Interfacial mechanisms in active emulsions

Abstract

Active emulsions, i.e., emulsions whose droplets perform self-propelled motion, are of tremendous interest for mimicking collective phenomena in biological populations such as phytoplankton and bacterial colonies, but also for experimentally studying rheology, pattern formation, and phase transitions in systems far from thermal equilibrium. For fuelling such systems, molecular processes involving the surfactants which stabilize the emulsions are a straightforward concept. We outline and compare two different types of reactions, one which chemically modifies the surfactant molecules, the other which transfers them into a different colloidal state. While in the first case symmetry breaking follows a standard linear instability, the second case turns out to be more complex. Depending on the dissolution pathway, there is either an intrinsically nonlinear instability, or no symmetry breaking at all (and hence no locomotion).

Chemotaxis and autochemotaxis of self-propelling droplet swimmers

“…The droplet locomotion is caused by a self-sustained Marangoni flow due to the inhomogeneous interfacial surfactant coverage and only observable in the nonequilibrium state of solubilization ( Fig. 1) (35). While incorporating oil molecules, micelles grow in size, incorporating free surfactant molecules from the aqueous phase and increasing the total area of oil-water interfaces in the system.…”

“…Studies exist on collective effects like autochemotaxis-induced clustering (31)(32)(33), but generally, there is still a lack of well-controllable and quantifiable experimental realizations of autochemotactic artificial swimmers. We demonstrate chemotaxis and autochemotaxis in microfluidic geometries for a highly symmetric and tunable artificial model swimmer system: self-propelling oil droplets in an aqueous surfactant solution (15,34,35).…”

“…Studies exist on collective effects like autochemotaxis-induced clustering (31-33), but generally, there is still a lack of well-controllable and quantifiable experimental realizations of autochemotactic artificial swimmers. We demonstrate chemotaxis and autochemotaxis in microfluidic geometries for a highly symmetric and tunable artificial model swimmer system: self-propelling oil droplets in an aqueous surfactant solution (15,34,35).The quantitative study of chemotaxis with traditional methods such as micropipette assays has been limited to observational studies (36, 37), as experimental conditions such as gradient strength are difficult to set in such geometries. Using microfluidic techniques, experimental conditions can be much better controlled; e.g., a linear gradient can be generated and kept constant, or even fast switched (38, 39), the object distribution can be easily analyzed (40), and the objects can be tracked individually (41-43).…”

Self-Propulsion of Pure Water Droplets by Spontaneous Marangoni-Stress-Driven Motion

“…The latter often take the form of Janus-like colloids [12], named after the twofaced Roman god, because their motion originates from the asymmetry of their surface properties [9,10,13]. An alternative design of artificial swimmers consists of active droplets, either on interfaces [14,15] or in bulk fluid [16][17][18][19]. Droplets are particularly interesting systems since they are extensively used in microfluidic devices as (bio-)chemical reactors [20,21].…”

“…The self-propulsion mechanism of swimming droplets [16][17][18][19] has its origin in the Marangoni flow induced by a surface-tension gradient. In most cases, this gradient is maintained through specific chemical reactions, including the hydrolysis [16,17] or the bromination of the surfactant [18].…”