  We present a fluid dynamics video showing the behavior of Leidenfrost
droplets composed by a mixture of water and surfactant (SDS, Sodium Dodecyl
sulfate).
  When a droplet is released on a plate heated above a given temperature a thin
layer of vapor isolates the droplet from the plate. The droplet levitates over
the plate. This is called the Leidenfrost effect.
  In this work we study the influence of the addition of a surfactant on the
Leidenfrost phenomenon. As the droplet evaporates the concentration of SDS
rises up to two orders of magnitude over the Critical Micelle Concentration
(CMC). An unexpected and violent explosive behavior is observed. The video
presents several explosions taken with a high speed camera (IDT-N4 at 30000
fps). All the presented experiments were performed on a plate heated at
300{\deg}C. On the other hand, the initial quantity of SDS was tuned in two
ways: (i) by varying the initial concentration of SDS and (ii) by varying the
initial size of the droplet. By measuring the volume of the droplet just before
the explosion, we were able to estimate the final concentration of SDS. We
found that the explosion always occurs around a critical concentration, about
100 times the CMC.
  The droplets have also been studied just before the explosion. By isolating
the droplet on a cold plate just before the explosion, we evidenced the
presence of a shell surrounding a liquid core.
  We conclude that above a critical concentration a solid shell is formed. This
leads to an increase of pressure into the droplet until the shell breaks. The
release of the pressure is accompanied by a violent explosion, and in some
cases foaming.
