A droplet immersed in another liquid does not necessarily behave like a rigid sphere; it can, through momentum transfer from its surroundings, develop pronounced internal dynamics. Numerical modelling or tracer studies convey only an incomplete picture of such circulations. Writing in Journal of Magnetic Resonance, Andrea Amar and collegues1 report how magnetic resonance imaging, or MRI, can offer a direct view of the inner life of a droplet.

Just as a patient is required to keep still during an MRI scan, the drops have to be kept in place to obtain sharp images. By finely balancing the flow of the surrounding liquid against the buoyant forces on the droplets, Amar et al. were able to levitate single drops of 2–4 millimetres in diameter with such precision that the drops, on average, moved by less than 20 micrometres over several hours. This high degree of stability meant that the fluid motion inside each drop could be analysed closely.

Using MRI techniques, the authors were able to record high-resolution images of the droplets and glean information about flow velocities. This approach revealed an overall rolling of the drop and also internal vortex patterns, indicating complex three-dimensional flow structures driven by the interaction with the ambient fluid.

This kind of interplay can result in mass transfer between the two immiscible liquids — for example, for the extraction of contaminants and cleaning procedures. Although such processes are not fully understood, they could now be explored in more detail using this MRI technique.