Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Effects of Gravity on Colloidal Aggregation

by Suliana Manley
 
 

Aggregation is an important process which can be used to control the viscoelastic properties of colloidal materials. Much has been done to understand the effect of interparticle interactions on aggregation. For strong attractions, aggregation can be described as diffusion-limited, or in the case of a repulsive barrier, reaction-limited. However, in the presence of gravity, diffusion becomes less important as the cluster Peclet number increases. We use a novel, CCD-based low angle dynamic light scattering apparatus to investigate cluster growth under varying buoyancy mismatch. We have also studied the growth of the aggregates under microgravity with the Physics of Colloids in Space project .
 

 
 

We study polystyrene and silica colloidal aggregates, formed by destabilization by the addition of salt. At a high volume fraction of colloids, the aggregates grow to fill the sample volume, thus forming an elastic network, or gel. When the buoyancy mismatch is high, the gel cannot support its own weight and collapses in time. This effect is seen in many gelling systems (J. R. Weeks et al., J. Phys. Cond. Mat. 12 (2000), C. Allain et al., Phys. Rev. Lett. 74 (1995)).

 
 
 

At lower volume fractions, we can watch the effect of gravity on the aggregation itself. This leads to a change in the shape of the measured correlation functions, because the aggregates are displaced by both diffusion and gravity. This effect becomes more and more pronounced as the aggregates grow, reflected in the correlation functions which deviate more and more from a single-exponential decay characteristic of diffusive motion. In addition, the aggregation rate increases as the larger aggregates settle faster than the smaller ones, thereby growing faster than they would by diffusion alone.

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