Viscoelasticity of liquid organic foam: relaxations, temporal dependence, and bubble loading effects on flow behavior

Liquid organic foams are prepared using a new blowing process based on the chemical generation of carbon dioxide. The foams are volumetrically stable for periods up to hours and can be fabricated with gas volume fractions ranging from 0.10 to 0.95. Both the "fresh" and temporal dependences of the linear viscoelastic response of these materials are evaluated. The organic foams exhibit rheological behavior characteristic of their aqueous counterparts: a weak dependence of the shear moduli over an extended frequency/time regime that is bounded by both a fast and slow relaxation. The onset of the fast mechanical response of the organic foams occurs at approximately the same frequency as in aqueous foams despite the continuous phase viscosity differing by orders of magnitude between the systems. This suggests that the viscosity does not affect the time scale of the "anomalous" viscous loss characteristic of these materials, which challenges currently proposed mechanisms for this dissipation and leaves the origin of the loss behavior unclear. The relative contribution of cell growth and bubble motion to the slow relaxation is also discerned by evaluating the relation between the transient and dynamic responses of the foam. Finally, the development of elasticity in the foam due to bubble interactions is analyzed and a bubble slip process is postulated to account for the lack of a true elastic response of the foam at intermediate time scales (between the fast and slow mechanical response) when gas fractions exceed 0.64.