Bubble-particle dynamics in multiphase flow of capillary foams in a porous micromodel.

Surfactant-free capillary foams (CFs) are known to be remarkably tolerant to oil, and possess unique stability and flow properties. These properties result from the presence of oil-and-particle-coated bubbles that are interconnected by a dense particle-oil capillary network. In this work, we present a study of the dynamics of capillary foams flowing through a porous micromodel. We determine that despite the presence of oil-particle networks, CFs can flow through a microporous environment and that above a threshold flowrate, >80% of foam pumped through the micromodel can be recovered. In addition, we highlight the absence of steady state in CF flow and identify the underlying phenomena including the increasing apparent viscosity, reconfigurable flow paths, and intermittent clogging of the micromodel from an oil-particle composite and bubbles trapped in pores. We also characterize bubble dynamics and show that CFs surprisingly exhibit the same bubble generation and destruction mechanisms as classical foams despite the absence of surfactants. Our observations suggest that the porous medium plays a key role in generating uniformly sized bubbles and that capillary foams in a microporous environment tend to reconfigure their flow paths in a manner that may provide opportunities for increased sweep efficiency in enhanced oil recovery.

Effect of Shear on Pumped Capillary Foams.

Foam flow in many applications, like firefighting and oil recovery, requires stable foams that can withstand the stress and aging that result from both shear and thermodynamic instability. Events of drainage and coarsening drive the collapse of foams and greatly affect foam efficacy in processes relying on foam transport. Recently, it was discovered that foams can be stabilized by the synergistic action of colloidal particles and a small amount of a water-immiscible liquid that mediates capillary forces. The so-called capillary foams contain gas bubbles that are coated by a thin oil-particle film and integrated in a network of oil-bridged particles; the present study explores how this unique architecture impacts the foams' flow dynamics. We pumped capillary foams through millimeter-sized tubing (ID: 790 µm) at different flow rates and analyzed the influence of stress and aging on capillary foam stability. We find that the foams remain stable when pumped at higher flow rates but undergo phase separation when pumped at low flow rates. Our observations further show that the particle network is responsible for the observed stability in capillary foams and that network strength and stability of an existing foam can be increased by shearing.

Structure-Property Relationship in Capillary Foams.

The recently discovered capillary foams are aqueous foams stabilized by the synergistic action of colloidal particles and a small amount of oil. Characteristically, their gas bubbles are coated by a particle-stabilized layer of oil and embedded in a gel network of oil-bridged particles. This unique foam architecture offers opportunities for engineering new foam-related materials and processes, but the necessary understanding of its structure-property relations is still in its infancy. Here, we study the effects of particle wettability, particle volume fraction, and oil-to-particle ratio on the structure and selected properties of capillary foams and use our findings to relate measured foamability, foam stability, and rheological key parameters to the observed foam microstructure. We see that particle wettability not only determines the type of gel network formed but also influences the prevalence of oil droplets included within the foam. Our results further show that the stability and rheology of capillary foams are mainly a function of the particle volume fraction whereas the foamability and observed microstructure are sensitive also to the oil-to-particle ratio. These insights expand our fundamental understanding of capillary foams and will greatly facilitate future work on new foam formulations.

Rheology of capillary foams.

Aqueous foams are ubiquitous; they appear in products and processes that span the cosmetics, food, and energy industries. The versatile applicability of foams comes as a result of their intrinsic viscous and elastic properties; for example, foams are exploited as drilling fluids in enhanced oil recovery for their high viscosity. Recently, so-called capillary foams were discovered: a class of foams that have excellent stability under static conditions and whose flow properties have so far remained unexplored. The unique architecture of these foams, containing oil-coated bubbles and a gelled network of oil-bridged particles, is expected to affect foam rheology. In this work, we report the first set of rheological data on capillary foams. We study the viscoelastic properties of capillary foams by conducting oscillatory and steady shear tests. We compare our results on the rheological properties of capillary foams to those reported for other aqueous foams. We find that capillary foams, which have low gas volume fractions, exhibit long lasting rheological stability as well as a yielding behavior that is reminiscent of surfactant foams with high gas volume fractions.