microswimmers for single-particle manipulation. Sci. Adv. 5, eaax3084 (2019).
Ghosh, A. & Fischer, P. Controlled propulsion of artificial magnetic nanostructured propellers. Nano Lett. 9, 2243–2245 (2009).
Chang, S. T., Paunov, V. N., Petsev, D. N. & Velev, O. D. Remotely powered self-propelling particles and micropumps based on miniature diodes. Nat. Mater. 6, 235–240 (2007).
Takatori, S. C. & Brady, J. F. Towards a thermodynamics of active matter. Phys. Rev. E 91, 032117 (2015).
Buttinoni, I., Volpe, G., Kümmel, F., Volpe, G. & Bechinger, C. Active Brownian motion tunable by light. J. Phys. Condens. Matter 24, 284129 (2012).
Xuan, M. et al. Near infrared light-powered Janus mesoporous silica nanoparticle motors. J. Am. Chem. Soc. 138, 6492–6497 (2016).
Lv, H., Xing, Y., Du, X., Xu, T. & Zhang, X. Construction of dendritic Janus nanomotors with H2O2 and NIR light dual-propulsion via a Pickering emulsion. Soft Matter 16, 4961–4968 (2020).
Shao, J. et al. Erythrocyte membrane modified Janus polymeric motors for thrombus therapy. ACS Nano 12, 4877–4885 (2018).
Peyer, K. E., Tottori, S., Qiu, F., Zhang, L. & Nelson, B. J. Magnetic helical micromachines. Chem. Eur. J. 19, 28–38 (2013).
Lee, S. et al. A capsule-type microrobot with pick-and-drop motion for targeted drug and cell delivery. Adv. Healthcare Mater. 7, 1–6 (2018).
Medina-Sánchez, M., Schwarz, L., Meyer, A. K., Hebenstreit, F. & Schmidt, O. G. Cellular cargo delivery: toward assisted fertilization by sperm-carrying micromotors. Nano Lett. 16, 555–561 (2016).
Cates, M. E. Diffusive transport without detailed balance in motile bacteria: does microbiology need statistical physics? Rep. Prog. Phys. 75, 042601 (2012).
Robinson, D. G., Hoppenrath, M., Oberbeck, K., Luykx, P. & Ratajczak, R. Localization of pyrophosphatase and V-ATPase in Chlamydomonas reinhardtii. Bot. Acta 111, 108–122 (1998).
Goldstein, R. E. Green algae as model organisms for biological fluid dynamics. Annu. Rev. Fluid Mech. 47, 343–375 (2015).
Denkov, N., Tcholakova, S., Lesov, I., Cholakova, D. & Smoukov, S. K. Self-shaping of oil droplets via the formation of intermediate rotator phases upon cooling. Nature 528, 392–395 (2015).
Cholakova, D. & Denkov, N. Rotator phases in alkane systems: in bulk, surface layers and micro/nano-confinements. Adv. Colloid Interface Sci. 269, 7–42 (2019).
Cholakova, D., Denkov, N., Tcholakova, S., Lesov, I. & Smoukov, S. K. Control of drop shape transformations in cooled emulsions. Adv. Colloid Interface Sci. 235, 90–107 (2016).
Denkov, N., Cholakova, D., Tcholakova, S. & Smoukov, S. K. On the mechanism of drop self-shaping in cooled emulsions. Langmuir 32, 7985–7991 (2016).
Burrows, S. A., Korotkin, I., Smoukov, S. K., Boek, E. & Karabasov, S. Benchmarking of molecular dynamics force fields for solid–liquid and solid–solid phase transitions in alkanes. J. Phys. Chem. B 125, 5145–5159 (2021).
Haas, P. A., Goldstein, R. E., Smoukov, S. K., Cholakova, D. & Denkov, N. Theory of shape-shifting droplets. Phys. Rev. Lett. 118, 1–5 (2017).
Haas, P. A., Cholakova, D., Denkov, N., Goldstein, R. E. & Smoukov, S. K. Shape-shifting polyhedral droplets. Phys. Rev. Res. 1, 023017 (2019).
Gordon, R., Hanczyc, M. M., Denkov, N. D., Tiffany, M. A. & Smoukov, S. K. in Habitability of the Universe before Earth: Astrobiology: Exploring Life on Earth and Beyond (eds Gordon, R. & Sharov, A.) 427–490 (Academic Press, 2018).
Fuller, R. B. & Applewhite, E. J. Synergetics 2: Further Explorations in the Geometry of Thinking (MacMillan 1979).
Gosselin, F. P., Neetzow, P. & Paak, M. Buckling of a beam extruded into highly viscous fluid. Phys. Rev. E 90, 052718 (2014).
De Canio, G., Lauga, E. & Goldstein, R. E. Spontaneous oscillations of elastic filaments induced by molecular motors. J. R. Soc. Interface 14, 20170491 (2017).
Ui, T. J., Hussey, R. G. & Roger, R. P. Stokes drag on a cylinder in axial motion. Phys. Fluids 27, 787–795 (1984).
Gueron, S. & Levit-Gurevich, K. Energetic considerations of ciliary beating and the advantage of metachronal coordination. Proc. Natl Acad. Sci. USA 96, 12240–12245 (1999).
Kotar, J., Leoni, M., Bassetti, B., Cosentino, M. & Cicuta, P. Hydrodynamic synchronization of colloidal oscillators. Proc. Natl Acad. Sci. USA 107, 7669–7673 (2010).
Brumley, D. R., Wan, K. Y., Polin, M. & Goldstein, R. E. Flagellar synchronization through direct hydrodynamic interactions. eLife 3, 1–15 (2014).
Wan, K. Y. & Goldstein, R. E. Coordinated beating of algal flagella is mediated by basal coupling. Proc. Natl Acad. Sci. USA 113, E2784–E2793 (2016).
Geyer, V. F., Jülicher, F., Howard, J. & Friedrich, B. M. Cell-body rocking is a dominant mechanism for flagellar synchronization in a swimming alga. Proc. Natl Acad. Sci. USA 110, 18058–18063 (2013).
Small, D. M. The Physical Chemistry of Lipids: From Alkanes to Phospholipids (Plenum Press, 1986).
May, G. J., Davidson, A. & Monahov, B. Lead batteries for utility energy storage: a review. J. Energy Storage 15, 145–157 (2018).
Yang, X., Xiang, L., Dong, Y., Cao, Y. & Wang, C. Effect of nonionic surfactant Brij 35 on morphology, cloud point, and pigment stability in Monascus extractive fermentation. J. Sci. Food Agric. 100, 4521–4530 (2020).
Yang, X. et al. Effects of nonionic surfactants on pigment excretion and cell morphology in extractive fermentation of Monascus sp. NJ1. J. Sci. Food Agric. 100, 1832–1832 (2020).
Yang, X. et al. Effects of nonionic surfactants on pigment excretion and cell morphology in extractive fermentation of Monascus sp. NJ1. J. Sci. Food Agric. 99, 1233–1239 (2019).
Tang, J. et al. Solid lipid nanoparticles with TPGS and Brij 78: a co-delivery vehicle of curcumin and piperine for reversing P-glycoprotein-mediated multidrug resistance in vitro. Oncol. Lett. 13, 389–395 (2017).
Tomasi, R. F.-X., Sart, S., Champetier, T. & Baroud, C. N. Individual control and quantification of 3D spheroids in a high-density microfluidic droplet array. Cell Rep. 31, 107670 (2020).
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. (2021). https://doi.org/10.1038/s41567-021-01291-3