An emulsion is a mixture of oil and water which is stabilised by a third component (such as a surfactant) which allows these typically adversarial liquids to coexist. An active emulsion is an unusual system, where, as a microscopic oil droplet solubilizes into a water-surfactant mixture to form a dispersion of millions nanodroplets (a nanoemulsion), it swims!
The mechanism behind this swimming is believed to be driven by a surface tension gradient - the same driving force which creates the famous "Tears of Wine". As micelles advect around a stationary droplet, they drive a gradient in the surfactant coverage around the droplet, which induces flows that push the droplet forward towards more micelles, which sustains the flow. This mechanism is stable for several hours, making these droplets a highly useful breed of micro-swimmer.
Micro-swimmers are an exciting and broad subject of research for several reasons. Firstly, a lot of the physics of swimming at the micr-scale is unintuitive; unlike a blue whale which can drift along under its own momentum for very long times, viscosity dominates the microscopic world, so drift times are negligible, so the swimmer must constantly pump the fluid around it to propel itself, making micro-swimming an energetically expensive trait. This high cost is possibly why many micro-organisms have not bothered to evolve the ability to swim, even though it would be universally useful to be able to actively move towards a food or light source, or away from danger. Interestingly, around 95% of the ecosystem-destroying toxic algal blooms are micro-swimmers; whether there is a causal link between the ability to swim and the tendency to devastate an ecosystem is, to my knowledge, not known. Secondly, and possibly most importantly, understanding micro-swimming techniques would be beneficial for medicine, in developing intelligent, self-propelling drug-delivery machines that seeks out the unhealthy target tissues, rather than the current inefficient mechanism, where the drug diffuses around the body and can at best only partially finds its target.
As a model micro-swimmer, our droplets have several advantages over other areas of micro-swimmer research: they are spherically symmetric with no moving parts, and so are computationally simple; they are non-biological, so experiments do not need to consider sleep-cycles, mutation, mating, or death; and despite being non-biological, they display a variety of bio-mimetic phenomena. For example, they are both chemo- and rheo-tactic, in that they have the ability to detect and respond to chemical signals, flows, and walls, and can even follow these signals in order to solve mazes. They also display convective clustering, similar to bioconvection observed in oceanic plankton.
My research is currently focused on probing this droplet-swimmer system under a variety of complex geometries, such as: active double emulsions, where the swimming oil droplet contains a secondary internal aqueous droplet, which swims until the oil shell gets sufficiently thin that it bursts, and releases its cargo; squeezed droplets, where two axes of the droplet are sufficiently large that multiple surface tension stress points form, and the droplet self-divides; and swimming in a drying drop, where the swimmers react to, and even modify, the usual flow and solidification processes that occur in an evaporating drop, e.g., the coffee-ring effect.
Chemotactic droplet swimmers in complex geometries, Journal of Physics: Condensed Matter, 2018.