The effect of plastic rearrangements on the flow of two-dimensional wet foam

Dislocation mechanisms in the plastic deformation of monodisperse wet foams within an expansion-contraction microfluidic geometry

Densely packed wet foam was subjected to gradual expansion and contraction in a wide (1400-1800 μm) microfluidic channel to study localized plastic deformation events within the monodisperse bubble matrix. Dislocation glide, reflection, nucleation, and dipole transformations from extensional and compressive stresses were observed across a range of fluid flow rates and bubble packing densities. Disparate, cyclic reflections occur in two independent regions of the flowing foam, and the mechanisms of dislocation reflection under tension are expanded. The use of an asymmetric channel created a dichotomy in the model crystalline system between straighter, aligned bubble rows and curved, misaligned rows due to the corresponding streamlines within the channel. The resulting gradient in crystalline alignment had numerous effects on dislocation mobility and plastic deformation. 7/7 dipoles were found to rearrange to a more stable configuration aligned with the foam flow before dissociating. Dislocations comprising 5/5 dipoles (resembling the inverse-Stone-Wales defect in carbon nanostructures) were discovered to pass through one another via intermediate ring structures, which most commonly consisted of three dislocation pairs around a triangular-shaped central bubble.

Stress and bubble pressure response of wet foam to continuous and oscillatory sinusoidal shear

Wet foam, as a typical multiphase soft material, has complex spatial structure. Foam quality (i.e., gas fraction of a foam fluid), one of fundamental structure parameters of a foam system, generally has a significant influence on the mechanical response of the wet foam to the continuous and oscillatory shear. This study shows that the stress level of the wet foam, including the shear stress and the normal stress difference, rises with the foam quality. An exponential link between the yield stress of wet foam and the foam quality is demonstrated. In the oscillatory sinusoidal shear, a frequent fluctuation of the stress curve mainly occurs at the relatively higher strain rate, and the stress state in the foam is still maintained at the end of the oscillatory shear. Further, with the increase of foam quality, the loss modulus decreases when the foam does not yield, while the storage modulus as well as the loss modulus increases as the strain amplitude exceeds a certain value. Additionally, a nonlinear stress response of the wet foam is mainly attributed to the third harmonic component as the strain amplitude increases in the oscillatory shear. In the shear, the average level of bubble pressure in the foam increases with the foam quality, and it fluctuates with the strain owing to the elastic-plastic deformations of the films. Especially, in the oscillatory shear, the average bubble pressure fluctuates more frequently as the strain rate reaches a relatively higher value.

Ideal wet two-dimensional foams and emulsions with finite contact angle

We present simulations that show that the equilibrium structure of an ideal two-dimensional foam with a finite contact angle develops an inhomogeneity for high liquid fraction φ. In liquid-liquid emulsions this inhomogeneity is known as flocculation. In the case of an ordered foam this requires a perturbation, but in a disordered foam inhomogeneity grows steadily and spontaneously with φ, as demonstrated in our simulations performed with the Surface Evolver.

Origin of accelerated and hindered sedimentation of two particles in wet foam

To explore the origin of interactional settling behaviors of multi-particles in wet foam, the sedimentation of two particles placed one above the other as well as placed side by side is studied. According to the average settling velocity in experiment and the average settling drag force of the two particles in numerical simulation, we show that the particles display accelerated sedimentation as placed one above the other while they display hindered sedimentation in the case of the ones positioned side by side. Furthermore, the evolution of structure and force parameters of the bubbles, such as T1 topological events, displacement vector and principal stress fields, shows that the reciprocal action between the foam and the settling particles placed side by side is more significant. The different levels of interplay for these two settling cases also give rise to the diverse changes of bubble pressure response. The bubble pressure component of the average drag force is higher for the particles placed side by side. Especially, for the first time, it reveals that these interactional sedimentation behaviors in the foam are mainly attributed to the changed pressure of bubbles caused by these settling particles at the mesoscopic level. The present results may suggest potential explanations to the cause of the complex accelerated or hindered sedimentation of more particles in wet foam.

Detailed Structural and Mechanical Response of Wet Foam to the Settling Particle

Liquid foam, as a complex fluid, provides an observable prototype for studying a discrete fluid system. In this work, a numerical study on the settling behavior of a round particle in wet polydisperse foam has been conducted on the bubble scale. The local and nonuniform distribution of bubble pressure, as well as the localized plastic events, is presented. It shows a foam region of higher pressure in front of the settling particle due to the extrusion deformation of the bubbles applied by the particle. Additionally, the forces exerted on the particle by the disordered wet foam are measured during the sedimentation. It exhibits in particular a power-law dependence of the drag force caused by the bubble as a function of the foam quality. Moreover, sedimentation experiments are demonstrated to verify this power-law relation. The evolution of the components of drag force is demonstrated when a plastic event occurs in front of the settling particle. The result shows that both the contributions of the pulling force of foam films and the bubble pressure to the drag force decrease in that case. Likewise, the variation of both these contributions to the drag force is illustrated as well when a bubble in the wake detaches from the particle. These results assist in understanding the mesoscopic response of wet foam to a settling particle.