C. The physicochemical basis of self-propulsion

After all the above discussions, it is evident that, by analogy to living cells, artificial cells must be designed to convert chemical energy into kinetic energy, namely, to achieve autonomous movement. Of course, motility is not the only relevant functionality in artificial cells,2,302. B. C. Buddingh and J. C. M. van Hest, “Artificial cells: Synthetic compartments with life-like functionality and adaptivity,” Acc. Chem. Res. 50, 769 (2017). https://doi.org/10.1021/acs.accounts.6b0051230. N. A. Yewdall, A. F. Mason, and J. C. M. van Hest, “The hallmarks of living systems: Towards creating artificial cells,” Interface Focus 8, 20180023 (2018). https://doi.org/10.1098/rsfs.2018.0023 but one of the most challenging to realize, as it involves the integration of environment signal processing and energy transduction. Here, we focus on the autonomous locomotion; hence, we leave aside the externally actuated mechanisms to produce motion. We concentrate our attention on the most used strategy for artificial swimmers: the Marangoni effect.3131. L. E. Schriven and C. V. Sternling, “The Marangoni effects,” Nature 187, 186 (1960). https://doi.org/10.1038/187186a0 Indeed, the vast majority of cases here reported involve Marangoni flows as the mechanism to convert localized chemical changes into fluid transport. The effect, schematically shown in Fig. 3, can be summarized as follows: given an interface separating two fluids (either air–liquid or liquid–liquid), if the surface tension varies from point to point, a tangential force per unit area (Marangoni stress) arises on the interface, which drags fluid toward the region of larger surface tension.3232. R. F. Probstein, Physicochemical Hydrodynamics: An Introduction (John Wiley & Sons, 2005). The generation of a surface tension gradient is due to local variation in the concentration of dissolved species and/or temperature. Therefore, the displacement of mobile interfaces ultimately depends on the heat and mass transfer processes that take place in the system: for example, the transport of oil between droplets of differing composition, in the case of Ref. 1414. C. H. Meredith, P. G. Moerman, J. Groenewold, Y.-J. Chiu, W. K. Kegel, A. Van Blaaderen, and L. D. Zarzar, “Predator–prey interactions between droplets driven by non-reciprocal oil exchange,” Nat. Chem. 12, 1136 (2020). https://doi.org/10.1038/s41557-020-00575-0 [Fig. 3(a)], or the consumption of reagents and increase of reaction products, in the case of Ref. 2727. A. D. Pizarro, C. L. A. Berli, G. J. A. A. Soler-Illia, and M. G. Bellino, “Droplets in underlying chemical communication recreate cell interaction behaviors,” Nat. Commun. 13, 3047 (2022). https://doi.org/10.1038/s41467-022-30834-2 [Fig. 3(b)]. Another aspect of the problem is the region where the hydrodynamic effect occurs: for example, the Marangoni flow may develop at the droplet surface (liquid–liquid interface), such as in the case of Ref. 1414. C. H. Meredith, P. G. Moerman, J. Groenewold, Y.-J. Chiu, W. K. Kegel, A. Van Blaaderen, and L. D. Zarzar, “Predator–prey interactions between droplets driven by non-reciprocal oil exchange,” Nat. Chem. 12, 1136 (2020). https://doi.org/10.1038/s41557-020-00575-0 [Fig. 3(a)], or at the droplet periphery (liquid–air interface), such as in Refs. 2525. B. Majhy and A. K. Sen, “Evaporation-induced transport of a pure aqueous droplet by an aqueous mixture droplet,” Phys. Fluids 32, 032003 (2020). https://doi.org/10.1063/1.5139002 and 2727. A. D. Pizarro, C. L. A. Berli, G. J. A. A. Soler-Illia, and M. G. Bellino, “Droplets in underlying chemical communication recreate cell interaction behaviors,” Nat. Commun. 13, 3047 (2022). https://doi.org/10.1038/s41467-022-30834-2 [Fig. 3(b)]. In any case, the relative displacement of fluid produces the droplet propulsion.In summary, although the generation of Marangoni flows is the common mechanism, each one of the discussed systems has a particular chemical process to generate the required energy for locomotion. It is worth to remark that another artificial swimmers (e.g., micro and nano-motors build on Janus particles) also displaying autonomous locomotion have been developed,3333. H. Su, C. A. Hurd Price, L. Jing, Q. Tian, J. Liu, and K. Qian, “Janus particles: Design, preparation, and biomedical applications,” Mater. Today Bio 4, 100033 (2019). https://doi.org/10.1016/j.mtbio.2019.100033 but not necessarily react or move toward a neighbor particle, in contrast to the droplet pair interactions illustrated along this Perspective. Nevertheless, coupling the locomotion of micro and nano-motors to chemical communication systems is quickly advancing.3434. L. Wang, S. Song, J. van Hest, L. K. E. A. Abdelmohsen, X. Huang, and S. Sánchez, “Biomimicry of cellular motility and communication based on synthetic soft-architectures,” Small 16, 1907680 (2020). https://doi.org/10.1002/smll.201907680Finally, it is worth to mention that, to our best knowledge, there are no scientific reports relating the Marangoni effect to the propulsion of individual cells in the natural world. In spite of that, it is known that some of the arthropods that are known to “walk on the water” are also able to “surf” the water by exploiting the Marangoni stress.3535. J. W. M. Bush and W. L. Wu, “Walking on water: Biolocomotion at the interface,” Annu. Rev. Fluid Mech. 38, 339–369 (2006). https://doi.org/10.1146/annurev.fluid.38.050304.092157 These small insects generate surface tension gradients by locally modifying the chemical composition of the water surface. The remarkable physicochemical mechanism has inspired researchers to develop miniaturized aquatic robots by mimicking the mechanism: the small robots make use of self-powered microfluidic pumps to secret surfactants that induce the Marangoni stress.3636. B. Kwak, S. Choi, J. Maeng, and J. Bae, “Marangoni effect inspired robotic self-propulsion over a water surface using a flow-imbibition-powered microfluidic pump,” Sci. Rep. 11, 17469 (2021). https://doi.org/10.1038/s41598-021-96553-8 Furthermore, it is well-understood in biomicrofluidics that microorganisms are predestined to swim at very low Reynolds numbers, where inertia is totally irrelevant and viscous flows have to be generated by irreversible and asymmetric forces.3737. E. M. Purcell, “Life at low Reynolds numbers,” Am. J. Phys. 45, 3–11 (1977). https://doi.org/10.1119/1.10903 The bioinspired design of artificial micro-swimmers essentially follows the same strategy, integrating functional microstructures to drive mechanical propulsion,3838. S. Palagi and P. Fischer, “Bioinspired microrobots,” Nat. Rev. Mater. 3, 113–124 (2018). https://doi.org/10.1038/s41578-018-0016-9 including soft, shape-changing, responsive materials.3939. G. Yan, A. A. Solovev, G. Huang, J. Cui, and Y. Mei, “Soft microswimmers: Material capabilities and biomedical applications,” Curr. Opin. Colloid Interface Sci. 61, 101609 (2022). https://doi.org/10.1016/j.cocis.2022.101609 Instead, the above-mentioned artificial “surfer” exploits the local generation of surface tension gradients, which drive interfacial (Marangoni) flows that propel themselves through the surrounding liquid. Notably, this is precisely the case of self-propelled droplets (both “swimmers” and “crawlers”) that we are discussing in the present perspective.