(88b) Stability and Viscoelasticity of Responsive Magneto-Pickering Foams

We will discuss the properties of a new class of Pickering foams, which can be manipulated using a magnetic field. Such foam systems can be applied in a range of industrial and environmental applications that require controlled defoaming on demand. These foams are stabilized by hypromellose phthalate (HP-55) and contain oleic acid coated carbonyl iron particles embedded in the HP-55 matrix. They exhibit exceptional stability in the absence of a magnetic field, but can also be destroyed rapidly and on demand above a certain concentration threshold. We analyze foam stability in the absence of a field by measuring the rate of water drainage from the foam as a function of time. We also correlate the collapse behavior of the foam to the liquid volume fraction as well as the concentration of magnetic particles in the system. Aging effects associated with film wetness lead to different collapse mechanisms. Films in fresher foams are thicker, and upon the application of a magnetic field, the particles and bubbles are able to move around and rearrange. Aged foams, in which the films have drained and the particles have decreased mobility, are more susceptible to magnetic field destabilization. This behavior can be correlated to the evolution of the viscoelastic properties of the foam. Rheological data suggests that Pickering foams are soft glassy materials, which behave like elastic solids when sheared due to the capacity of the bubbles to re-arrange under mechanical deformation. However, the dynamic elastic modulus and yield stress are functions of the liquid fraction and drainage time. In summary, we demonstrate that the drainage and stiffening of the foam can be correlated to the foam lifetime in the presence and absence of magnetic field.