Soft, Softer, Softest: Strain softening in foams and emulsions

Foams and emulsions are solids, as is evident from the fact that a dollop of mayonnaise or shaving cream keeps its shape. Yet they are very easy to deform -- why is this so? This is a practical question, because foams, emulsions, pastes, and other soft disordered materials appear frequently in the oil, food, and personal care industries.

Foams and emulsions are soft, in part, because the individual bubbles and droplets are themselves very soft. While the structure of a foam and a pile of steel ball bearings are very similar, bubbles in a soapy solution made from a detergent like Dreft are orders of magnitude softer than ball bearings.

But when foams or emulsions are subjected to large deformations, a strange new effect called strain softening appears. When materials show strain softening, their stiffness depends on how far they have already deformed: the more they deform, the softer they get. Strain softening is well-known but poorly understood, and it cannot be explained by physics on the scale of a single droplet or bubble.
 

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To understand and explain strain softening in foams and emulsions, the student will:

1. perform computer simulations of foams and emulsions undergoing shear, varying both their volume fraction and how far they are deformed.

2. perform numerical analysis of the model materials, including calculations of their apparent shear modulus.

3. learn the theoretical methods of critical scaling analysis, and apply them to sheared foams and emulsions.

This project is best suited to students who have enthusiasm for computer simulations and numerical analysis.