An Oil Droplet Division–Fusion Cycle

Abstract

Cell division extolls an energetic cost on the system, which makes fission nonspontaneous. In efforts to engineer self-replicating experimental systems, the division step is often affected by purely external forces or through speculated mechanisms. Herein, we demonstrate an oil droplet division event that is intrinsic, coupling interfacial phenomena with internal flow structures that literally pull the original droplet into two or more smaller stable aggregates. Self-division is based on interfacial instabilities driven by a far from equilibrium catanionic surfactant system. Salt-induced droplet fusion initiates feeding and embedded chemical reactions. Together fusion and division demonstrate a rudimentary droplet replication cycle. The droplet as an embodied model of artificial life possesses some relevant characteristics and qualities of living systems, such as a perpetuated body or identity, an embedded metabolism, and the ability to avoid equilibrium through directed movement or chemical chemotaxis. 4] A simple oil-droplet system may also possess a chemical language shared by a population of droplets resulting in group dynamics and higher order behaviors. The oil phase can host chemical reactions, especially those sensitive to water. Enzymatic function is possible as well as very strong hydrogen bonding in the oil phase. The oil droplet can protect water-labile fuel sources encapsulated within the oil phase and can regulate fuel consumption by controlled convective flow and movement. Reverse micelles spontaneously form in oil-in-water systems that contain surfactants and may serve as internal compartments and reservoirs. In addition, monolayers of simple surfactants at an oil–water interface can serve to regulate the exchange of material as well as heat, thus creating gradients. The droplet considered here is distinct from protocell models typically based on lipid membranes with an encapsulated aqueous phase. The droplet replication cycle is based on transient non-equilibrium conditions in catanionic surfactant systems. Catanionic systems at an interface: 1) lower the interfacial tension, 2) locally enhance the surface expansion rate, 3) support internal flows under non-equilibrium conditions, an example of a Marangoni instability, and importantly in this case raise the interfacial tension when the system approaches equilibrium leading to a stable system. We exploit this process for spontaneous fission of droplets. To enforce non-equilibrium initial conditions, we separate the cationic and anionic surfactants into the oil and water phases. From this starting point, the surfactant in the aqueous phase must absorb to the interface from the outside and the surfactant in the oil phase must reach the interface from the inside to approach equilibrium. A droplet of nitrobenzene is first impregnated with 20 mm cationic surfactant (see the Supporting Information). Then the droplet is added to a solution of 5 mm anionic surfactant at pH 12, Figure 1. The droplet becomes unstable and begins to deform (Figure 1, 6–9 sec). The instabilities and slight movements are due to the interface-driven flow structures generated inside the oil droplet. After a few seconds the droplet spreads out further into a torus and dramatically breaks up into two or more smaller droplets (Figure 1, 9–18 sec). The droplets then transform from an aspherical shape to a spherical shape (Figure 1, 18 sec). After the spontaneous fission event the droplets do not fuse back into one droplet even after many minutes, see Movie S1. Spontaneous fission is driven by a transient decrease in droplet interfacial tension. When the droplet is in the fission condition it shows a lower interfacial tension than in its equilibrium condition (5 mm oleate (OA) and 2 mm CTAB at pH 12) or with either surfactant alone, see Figure 2. The maximum volume measurement for the fission condition was repeatedly measured and suggests an interfacial tension of 0.4 mNm ,