2011 Annual Meeting

(191j) Sterically Stabilized Emulsions: How Important Is the Polymer Brush?

Authors

Ofer Manor - Presenter, The University of Melbourne
G. W. Stevens - Presenter, The University of Melbourne
Franz Grieser - Presenter, The University of Melbourne
D.Y.C. Chan - Presenter, The University of Melbourne
Tam T. Chau - Presenter, The University of Melbourne


Polymeric stabilisers are common in many processed products ranging from pharmaceuticals and engine lubricants to formulated foods and shampoos1,2. In particulate systems, steric stabilization provides a repulsive force that arises from the compression of the polymer brush, which screens the influence of attractive surface forces, such as the van der Waals force. This mechanism has dictated polymers design and selection for more than thirty years, and yet the mechanisms behind steric stabilization in emulsions systems may be fundamentally different or even counter intuitive compared to particulate systems. For the interactions between two drops, we show that the interplay between drop deformation, hydrodynamic drainage and steric forces combine to make their behaviour and emulsion stability much less sensitive to polymer architecture and yet a far more complex interaction than in rigid systems.

We present direct force measurements between two oil droplets coated with an amphiphilic tri-block co-polymer as a function of collision speed using Atomic Force Microscopy (AFM).   At slow collision speeds we show the contributions to the observed forces show little dependence on the steric force law and are far more dependent on the drop deformation.  At higher collision speeds, the observed force is sensitive to the amount of drainage of liquid through the brush.  Simple constitutive models for both force2 and drainage3 behaviour of the steric layer have been incorporated into an existing physical model4 to describe the interplay between droplet deformation, surface forces and hydrodynamic drainage to quantitatively analyze the AFM force measurements.  These results demonstrate the importance of accurately understanding the role of deformation when involving steric stabilization in soft matter systems.

1. Israelachvili, J. N., Intermolecular and surface forces, 2 ed. (Academic press limited, San Diego, (1991).

 2. De Gennes, P. G., Polymers At An Interface - A Simplified View. Adv. Colloid Interface Sci. 27 (3-4), 189 (1987).

3. Klein, J., Shear, friction, and lubrication forces between polymer-bearing surfaces. Annual Review Of Materials Science 26, 581 (1996).

4. Dagastine, R. R. et al., Dynamic Forces Between Two Deformable Oil Droplets in Water. Science 313 (5784), 210 (2006).