Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Aging of Concentrated Emulsions

by Suliana Manley
 
 

Aging is a quite general phenomenon which occurs in a wide variety of off-equilibrium materials, such as glasses and colloidal gels. Aging affects the properties (mechanical, rheological, magnetic...) of materials. As well as having potential industrial applications, it is of great physical interest to understand the principal mechanisms for aging. The most direct way to assess aging is to measure the change in the sample dynamics. We are exploring the use of dynamic light scattering in conjunction with microscopy to study the aging of concentrated emulsions. We use a novel, CCD-based low angle dynamic light scattering apparatus to take "snapshots" of the dynamics, thereby probing directly the aging. We use a confocal microscope to take 3-D images of the emulsions.
 

 
 

The system we study is a gel composed of emulsion droplets, formed by water droplets in oil with a surfactant-stabilized interface. Despite being composed solely of fluids, the system is a solid "gel", with an elasticity. The source of the elastic behavior lies in the deformation at the interface of the droplets, which results when the emulsion is compressed. These emulsions are at a volume fraction of 0.75, which is considerably higher that the monodisperse hard sphere close-packing limit of 0.56. Polydispersity allows the droplets to pack more efficiently, but these emulsions do show an elastic modulus.

 
 
 

We study concentrated emulsions at various surfactant concentrations. We can see from the microscope images below that droplet size is strongly dependent on surfactant concentration.  

 
 

We propose that the droplet size is constrained by the amount of surfactant available to stabilize the surface. If this is the case, the droplet size should be inversely proportional to the surfactant concentration. This is in good agreement with what we see from a rough analysis of the particle sizes taken from microscopy data. A slope of 1 is indicated by the red line.

 
 

Previous studies (Mason et al., Phys. Rev. Lett. 1995) have shown that the elasticity of concentrated emulsions is inversely dependent on droplet size. To confirm that our picture is correct, we show here the elastic modulus as a function of surfactant concentration. Indeed, the elasticity increases approximately linearly (indicated by the red line) with surfactant concentration.

 
 

In order to understand how these changes in sample morphology result in changes in sample stability, we did dynamic light scattering. All of the samples showed aging, in that the decay time of the correlation function increased with time. We found that we could initialize the gel by centrifugation, and re-centrifuging the sample returned the pre-aged behavior. We plot here the correlation functions at two angles, and show the evolution in time as the sample relaxes after centrifugation.

 
 

We find that the correlation functions are fit well by a stretched exponential, many of them with stretching exponent 1.5-2. Furthermore, the scaling of the relaxation time with scattering angle is consistent with ballistic motion. This is reminiscent of what we have seen for the aging of fractal colloidal gels.  

 
 

We watched the aging for several different surfactant concentrations and centrifugation rates. The impact of the surfactant concentration on aging is not clear from these experiments. However, it appears that the relaxation time decreases as the centrifugation rate is increased. This suggests that the gel is something like a spring, and the more it is compressed, the faster it relaxes when it is released.

 

We have also been able to detect phase separation using light scattering in some unstable emulsions. It seems that the combined tools of light scattering and microscopy can be very useful in understanding the relaxation mechanisms of concentrated emulsions. These techniques are being extended to study other systems as well.

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