title: Stability and viscoelasticity of magneto-Pickering foams.
abstract: We have developed a new class of bistable Pickering foams, which can remain intact for weeks at room temperature but can be destroyed rapidly and on-demand with the use of a magnetic field. Such responsive foam systems can find application in various industrial and environmental processes that require controlled defoaming. These foams are stabilized by particles of hypromellose phthalate (HP-55) and contain oleic acid-coated carbonyl iron particles embedded in the HP-55 matrix. The complex behavior of these foams arises from several factors: a robust anisotropic particle matrix, the capacity to retain a high amount of water, as well as an age-dependent response to an external field. We report how the structure and viscoelastic properties of the foams change with time and affect their collapse characteristics. The evolution of foam properties is quantified by measuring the rate of liquid drainage from the foam as well as the rate of bubble growth in the foam with respect to time elapsed (in the absence of a magnetic field). We also evaluate the time necessary for foam collapse in magnetic fields as a function of magnetic particle content. A decreasing liquid volume fraction in the foam during aging leads to an increase in the elasticity and rigidity of the foam structure. These data allow us to identify a transition time separating two distinct stages of foam development in the absence of field. We propose different mechanisms which control foam collapse for each stage in a magnetic field. The stiffening of foam films between air bubbles with age plays a key role in distinguishing between the two destabilization regimes.
authors.affiliation: Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States.
authors.forename: Elena
authors.initials: E
authors.lastname: Blanco
authors.forename: Stephanie
authors.initials: S
authors.lastname: Lam
authors.forename: Stoyan K
authors.initials: SK
authors.lastname: Smoukov
authors.forename: Krassimir P
authors.initials: KP
authors.lastname: Velikov
authors.forename: Saad A
authors.initials: SA
authors.lastname: Khan
authors.forename: Orlin D
authors.initials: OD
authors.lastname: Velev
mesh_terms: D054354:Iron Carbonyl Compounds / Q000737:chemistry*; D060526:Magnetic Fields; D008747:Methylcellulose / Q000031:analogs & derivatives* / Q000737:chemistry; D009829:Oleic Acids / Q000737:chemistry*; D010316:Particle Size; D013499:Surface Properties; D014783:Viscosity
chemical_list: D009829:Oleic Acids; D054354:Iron Carbonyl Compounds; D008747:Methylcellulose; C053309:hydroxypropyl methylcellulose phthalate