The sensitive material of PICASSO detectors are millions of tiny droplets of a superheated liquid. A liquid is superheated when it is heated well above its boiling point. Such a liquid is extremely unstable, as you can imagine if you try to heat water to well above 1000 C. Any slight perturbation will trigger an explosive transformation of the liquid into vapour.
The PICASSO experiment uses as a fluorine loaded active liquid C4F10, which is dispersed in the form of 50-100 Ám diameter droplets in a polymerized or viscous medium. If a dark matter particle, e.g. a neutralino hits e.g. a F- atom in a droplet, the recoiling F-atom deposits its kinetic energy on its track in the surrounding liquid. A tiny proto-bubble forms and grows explosively until the entire droplet is transformed into a vapor bubble (Fig.1). This mini-explosion is accompanied by an acoustic pulse, which can be picked up by piezo electric sensors as shown in Fig.2.
Here is how such an event looks like under the microscope! (.avi file 6MB)
...and here you can watch a "WIMP" hitting a bubble:
Fig. 1: If a dark matter particle hits a nucleus in a tiny superheated droplet,
the atom recoils and deposits its energy in a heat spike, which in turn triggers
a phase transition.
Fig. 2: This mini-explosion gives an acoustic signal lasting about 4 milliseconds
and can be recorded easily with piezoelectric transducers.
...here are more details about the superheated liquid technique