•Solubilizing Janus oil droplets can propel >10× faster than isotropic drops•Partitioning across droplets’ oil–oil interface is key to swim speed and direction•Applied thermal gradients are shown to effectively control droplet swimming behavior•Droplets can spontaneously form spinning clusters due to collective interactions The study of active colloidal microswimmers with tunable phoretic and self-organizational behaviors is important for understanding out-of-equilibrium systems and the design of functional, adaptive matter. We elucidate the underlying drivers and key design principles that govern Janus droplet motion through a systematic investigation of oil chemical structures and partitioning, droplet shape, surfactant conditions, and temperature. Our experimental results point to the pivotal role played by oil partitioning between the Janus droplet compartments in determining droplet swimming speed and direction. In addition, we demonstrate approaches for controlling droplet swimming behaviors through the top-down application of localized thermal gradients and bottom-up collective interactions. Our findings provide insights as to how the chemistry and structure of multiphase fluids can be harnessed to design microswimmers with programmable active and collective behaviors. Solubilizing, self-propelling droplets have emerged as a rich chemical platform for the exploration of active matter, but isotropic droplets rely on spontaneous symmetry breaking to sustain motion. The introduction of permanent asymmetry, e.g., in the form of a biphasic Janus droplet, has not been explored as a comprehensive design strategy for active droplets, despite the widespread use of Janus structures in motile solid particles. Here, we uncover the chemomechanical framework underlying the self-propulsion of biphasic Janus oil droplets solubilizing in aqueous surfactant. We elucidate how droplet propulsion is influenced by the degree of oil mixing, droplet shape, and oil solubilization rates for a range of oil combinations. In addition, spatiotemporal control over droplet swimming speed and orientation is demonstrated through the application of thermal gradients applied via joule heating and laser illumination. We also explore the interactions between collections of Janus droplets, including the spontaneous formation of spinning multi-droplet clusters. Solubilizing, self-propelling droplets have emerged as a rich chemical platform for the exploration of active matter, but isotropic droplets rely on spontaneous symmetry breaking to sustain motion. The introduction of permanent asymmetry, e.g., in the form of a biphasic Janus droplet, has not been explored as a comprehensive design strategy for active droplets, despite the widespread use of Janus structures in motile solid particles. Here, we uncover the chemomechanical framework underlying the self-propulsion of biphasic Janus oil droplets solubilizing in aqueous surfactant. We elucidate how droplet propulsion is influenced by the degree of oil mixing, droplet shape, and oil solubilization rates for a range of oil combinations. In addition, spatiotemporal control over droplet swimming speed and orientation is demonstrated through the application of thermal gradients applied via joule heating and laser illumination. We also explore the interactions between collections of Janus droplets, including the spontaneous formation of spinning multi-droplet clusters. Active, colloidal microswimmers of diverse compositions and propulsion mechanisms are of interest as minimal models to probe out-of-equilibrium behavior and collective organization. For a colloid to exhibit self-propulsion, asymmetric forces must be continually present. A common microswimmer design strategy is to create solid colloids with permanent geometric or chemical asymmetry, i.e., Janus particles.1Jurado-Sanchez B. Pacheco M. Maria-Hormigos R. Escarpa A. Perspectives on Janus micromotors: materials and applications.Appl. Mater. Today. 2017; 9: 407-418Crossref Scopus (61) Google Scholar,2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1437) Google Scholar Localized chemical reactions occurring at the particle surface lead to gradient distributions of products that cause particle propulsion by mechanisms such as self-electrophoresis,3Wong F. Dey K.K. Sen A. Synthetic micro/nanomotors and pumps: fabrication and applications.Annu. Rev. Mater. Res. 2016; 46: 407-432Crossref Scopus (60) Google Scholar self-diffusiophoresis,4Moran J.L. Posner J.D. 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Soc. 2010; 132: 1198-1199Crossref PubMed Scopus (189) Google Scholar Liquid Janus droplets, wherein the droplet is composed of two immiscible fluids, each having an interface with the continuous phase, have been widely explored as particle templates,10Lone S. Cheong I.W. Fabrication of polymeric Janus particles by droplet microfluidics.RSC Adv. 2014; 4: 13322-13333Crossref Google Scholar,11Frank B.D. Antonietti M. Zeininger L. Structurally anisotropic Janus particles with tunable amphiphilicity via polymerization of dynamic complex emulsions.Macromolecules. 2020; 54: 981-987Crossref PubMed Scopus (2) Google Scholar for optics,12Goodling A.E. Nagelberg S. Kaehr B. Meredith C.H. Cheon S.I. Saunders A.P. Kolle M. Zarzar L.D. Colouration by total internal reflection and interference at microscale concave interfaces.Nature. 2019; 566: 523-527Crossref PubMed Scopus (69) Google Scholar,13Nagelberg S. Zarzar L.D. Nicolas N. Subramanian K. Kalow J.A. Sresht V. Blankschtein D. Barbastathis G. Kreysing M. Swager T.M. Reconfigurable and responsive droplet-based compound micro-lenses.Nat. Commun. 2017; 8: 14700Crossref PubMed Scopus (85) Google Scholar and as sensors,14Balaj R.V. Zarzar L.D. Reconfigurable complex emulsions: design, properties, and applications.Chem. Phys. Rev. 2020; 1: 011301Crossref Google Scholar,15Zarzar L.D. Kalow J.A. He X. Walish J.J. Swager T.M. Optical visualization and quantification of enzyme activity using dynamic droplet lenses.Proc. Natl. Acad. Sci. 2017; 114: 3821-3825Crossref PubMed Scopus (28) Google Scholar but few examples of self-propelled Janus droplets have been described.16Li M. Brinkmann M. Pagonabarraga I. Seemann R. Fleury J.-B. Spatiotemporal control of cargo delivery performed by programmable self-propelled Janus droplets.Commun. Phys. 2018; 1: 23Crossref Scopus (15) Google Scholar, 17Jeong J. Gross A. Wei W.-S. Tu F. Lee D. Collings P.J. Yodh A. Liquid crystal Janus emulsion droplets: preparation, tumbling, and swimming.Soft Matter. 2015; 11: 6747-6754Crossref PubMed Google Scholar, 18Wang X. Zhang R. Mozaffari A. de Pablo J.J. Abbott N.L. Active motion of multiphase oil droplets: emergent dynamics of squirmers with evolving internal structure.Soft Matter. 2021; 17: 2985-2993Crossref PubMed Google Scholar However, Janus droplets present exciting opportunities for control of chemotactic colloidal interactions, given the presence of multiple fluid phases that enable dynamic changes in chemical partitioning and the tunability of droplet morphology via modification of droplet and surfactant chemistry. Here, we explore systematically how chemical composition, oil partitioning, droplet shape, and solubilization cooperatively influence the propulsion of Janus oil droplets in aqueous surfactant. By providing a deeper understanding of the principles and trends influencing Janus droplet swimming behaviors, we aim to demonstrate that such droplets can serve as a robust material platform for rational design of chemically responsive, self-propulsive colloids. The simplest droplet swimmers consist of a single dispersed fluid phase within a continuous phase, such as spherical oil droplets in water19Jin C. Krüger C. Maass C.C. Chemotaxis and autochemotaxis of self-propelling droplet swimmers.Proc. Natl. Acad. Sci. 2017; 114: 5089-5094Crossref PubMed Scopus (111) Google Scholar,20Moerman P.G. Moyses H.W. Van Der Wee E.B. Grier D.G. Van Blaaderen A. Kegel W.K. Groenewold J. Brujic J. Solute-mediated interactions between active droplets.Phys. Rev. E. 2017; 96: 032607Crossref PubMed Scopus (37) Google Scholar or water droplets in oil.21Izri Z. van der Linden M.N. Michelin S. Dauchot O. Self-propulsion of pure water droplets by spontaneous Marangoni-stress-driven motion.Phys. Rev. Lett. 2014; 113: 248302Crossref PubMed Scopus (144) Google Scholar Active behavior is typically generated either by a reaction occurring at the droplet interface22Hanczyc M.M. Toyota T. Ikegami T. Packard N. Sugawara T. Fatty acid chemistry at the oil− water interface: self-propelled oil droplets.J. Am. Chem. Soc. 2007; 129: 9386-9391Crossref PubMed Scopus (212) Google Scholar or by droplet solubilization into surfactant micelles23Christian S.D. Scamehorn J.F. Solubilization in Surfactant Aggregates. CRC Press, 1995Google Scholar that creates interfacial tension gradients and propels the drops via the Marangoni effect.24Saville D. The effects of interfacial tension gradients on the motion of drops and bubbles.Chem. Eng. J. 1973; 5: 251-259Crossref Scopus (57) Google Scholar In the case of solubilization-driven propulsion, higher concentrations of solubilizate-“filled” micelles have been associated with higher interfacial tensions, causing droplets to move toward regions of solution containing more “empty” micelles and minimize interfacial energy.25Herminghaus S. Maass C.C. Krüger C. Thutupalli S. Goehring L. Bahr C. Interfacial mechanisms in active emulsions.Soft Matter. 2014; 10: 7008-7022Crossref PubMed Google Scholar Droplets can become self-propelled due to repulsion from their own solubilizate gradient when hydrodynamic instabilities result in spontaneous symmetry breaking of the fluid flows surrounding the droplet.21Izri Z. van der Linden M.N. Michelin S. Dauchot O. Self-propulsion of pure water droplets by spontaneous Marangoni-stress-driven motion.Phys. Rev. Lett. 2014; 113: 248302Crossref PubMed Scopus (144) Google Scholar,26Guyon E. Hulin J.-P. Petit L. Mitescu C.D. Physical Hydrodynamics. Oxford University Press, 2001Google Scholar Overlap of chemical gradients resultant from nearby solubilizing droplets,20Moerman P.G. Moyses H.W. Van Der Wee E.B. Grier D.G. Van Blaaderen A. Kegel W.K. Groenewold J. Brujic J. Solute-mediated interactions between active droplets.Phys. Rev. E. 2017; 96: 032607Crossref PubMed Scopus (37) Google Scholar,27Hokmabad B.V. Saha S. Canalejo J.A. Golestanian R. Maass C.C. Quantitative characterization of chemorepulsive alignment-induced interactions in active emulsions.arXiv preprint. 2020; (arXiv:2012.05170)Google Scholar,28Soto R. Golestanian R. Self-assembly of catalytically active colloidal molecules: tailoring activity through surface chemistry.Phys. Rev. Lett. 2014; 112: 068301Crossref PubMed Scopus (126) Google Scholar externally applied chemical gradients,9Lagzi I. Soh S. Wesson P.J. Browne K.P. Grzybowski B.A. Maze solving by chemotactic droplets.J. Am. Chem. Soc. 2010; 132: 1198-1199Crossref PubMed Scopus (189) Google Scholar,19Jin C. Krüger C. Maass C.C. Chemotaxis and autochemotaxis of self-propelling droplet swimmers.Proc. Natl. Acad. Sci. 2017; 114: 5089-5094Crossref PubMed Scopus (111) Google Scholar internal droplet asymmetries,29Hokmabad B.V. Baldwin K.A. Krüger C. Bahr C. Maass C.C. Topological stabilization and dynamics of self-propelling nematic shells.Phys. Rev. Lett. 2019; 123: 178003Crossref PubMed Scopus (16) Google Scholar and interfacially adsorbed particles30Cheon S.I. Batista Capaverde Silva L. Khair A. Zarzar L. Interfacially-adsorbed particles enhance the self-propulsion of oil droplets in aqueous surfactant.Soft Matter. 2021; 17: 6742-6750Crossref PubMed Google Scholar are also possible sources of chemical asymmetry that can induce droplet motion. Recently, we described how oil droplets exhibit predator-prey chasing interactions when placed in a system containing at least two droplet “species” of different chemical composition that act together in a source-sink framework.31Meredith C.H. Moerman P.G. Groenewold J. Chiu Y.-J. Kegel W.K. van Blaaderen A. Zarzar L.D. Predator–prey interactions between droplets driven by non-reciprocal oil exchange.Nat. Chem. 2020; 12: 1136-1142Crossref PubMed Scopus (18) Google Scholar A solubilizing predator droplet acts as an oil-filled micelle source, while nearby prey droplets uptake the predator’s oil, acting as a sink. The prey’s attenuation of the oil concentration attracts the predator drops while the prey drops are repulsed by the approaching predator, resulting in a chase. Inspired by the synergy resultant from the dynamic exchange of oils with different solubilization profiles and chemistry, and the inherent asymmetry that can be attained in a Janus colloid structure,1Jurado-Sanchez B. Pacheco M. Maria-Hormigos R. Escarpa A. Perspectives on Janus micromotors: materials and applications.Appl. Mater. Today. 2017; 9: 407-418Crossref Scopus (61) Google Scholar we sought to investigate the active behavior of solubilizing, biphasic Janus droplets. In this work, we examine how the active swimming behaviors of Janus, biphasic oil-in-water droplets in surfactant are affected by droplet shape, droplet-internal oil partitioning, and oil solubilization kinetics. By characterizing Janus droplets containing many different combinations of oils in Triton X-100 surfactant, we elucidate the relationships between the degree of oil mixing, interfacial tensions, and oil solubilization rates and their effects on droplet propulsion. A key conclusion is that for droplets containing both a mobile (solubilizing) and non-mobile oil, the degree of partitioning of the mobile oil across the Janus droplets’ oil–oil interface critically influences droplet speed and swimming direction. As a result, the propulsion speeds of Janus droplets containing an oil–oil interface can be over an order of magnitude faster compared with those of chasing pairs of single emulsion droplets of the same chemical composition and size. Tuning of the Janus droplet compartment volume ratios, surfactant concentration, and solution temperature allows further control over the Janus droplet propulsion. Spatiotemporal control over the swimming speed and orientation of droplets is demonstrated by applying thermal gradients induced via joule heating and laser spot illumination. We also explore interactions between Janus droplets, including the spontaneous formation of multi-droplet spinning clusters that rotate predictably based on cluster symmetry. Our findings provide insight as to how the chemistry and structure of multiphase droplets can be harnessed to design microswimmers with programmable self-propulsive and collective behaviors. To investigate swimming Janus droplet behavior, we began by exploring Janus droplets containing a pair of oils previously known to exhibit active behavior through chasing interactions31Meredith C.H. Moerman P.G. Groenewold J. Chiu Y.-J. Kegel W.K. van Blaaderen A. Zarzar L.D. Predator–prey interactions between droplets driven by non-reciprocal oil exchange.Nat. Chem. 2020; 12: 1136-1142Crossref PubMed Scopus (18) Google Scholar when present together as separate single emulsion droplets: 1-iododecane and ethoxynonafluorobutane (EFB), with drop diameters of approximately 60 μm in 0.5 wt % Triton X-100 (Triton) nonionic surfactant (Figure 1A). Both oils sink in the aqueous solution, lending to ease of experimentation. Iododecane predator droplets readily undergo micellar solubilization (solubilization rate defined as change in droplet diameter D over time t, −dD/dt = 0.22 μm/min), whereas EFB prey droplets have negligible solubilization (−dD/dt ≈ 0 μm/min). Once iododecane catches the EFB, the droplets form a pair that translates at speeds (v) of up to v = 13 μm/s via the source-sink oil exchange framework, previously described31Meredith C.H. Moerman P.G. Groenewold J. Chiu Y.-J. Kegel W.K. van Blaaderen A. Zarzar L.D. Predator–prey interactions between droplets driven by non-reciprocal oil exchange.Nat. Chem. 2020; 12: 1136-1142Crossref PubMed Scopus (18) Google Scholar (Figure 1A1). The droplets are stable when they touch and do not coalesce each other or wet the underlying substrate. We prepared stable Janus droplets of iododecane and EFB in 0.5 wt % Triton using flow-focusing microfluidics, which allowed us to control the size of the droplets and the volume ratio of the oils (Figures 1Aii and S1). The resultant shape of the droplets is governed by the balance of interfacial tensions.32Guzowski J. Korczyk P.M. Jakiela S. Garstecki P. The structure and stability of multiple micro-droplets.Soft Matter. 2012; 8: 7269-7278Crossref Scopus (137) Google Scholar,33Zarzar L.D. Sresht V. Sletten E.M. Kalow J.A. Blankschtein D. Swager T.M. Dynamically reconfigurable complex emulsions via tunable interfacial tensions.Nature. 2015; 518: 520-524Crossref PubMed Scopus (238) Google Scholar We found that the iododecane-EFB Janus droplets dispersed in 0.5 wt % Triton swam in the same directional manner as the chasing droplet pair (i.e., iododecane in the back and EFB in the front) but moved at speeds more than an order of magnitude faster, v ≈ 200 μm/s, with long persistence lengths, many times the droplet length (Figure 1Aii, Video S1). (For a description of how drop speeds are calculated, please refer to the supplemental information section, “protocol to observe Janus droplet individual and multibody droplet swimming behavior.”) The Janus droplet speed gradually decreased over the droplet lifetime, which could be over an hour, as the iododecane tail of the droplet shrank and eventually disappeared due to solubilization (Figure 1B). Based on the observed droplet speeds and rate of volume change, the propulsion efficiency and Péclet number of the Janus droplet swimmers are estimated to be approximately 14 and 15 times greater, respectively, than those of the chasing single emulsion droplets of the same chemistry and size (see supplemental information sections, “Propulsion efficiency comparison” and “Péclet number calculation”). Higher surfactant concentrations correlated with faster swimming (Figure 1C). No droplet motion was observed at Triton concentrations below the critical micelle concentration (CMC) or when surfactant solution was pre-saturated with both oils, indicating that the droplet swimming mechanism depended on the kinetics of micelle-mediated oil solubilization. The interfacial tensions of the oils comprising the Janus droplet were higher in the presence of oil-filled Triton micelles compared with empty micelles (Table S1), supporting the idea that droplets are propelled via the Marangoni effect toward solution containing less oil. The ratio of the Janus droplet compartment sizes (Figures S2A and S2B) and overall droplet size (Figure S2C) did influence the speeds, but all of these Janus droplets examined were still much faster than the chasing single emulsion droplet pair. https://www.cell.com/cms/asset/1d6cb7bf-62e1-42fe-93a3-7bfe72b05a03/mmc2.mp4Loading ... Download .mp4 (4.8 MB) Help with .mp4 files Video S1. Janus droplets composed of iododecane and EFB swim in 0.5 wt % Triton X-100 surfactant solutionSpeed, 5×. Scale, 250 μm. As such, it appeared that the mere presence of the oil–oil interface was a critical contributor to Janus drop swimming. An oil–oil interface would be expected to enable the direct partitioning of the oils between the Janus droplet compartments, whereas micelle-mediated oil transfer through the water is required for chasing single emulsion droplets to exchange oil. These different pathways for oil transport, we suspected, could be largely influential over the droplet swimming dynamics and required further investigation. To explore the influence of chemical partitioning across the oil–oil interface on the Janus droplet propulsion, we systematically varied the oil compositions and examined trends in droplet speed and shape (Figure 2). We note that when changing oil compositions, we are altering more than just chemical partitioning; the droplet shape can change due to alterations in the balance of interfacial tensions,32Guzowski J. Korczyk P.M. Jakiela S. Garstecki P. The structure and stability of multiple micro-droplets.Soft Matter. 2012; 8: 7269-7278Crossref Scopus (137) Google Scholar,33Zarzar L.D. Sresht V. Sletten E.M. Kalow J.A. Blankschtein D. Swager T.M. Dynamically reconfigurable complex emulsions via tunable interfacial tensions.Nature. 2015; 518: 520-524Crossref PubMed Scopus (238) Google Scholar and oil solubilization rates34Carroll B.J. The kinetics of solubilization of nonpolar oils by nonionic surfactant solutions.J. Colloid Interface Sci. 1981; 79: 126-135Crossref Scopus (142) Google Scholar—which provide the fuel for propulsion—can change as well. We attempt to consider these factors when designing the experiments and interpreting the results as described below. We first examined Janus droplets containing iododecane paired with several other fluorinated oils of varying degrees of fluorination: methoxyperfluorobutane (MFB), 2-(trifluoromethyl)-3-ethoxydodecafluorohexane (HFE-7500), perfluorohexane (PFH), perfluorooctane (PFO), and perfluorotributylamine (FC-43) (Figure 2). None of these fluorinated oils solubilize to a measurable extent in 0.5 wt % Triton (−dD/dt ≈ 0 μm/min). These fluorinated oils have, respectively, a decreasing degree of partitioning, x, into iododecane, ranging from 8 vol % to <0.1 vol %, at room temperature as measured by changes in refractive index (Figure S3; supplemental information, “oil partitioning measurement”). We define degree of oil partitioning (x) such that fully miscible oils have x = 50 vol %. We presume that the same partitioning trends also hold when considering the amount of iododecane in the fluorinated oil. All the fluorinated oil pairings with iododecane produced stable Janus droplets with similar shapes, where we define how dumbbell-like the droplet is by using the outer three-phase contact angle; a perfectly spherical Janus drop we define as having a contact angle of 180° and touching single emulsions a contact angle of 0°. All iododecane Janus drops had contact angles between 110° and 130°. All the Janus droplets swam in the same direction, with the solubilizing iododecane oil in the back (Figure 2A, yellow). However, the droplets swam at very different speeds, depending on the non-solubilizing fluorinated oil; generally, the smaller the oil partitioning x, the slower the speed v. For instance, iododecane-EFB droplets swam at v = 196 ± 4 μm/s and had x = 7.9 vol %, whereas iododecane-FC-43 droplets swam at v = 12 ± 2 μm/s with x < 0.1 vol %. To correlate these trends with interfacial tensions, we measured the interfacial tensions of iododecane-saturated EFB, MFB, and FC-43 in 0.5 wt % Triton with and without iododecane-filled micelles. Although the FC-43 had almost no difference in interfacial tension, the EFB showed an increase of 0.2 mN/m in the presence of oil-filled micelles (Table S1). The fact that the interfacial tension difference increased with the oil pair’s x is consistent with the observation that greater partitioning correlated with faster speeds. When iododecane was replaced with the shorter chain length iodononane, droplet speeds increased overall (Figure 2A, orange), whereas replacement with longer chain length iodododecane led to reduced speeds (Figure 2A, blue). Most of the droplets had similar shapes (outer three-phase contact angles between 110° and 130°). Partitioning of the fluorinated oils with the iodononane and iodododecane was similar to that of iododecane (x = 10.0 vol %, 6.7 vol %, and 7.9 vol % for pairing with EFB, respectively) (Figure S3A). However, the solubilization rate of iodononane was much faster (−dD/dt = 0.34 ± 0.03 μm/min) and iodododecane much slower (−dD/dt = 0.09 ± 0.01 μm/min) than that of the iododecane (−dD/dt = 0.22 ± 0.02 μm/min) (Figure S4A). Thus, we conclude that, when controlling for droplet shape and degree of partitioning, faster micelle-mediated solubilization led to faster Janus droplet swimming for mobile oils with similar chemical functionality. When we changed the halogen atom on the haloalkane oil while maintaining carbon number (e.g., 1-chlorodecame, 1-bromodecane, and 1-iododecane), we saw a greater diversity of droplet morphologies and a wider range of degrees of partitioning, which corresponded to greater variation in the Janus droplet speeds (Figures 2B and S3B). We take chlorodecane as an example to discuss the observed effects of droplet shape on the speed (Figure 2B, gray). Chlorodecane and EFB are fully miscible, forming a single emulsion droplet that did not swim. Chlorodecane paired with MFB or HFE-7500 remained phase-separated but formed double or near-double emulsion droplets that swam slowly, if at all (v < 5 μm/s). The degree of partitioning in these two aforementioned oil pairs (x = 13.6 vol % and 7.4 vol %, respectively) is on par with that of iodononane paired with EFB or MFB (x = 5.7 vol % and 10.0 vol %, respectively), and chlorodecane also solubilizes at a similar rate to that of iodononane (−dD/dt = 0.41 μm/min versus 0.34 μm/min). However, dumbbell-shaped iodononane-EFB and iodononane-MFB Janus droplets (contact angles between 110° and 130°) swam an order of magnitude faster than the more nearly encapsulated chlorodecane-MFB and chlorodecane-HFE-7500 drops (v = 191 μm/s and 268 μm/s versus <5 μm/s), suggesting that the droplet shape asymmetry also plays a significant role in droplet speed. Chlorodecane paired with PFH and PFO formed more dumbbell-like droplets (contact angle ≈ 110°) and, correspondingly, swam significantly faster (v = 159 and 145 μm/s). Chlorodecane paired with FC-43 formed a near-double emulsion drop shape and again swam more slowly (v = 43 μm/s) with shorter persistence lengths that are similar to the droplet length, having both an unfavorable encapsulated drop shape as well as low degree of oil partitioning (≈0.1 vol %). Thus, we conclude that an anisotropic Janus droplet shape in combination with a higher partitioning (∼0.5 vol % or higher) and a faster rate solubilization (−dD/dt > 0.1 μm/min) contributes to faster propulsion. From the data presented in Figures 2A and 2B, it was clear that the oil partitioning x, oil solubilization rates, and droplet shape were all important factors governing the droplet speed v. However, these parameters are not independent, as they are all physical properties arising from the specific chemical interactions between the surfactants and oils. To empirically explore which parameter(s) might be the most directly useful in predicting droplet speeds, we examined the droplet speed as a function of each parameter (partitioning, solubilization rate, and drop shape). The only parameter that exhibited direct correlation with droplet speed was the oil degree of partitioning (Figure S5). The data for each haloalkane were fitted empirically using a function with the form, v=a∗x∗e−x/b, where v is the swim speed, x is the degree of partitioning, and a and b are the fitting parameters, which we propose are related to solubilization rate and the sensitivity of the interfacial tensions to solubilized oil (Figure S5). Using these empirical fits, we then plotted drop speed (normalized to the peak speed vmax as fitted for each haloalkane) versus the partitioning x (normalized to x associated with vmax for each haloalkane) for each Janus droplet in Figures 2A and 2B. We find all Janus droplets fit well to v=x∗e−x (Figure 2C). Thus, for a given mobile oil and surfactant condition, the partitioning of the mobile and non-mobile oil inside the Janus drop appears to be a good predictor of the droplet speed. A physical explanation for the fitted form possessing a local maximum at intermediate partitioning values is forthcoming as postulated by the framework discussed in Figure 4. For a given Janus drop oil composition, the equilibrium shape of the droplet can be altered by varying the specific surfactants to change the balance of interfacial tensions.14Balaj R.V. Zarzar L.D. Reconfigurable complex emulsions: design, properties, and applications.Chem. Phys. Rev. 2020; 1: 011301Crossref Google Scholar,33Zarzar L.D. Sresht V. Sletten E.M. Kalow J.A. Blankschtein D. Swager T.M. Dynamically reconfigurable complex emulsions via tunable interfacial tensions.Nature. 2015; 518: 520-524Crossref PubMed Scopus (238) Google Scholar To investigate the relationship between droplet shape and swimming speed, we conducted experiments in which we used varying concentrations of a fluorosurfactant, Capstone FS-30, along with 0.5 wt % Triton for three different oil pairings: io