(175g) Engineering of Novel Classes of “Smart” Stimuli Responsive Foams

Soft materials, such as foam and emulsion systems, which respond to external stimuli, are on the leading edge of materials research and have recently been of interest to many scientists. We present a novel class of magnetically responsive Pickering foams, which remain stable at ambient conditions but can be quickly destroyed upon exposure to a gradient magnetic field (S. Lam, et al., J. Am. Chem. Soc. 2011, 133, 13856). These foams are stabilized by particles made from hydrophobically modified cellulose (HP-55) with magnetic responsiveness imparted through the incorporation of carbonyl iron (CI) into the cellulose matrix. We studied the change in foam water fraction and collapse behavior over time, and correlated the evolution of these properties with foam viscoelasticity as a function of age. We observed differences in the mechanism of foam collapse based on the fraction of water in the foam as well as the age of the foam. This age-dependent response of the foams to a magnetic field was demonstrated using SQUID magnetometry as well as magnetorheology. By correlating the time-dependence of foam viscoelastic properties and magnetic response, we proposed two separate mechanisms of collapse - one for fresh foams and one for aged foams. In these mechanisms, the collapse of fresh foam is correlated with the expulsion of air bubbles from the foam matrix, whereas the collapse of aged foam results from the rupture of foam films. We will also discuss how these systems can be made to be photo- as well as thermally responsive; and, present in brief, a characterization of the collapse properties of photo-responsive foams as a function of system composition. Systems, such as those presented here, can find application in a wide range of industrial and environmental processes that require controlled, non-contact and on-demand defoaming.