Collapse of transient gels in colloid-polymer mixtures

Transient gelation in colloid-polymer mixtures is an interesting but poorly understood non-equilibrium phenomenon which has recently attracted the attention of experiment and theory. In a transient gel the particles aggregate, by the depletion interaction, to form a space-spanning network which maintains its structural intecrity for a finite period before suddenly collapsing. In this paper we present a study of the collapse process which provides new information on the way in which transient gels collapse. We have studied gel collapse in three ways. Firstly, we have observed the gel throughout its lifetime using dark-field optics and time-lapsed video recording. In a second experiment we have measured the concentration of gels throughout their entire height before, during and after collapse. This was performed using a non-intrusive ultrasonic technique which measures the speed of sound in the suspension. Finally, we have studied the effect of varying sample height and width on the lifetime of a gel. Our observations provide new information on the collapse process and have important implications for the formulation of a theoretical model. In discussing the effects of height and width we refer to the following companion paper which describes this particular effect.