On a roll

摘要

In a study published in 1896, the German physicist Georg Quincke demonstrated that, when suspended in an electrically conducting fluid, an insulating sphere will rotate if a sufficiently strong electric field is applied1. On page 95 of this issue, Bricard et al.2 use this little-known effect - now called Quincke rotation - as the energy-transfer mechanism, or motor, to drive populations of millions of microspheres to spontaneously form rolling herds (Fig. 1). In a racetrack-shaped enclosure, these 'Quincke roller' herds merge into a unidirectional swarm that circles the track (see Fig. 2 of the paper), and increase in size with increasing roller concentration. The authors interpret and rationalize their results using mathematical modelling to show that, at short distances, Quincke rollers interact mainly through two pairwise effects: a hydrodynamic interaction that promotes 'polar' alignment of rolling directions (that is, the movement of rollers in the same direction) and a repulsive electrostatic interaction.