Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Predator–prey interactions between droplets driven by non-reciprocal oil exchange


Chemotactic interactions are ubiquitous in nature and can lead to non-reciprocal and complex emergent behaviour in multibody systems. However, developing synthetic, inanimate embodiments of a chemomechanical framework to generate non-reciprocal interactions of tunable strength and directionality has been challenging. Here we show how chemotactic signalling between microscale oil droplets of different chemistries in micellar surfactant solutions can result in predator–prey-like non-reciprocal chasing interactions. The interactions and dynamic self-organization result from the net directional, micelle-mediated transport of oil between emulsion droplets of differing composition and are powered by the free energy of mixing. We systematically elucidated chemical design rules to tune the interactions between droplets by varying the oil and surfactant chemical structure and concentration. Through the integration of experiment and simulation, we also investigated the active behaviour and dynamic reorganization of multidroplet clusters. Our findings demonstrate how chemically minimal systems can be designed with controllable, non-reciprocal chemotactic interactions to generate emergent self-organization and collective behaviours reminiscent of biological systems.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Get just this article for as long as you need it


Prices may be subject to local taxes which are calculated during checkout

Fig. 1: BOct droplets chase EFB droplets in aqueous surfactant due to micelle-mediated oil transport.
Fig. 2: Interaction and displacement energies associated with predator–prey droplets are tunable by varying the chemical structure and concentration of surfactant and oil miscibility.
Fig. 3: Oil molecular structure influences the direction, speed and efficiency of chasing between droplets.
Fig. 4: Multibody non-reciprocal interactions between droplets cause emergent assembly and disassembly dynamics that are predictable based on measured two-body interactions.

Data availability

All relevant data generated or analysed for this study are included in the published article and its Supplementary Information files.


  1. Vicsek, T. & Zafeiris, A. Collective motion. Phys. Rep. 517, 71–140 (2012).

    Article  Google Scholar 

  2. Czirók, A., Ben-Jacob, E., Cohen, I. & Vicsek, T. Formation of complex bacterial colonies via self-generated vortices. Phys. Rev. E 54, 1791 (1996).

    Article  Google Scholar 

  3. Bellomo, N. & Soler, J. On the mathematical theory of the dynamics of swarms viewed as complex systems. Math. Models Methods Appl. Sci. 22, 1140006 (2012).

    Article  Google Scholar 

  4. Cira, N. J., Benusiglio, A. & Prakash, M. Vapour-mediated sensing and motility in two-component droplets. Nature 519, 446–450 (2015).

    Article  CAS  Google Scholar 

  5. Niu, R., Palberg, T. & Speck, T. Self-assembly of colloidal molecules due to self-generated flow. Phys. Rev. Lett. 119, 028001 (2017).

    Article  Google Scholar 

  6. Theurkauff, I., Cottin-Bizonne, C., Palacci, J., Ybert, C. & Bocquet, L. Dynamic clustering in active colloidal suspensions with chemical signaling. Phys. Rev. Lett. 108, 268303 (2012).

    Article  CAS  Google Scholar 

  7. Usta, O. B., Alexeev, A., Zhu, G. & Balazs, A. C. Modeling microcapsules that communicate through nanoparticles to undergo self-propelled motion. ACS Nano 2, 471–476 (2008).

    Article  CAS  Google Scholar 

  8. Sengupta, A., Kruppa, T. & Löwen, H. Chemotactic predator–prey dynamics. Phys. Rev. E 83, 031914 (2011).

    Article  Google Scholar 

  9. Lach, S., Yoon, S. M. & Grzybowski, B. A. Tactic, reactive, and functional droplets outside of equilibrium. Chem. Soc. Rev. 45, 4766–4796 (2016).

    Article  CAS  Google Scholar 

  10. Christian, S. D. & Scamehorn, J. F. Solubilization in Surfactant Aggregates (CRC Press, 1995).

  11. Schmitt, M. & Stark, H. Swimming active droplet: a theoretical analysis. Europhys. Lett. 101, 44008 (2013).

    Article  Google Scholar 

  12. Maass, C. C., Krüger, C., Herminghaus, S. & Bahr, C. Swimming droplets. Annu. Rev. Condens. Matter Phys. 7, 171–193 (2016).

    Article  CAS  Google Scholar 

  13. Lagzi, I., Soh, S., Wesson, P. J., Browne, K. P. & Grzybowski, B. A. Maze solving by chemotactic droplets. J. Am. Chem. Soc. 132, 1198–1199 (2010).

    Article  CAS  Google Scholar 

  14. Anderson, J. L. Colloid transport by interfacial forces. Annu. Rev. Fluid Mech. 21, 61–99 (1989).

    Article  Google Scholar 

  15. Izri, 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. 113, 248302 (2014).

    Article  Google Scholar 

  16. Moerman, P. G. et al. Solute-mediated interactions between active droplets. Phys. Rev. E 96, 032607 (2017).

    Article  Google Scholar 

  17. Jin, C., Krüger, C. & Maass, C. C. Chemotaxis and autochemotaxis of self-propelling droplet swimmers. Proc. Natl Acad. Sci. USA 114, 5089–5094 (2017).

    Article  CAS  Google Scholar 

  18. Gladysz, J. A., Curran, D. P. & Horváth, I. T. Handbook of Fluorous Chemistry (John Wiley & Sons, 2006).

  19. De Smet, Y., Deriemaeker, L. & Finsy, R. Ostwald ripening of alkane emulsions in the presence of surfactant micelles. Langmuir 15, 6745–6754 (1999).

    Article  Google Scholar 

  20. Hoang, T. K. N., Deriemaeker, L. & Finsy, R. Ostwald ripening and solubilization in alkane in water emulsions stabilized by different surfactants. Phys. Chem. Chem. Phys. 6, 1413–1422 (2004).

    Article  CAS  Google Scholar 

  21. Soto, R. & Golestanian, R. Self-assembly of catalytically active colloidal molecules: tailoring activity through surface chemistry. Phys. Rev. Lett. 112, 068301 (2014).

    Article  Google Scholar 

  22. Yalkowsky, S. H., He, Y. & Jain, P. Handbook of Aqueous Solubility Data (CRC Press, 2016).

  23. Carroll, B. J. The kinetics of solubilization of nonpolar oils by nonionic surfactant solutions. J. Colloid Interface Sci. 79, 126–135 (1981).

    Article  CAS  Google Scholar 

  24. Donegan, A. C. & Ward, A. J. Solubilization kinetics of n‐alkanes by a non‐ionic surfactant. J. Pharm. Pharmacol. 39, 45–47 (1987).

    Article  CAS  Google Scholar 

  25. Wang, W., Chiang, T.-Y., Velegol, D. & Mallouk, T. E. Understanding the efficiency of autonomous nano- and microscale motors. J. Am. Chem. Soc. 135, 10557–10565 (2013).

    Article  CAS  Google Scholar 

  26. Bibette, J., Roux, D. & Nallet, F. Depletion interactions and fluid–solid equilibrium in emulsions. Phys. Rev. Lett. 65, 2470 (1990).

    Article  CAS  Google Scholar 

  27. Israelachvili, J. N. Intermolecular and Surface Forces (Academic, 2011).

  28. Michelin, S., Lauga, E. & Bartolo, D. Spontaneous autophoretic motion of isotropic particles. Phys. Fluids 25, 061701 (2013).

    Article  Google Scholar 

  29. Palacci, J., Sacanna, S., Steinberg, A. P., Pine, D. J. & Chaikin, P. M. Living crystals of light-activated colloidal surfers. Science 339, 936–940 (2013).

    Article  CAS  Google Scholar 

  30. Rosslee, C. & Abbott, N. L. Active control of interfacial properties. Curr. Opin. Colloid Interface Sci. 5, 81–87 (2000).

    Article  CAS  Google Scholar 

  31. Thutupalli, S., Geyer, D., Singh, R., Adhikari, R. & Stone, H. A. Flow-induced phase separation of active particles is controlled by boundary conditions. Proc. Natl Acad. Sci. USA 115, 5403–5408 (2018).

    Article  CAS  Google Scholar 

  32. Illien, P., Golestanian, R. & Sen, A. ‘Fuelled’ motion: phoretic motility and collective behaviour of active colloids. Chem. Soc. Rev. 69, 5508–5518 (2017).

    Article  Google Scholar 

  33. Ross, T. D. et al. Controlling organization and forces in active matter through optically defined boundaries. Nature 572, 224–229 (2019).

    Article  CAS  Google Scholar 

  34. Joseph, A. et al. Chemotactic synthetic vesicles: design and applications in blood–brain barrier crossing. Sci. Adv. 3, e1700362 (2017).

    Article  Google Scholar 

  35. Somasundar, A. et al. Positive and negative chemotaxis of enzyme-coated liposome motors. Nat. Nanotechnol. 14, 1129–1134 (2019).

    Article  CAS  Google Scholar 

  36. Wohlfahrt, C. & Wohlfahrt, B. Refractive Indices of Organic Liquids: Optical Constants (Springer, 1996).

  37. Thielicke, W. & Stamhuis, E. J. PIVlab-time-resolved digital particle image velocimetry tool for MATLAB (programmed in MATLAB 7, R14) (BSD license, 2014).

  38. Ravera, F., Ferrari, M., Liggieri, L., Miller, R. & Passerone, A. Measurement of the partition coefficient of surfactants in water/oil systems. Langmuir 13, 4817–4820 (1997).

    Article  CAS  Google Scholar 

  39. Crocker, J. C. & Grier, D. G. Methods of digital video microscopy for colloidal studies. J. Colloid Interface Sci. 179, 298–310 (1996).

    Article  CAS  Google Scholar 

Download references


We thank C. Wentworth for her help obtaining interfacial tension measurements using the pendant drop method. L.D.Z., C.H.M. and Y.-J.C. acknowledge support from the Army Research Office through grant no. W911NF-18-1-0414 and the Penn State MRSEC funded by the National Science Foundation (DMR-1420620). C.H.M. acknowledges support from the Thomas and June Beaver Fellowship at Penn State and the Pennsylvania Space Grant Fellowship, and Y.-J.C. received support from the Erickson Discovery Grant Program at Penn State. P.G.M. acknowledges funding from the NWO (Dutch National Science Foundation) Graduate Program through the Debye Institute for Nanomaterials. J.G. wishes to thank the program for Chang Jiang Scholars and Innovative Research Teams in Universities (no. IRT 17R40) and the 111 Project of the PRC.

Author information

Authors and Affiliations



C.H.M., P.G.M. and L.D.Z. developed the concept for the research. C.H.M and Y.-J.C. conducted the droplet chasing experiments under different conditions and measured the oil solubilization rates. P.G.M. analysed data on the droplet motions and performed the simulations. C.H.M., P.G.M. and L.D.Z. wrote the manuscript. P.G.M. was supervised by W.K.K., J.G. and A.v.B. C.H.M. and Y-J.C. were supervised by L.D.Z. All the authors discussed the results and manuscript.

Corresponding author

Correspondence to Lauren D. Zarzar.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Observation of fluorinated oil transfer between droplets in Capstone surfactant solution.

The fluorosurfactant Capstone FS-30 preferentially solubilizes EFB over BOct (rate of 0.54 μm/min vs. 0.01 μm/min, respectively, in 5.0 wt% Capstone) and facilitates the net directional transfer of EFB oil into BOct droplets. BOct (n = 1.45) droplets surrounded by EFB in 5.0 wt% Capstone undergo a shift their appearance as the droplets’ refractive index changes due to the addition of EFB (n = 1.28), eventually becoming nearly transparent due to index matching with the aqueous phase (n = 1.33). Scale, 100 μm.

Extended Data Fig. 2 The pendant drop method is used to measure the dynamic interfacial tension (IFT) increase resultant from BOct oil saturation in Triton surfactant solutions.

a, The interfacial tension of BOct is higher in BOct-saturated 0.5 wt% Triton surfactant solution (red) compared to unsaturated surfactant solution (blue). b, A higher interfacial tension was also observed for EFB in BOct-saturated 0.5 wt% Triton (red), compared to oil-free Triton (blue). Experiments were produced in triplicate and the same trends were consistently observed.

Extended Data Fig. 3 Fluorescence was used to distinguish between chasing iodoalkanes or chasing alkanes and identify predator and prey.

a, Under transmission brightfield imaging, iodohexane and iodoheptane droplets are indistinguishable (top, and Supplementary Video 4); by dying the iodohexane droplet with fluorescent Lumogen F Red 305, the iodohexane droplet can be easily identified using fluorescence microscopy (middle). By simultaneously imaging the droplets in both transmission brightfield and fluorescence (bottom), we can visualize all droplets and also distinguish the two types of oil. This imaging method is used in (b,c). b, An iodoheptane droplet (outlined in red) chases an iodohexane droplet dyed with fluorescent Lumogen F Red 305 (outlined in blue) in 0.5 wt% Triton. The optical micrographs were taken with simultaneous transmission brightfield and fluorescence imaging as shown in (a). Scale, 100 µm. c, An octane droplet (outlined in red) chases a hexadecane droplet dyed with fluorescent Lumogen F Red 305 (outlined in blue) in 0.5 wt% Triton. Optical micrographs were taken with simultaneous transmission brightfield and fluorescence imaging as shown in (a). Scale, 100 µm.

Extended Data Fig. 4 Fluorescence was used to observe the exchange of oil between iodoalkane droplets.

1-iododecane droplets prepared with fluorescent pyrromethene 650 dye were mixed with 1-iodohexane droplets in 0.5 wt% Triton. 1-iodohexane droplets became increasingly fluorescent over time due to the surfactant-mediated transfer of dye, which we use to infer that transfer of oil also occurs. The 1-iodohexane droplets shrink faster than 1-iododecane droplets due to faster solubilization into the aqueous phase (0.41 μm/min vs. 0.24 μm/min). Scale, 100 μm.

Extended Data Fig. 5 External flow profile surrounding chasing iodoalkane droplet pair.

External flow field surrounding a chasing pair of 1-iodoheptane and 1-iodohexane droplets in 0.5 wt% Triton X-100 was visualized using neutrally buoyant tracer microparticles. The direction of pair motion is from left to right and 1-iodoheptane is the predator. The blue arrows in the left panel indicate the average local flow velocity of the fluid. Before averaging, each frame was rotated so that the line connecting the predator and prey droplets aligned with the x-axis. The red arrow indicates the average velocity of the chaser, as seen in Supplementary Video 5, \(v = 11.1\frac{{{\upmu m}}}{{\mathrm{s}}}\) The length of the blue arrows is scaled to the velocity of the droplet pair. The panel on the right shows streamlines around the same droplet pair. Scale bar is 100 µm.

Extended Data Fig. 6 BOct droplets become self-propelled at smaller diameters.

Droplet-tracking image analysis was used to determine the average droplet speed of BOct droplets as a function of droplet diameter as the drops were solubilizing in 0.5 wt% Triton. The average speed was calculated by measuring the nominal speed and subtracting the component of the speed that all droplets had in the same direction due to drift. Each data point represents between 20 and 50 droplet measurements, and the error bars represent the standard deviation. As the droplets become smaller, the average self-propulsion speed of the BOct droplets increases.

Extended Data Fig. 7 Trochoidal motion of a swimming predator-prey pair.

a, Speed of the predator BOct droplet (red) and prey methoxyperfluorobutane droplet (blue) as function of time. The schematic indicates how self-propulsion of the predator drop along the surface of the prey results in a circular trajectory of the chasing pair. b, Trochoidal trajectory of a BOct droplet chasing after an methoxyperfluorobutane droplet. Scale, 100 μm.

Supplementary information

Supplementary Information

Supplementary Discussion, Figs. 1–2 and Table 1.

Supplementary Video 1

A bromooctane droplet chases an ethoxynonafluorobutane droplet in 0.5wt% Triton X-100 surfactant. The bromooctane predator droplet (darker colored drop) accelerates as it moves towards the ethoxynonafluorobutane prey (lighter colored drop), and the prey droplet flees but is eventually ‘caught’. The resulting droplet pair continues to move in a direction always led by the ethoxynonafluorobutane prey droplet. Still frames from this video are included in Fig. 1a. Scale bar, 100 μm. Video speed, 10x real time.

Supplementary Video 2

Nonreciprocal chasing interactions produce droplet pairs and mixed multibody droplet clusters. Chasing pairs of bromooctane (darker colored drops) and ethoxynonafluorobutane (lighter colored drops) and larger multi-droplet clusters undergo various motions as a result of nonreciprocal interactions between droplets. The continuous phase is 0.5 wt% Triton X-100 surfactant solution. Scale bar, 250 μm. Video speed, 25x real time.

Supplementary Video 3

Ethoxynonafluorobutane droplets chase bromooctane droplets in 3wt% Capstone FS-30 surfactant. Ethoxynonafluorobutane predator droplets (lighter colored drops) chase bromooctane prey droplets (darker colored drops) in 3.0 wt% fluorosurfactant Capstone FS-30. This chasing direction is the reverse of that observed in 0.5% Triton X-100 (as seen in Supplementary Videos 1 and 2) due to the fact the fluorosurfactant preferentially solubilizes and transports the fluorinated oil. Scale bar, 250 μm. Video speed, 25x real time.

Supplementary Video 4

1-Iodoheptane droplets chase 1-iodohexane droplets in 0.5wt% Triton X-100 surfactant. The designation of chasing direction was achieved by fluorescent labeling (Extended Data Fig. 2) Scale bar, 250 μm. Video speed, 25x real time.

Supplementary Video 5

Marangoni-driven directional flows inside and outside the predator and prey droplets align with the droplets’ direction of motion. A 1-iodoheptane droplet chases a 1-iodohexane droplet in 0.5% wt% Triton X-100. The droplet-internal flows are visualized with iron oxide tracer particles suspended in both oils (top). The external flows surrounding a different droplet pair of the same two oils in 0.5 wt% Triton X-100 are visualized from the motion of latex tracer particles suspended in the aqueous phase. Scale bar, 100 μm. Video speed, 3x real time.

Supplementary Video 6

Collective assembly and disassembly dynamics of droplet clusters. A cascade of nonreciprocal interactions between bromooctane (darker drops) and ethoxynonafluorobutane (lighter drops) in 0.5 wt% Triton X-100 result in dynamic behaviour of larger droplet clusters. Exchange of droplets between different clusters is commonly observed. Scale bar, 500 μm. Video speed, 25x real time.

Supplementary Video 7

Dynamic cluster motions resultant from self-propelled droplets. Bromooctane predators (darker colored droplets) with diameters below approximately 40 μm in 0.5 wt% Triton can become self-propelled, resulting in trochoidal, run-and-tumble, and flapping motions. Scale bar, 250 μm. Video speed, 50x real time.

Supplementary Video 8

Simulations reproduce experimentally observed cluster dynamics. The circular motion of a droplet cluster containing two bromooctane drops (darker, red colored) and two ethoxynonafluorobutane drops (lighter, blue colored) was reproduced in a simulation by using droplet initial positions and measured two-body droplet interaction parameters as inputs. Continuous phase is of 0.5 wt% Triton X-100 surfactant solution. Scale bar, 100 μm. Video speed, 10x real time.

Supplementary Video 9

Individual droplets act as chemical signaling posts to direct the motion of chasing droplet pairs as seen in experiment and simulation. Seen in both experiment and simulation, individual bromooctane droplets (darker, red drops) act as chemical signaling posts via long-range solute-mediated interactions to direct the movement of a bromooctane and methoxyperfluorobutane chasing droplet pair. Continuous phase of 0.5 wt% Triton X-100 surfactant solution. Scale bar, 100 μm. Video speed, 10x real time.

Supplementary Video 10

Solute-mediated interactions between neighboring droplets cause cluster rearrangement as seen in experiment and simulation. Repulsive interactions between bromooctane predators (darker, red drops) chasing ethoxynonafluorobutane prey (lighter, blue drops) in separate clusters results in the destabilization and loss of a single bromooctane droplet from one of the clusters. Continuous phase of 0.5 wt% Triton X-100 surfactant solution. Scale bar, 100 μm. Video speed, 10x real time.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Meredith, C.H., Moerman, P.G., Groenewold, J. et al. Predator–prey interactions between droplets driven by non-reciprocal oil exchange. Nat. Chem. 12, 1136–1142 (2020).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing