Transient gelation by spinodal decomposition in colloid-polymer mixtures

Abstract

We have investigated with small angle light scattering and optical microscopy transient gelation phenomena which occur in phase-separating colloid-polymer mixtures. The scattering intensity distribution shows a peak at non-zero wave vector and satisfies the asymptotic q-4 Porod behaviour. Consistent with these observations, optical micrographs show an alternating pattern of dark and bright domains. These findings suggest that the polymer-induced depletion forces lead to the formation of a bicontinuous network of colloid-rich and colloid-poor domains, via a spinodal decomposition process. This bicontinuous network rapidly attains a gellike character as indicated by the arrest of speckle fluctuations. The occurrence of the gel is ascribed to polymer-induced aggregation between the colloids in the colloid-rich phase. Due to the reversible nature of the aggregation the network restructures and eventually the gel collapses, as is manifested by the rapid separation of the colloid-rich phase from the colloid-poor phase.

Transient gels in colloid–polymer mixtures studied with fluorescence confocal scanning laser microscopy

“…Taking into account that an array of 512 pixels measures 237 m in real space, this corresponds to a wavevector q = 3:9 × 10 3 cm −1 . The corresponding length scale in real space is given by L = 2 =q = 16 m. This is in good agreement with previous experiments on a closely related system [12]. As time elapses, the ring becomes elongated in the horizontal direction, indicating that the gel structure in real space exhibits anisotropy, presumably due to gravity.…”

“…So far, experimental information on the structural rearrangement leading to the collapse of transient gels is limited. In our previous work [12] the small angle light scattering proÿles showed a slow increase in intensity at low wavevector during the gel lifetime, indicating a loss of order in the system. We have tentatively ascribed this small angle behaviour to large scale rearrangement in the structure.…”

“…Indications have been given that these transient gels are made of fractal aggregates ÿll-ing all the available space. In a recent experiment transient gels in a colloid-polymer mixture with a larger range of attraction ( p = c = 0:25) have been investigated [12]. A stationary ring in the scattered intensity pattern is again observed during the gel life time.…”

“…This is presumably due to the fact that the energy of polymer-induced bonds is in the order of the particles' thermal energy. It has been argued that the reversible nature of the bonds allows a rearrangement of the gel structure, eventually leading to the collapse of the gel [9][10][11][12]. Computer simulations on colloidal aggregation have indeed established that in the case of ÿnite attractive interactions, restructuring occurs leading to local compactiÿcation [16][17][18][19].…”

Theoretical predictions of structures in dispersions containing charged colloidal particles and non-adsorbing polymers

“…Colloidal particles usually interact with each other via shortranged van der Waals (vdW) attractions, and when this attraction is strong, the system may form a gel 2,3 . Colloidal gels find a variety of applications as well 4 .…”