Volume 56, Issue 3, May 2012
Index of content:
On the effectiveness of a quasistatic bubble-scale simulation in predicting the constriction flow of a two-dimensional foam56(2012); http://dx.doi.org/10.1122/1.3687301View Description Hide Description
A comprehensive set of experiments on a two-dimensional constriction flow of a foam are described. Image analysis of the flow is used to ascertain bubble shape and flow dynamics. The bubble velocity, elongation (texture), stress, and deformation rate for a reference case are used to validate a quasistatic simulation. Changes to the constriction geometry, most especially in the rounding of the corners, have a significant effect on the response of the foam, captured in both experiment and simulation. On the other hand, foam properties such as bubble size have little impact on the rheological behavior of the foam in the range considered here.
56(2012); http://dx.doi.org/10.1122/1.3687415View Description Hide Description
The effect of the interplay between surface tension and gravity on the sedimentation of objects in structured fluids is investigated by simulating the quasi-static motion of a spherical particle through an ordered foam. We describe the path which a sphere takes as it descends through bamboo (1,1,0), staircase (2,1,1), chiral (3,2,1), and double staircase (4,2,2) foams, and measure the degree of control of the sphere’s motion that each foam offers. For an ordered foam contained within a vertical cylinder, the resulting sphere motion depends strongly on the structure itself, on how the films are deformed near the sphere, and on how the motion of the sphere deforms them further. For staircase and chiralfoams, the distance that a sphere is pulled away from the center-line of the cylinder by the foam is found to depend on the Bond number with a power-law relation. By tilting the cylinder at an angle to the vertical, we show that there exists a critical tilt angle above which the sphere falls out of the foam. This angle is dependent on the choice of foam structure and the Bond number. For a sphere of given size and given Bond number in the ordered foams studied here, the greatest tilt can be imposed on the double staircase foam.
56(2012); http://dx.doi.org/10.1122/1.3687425View Description Hide Description
A single layer of foam bubbles between parallel plates fails (fractures) when subjected to an applied pressure. The fracture can occur in both a ductile mode and a brittle mode. It is shown that dissipative effects during propagation of a brittlecrack lead to a spontaneous brittle-to-ductile transition, which occurs dynamically during fracture and can be experimentally resolved in its details. The transition occurs when the driving pressure at the crack tip drops below that necessary to maintain supersonic propagation of the brittlecrack. A theory taking into account the lateral crack opening together with air flow in the crack successfully predicts the occurrence and location of this novel spontaneous transition.
56(2012); http://dx.doi.org/10.1122/1.3687442View Description Hide Description
The two-dimensional, regular, hexagonal foam is a common benchmark system for studies of foam rheology under imposed shear. Traditionally, the hexagonal foam has been studied under quasistatic shear deformations. Here, however, nonequilibrium systems are considered. Specifically, the hexagonal foam is assumed to depart from physicochemical equilibrium (surfactant coverage varies and hence surface tension varies between films), but to remain in static mechanical equilibrium (as a consequence, films in the hexagonal foam remain straight, simplifying the geometrical description considerably). Regardless of the mechanism for departing from equilibrium, topological transformations (during which certain films in the hexagonal structure shrink to zero, and bubbles exchange neighbors) tend to be postponed compared to an equilibrium foam. Even when the rate of imposed shear is small, significant departures from physicochemical equilibrium are still observed on the approach to and in the immediate aftermath of a topological transformation. The nature of the relaxation post-topological transformation depends on the variation of film tension with surfactant coverage. It may be almost entirely mechanical (in the case of weakly varying film tension) or almost entirely physicochemical (for a strongly varying film tension). As the imposed shear rate increases, models incorporating weak departures from physicochemical equilibrium prove inadequate to predict the criteria for topological transformation: far-from-equilibrium physicochemical properties must be considered also.
56(2012); http://dx.doi.org/10.1122/1.3695152View Description Hide Description
The behavior of materials under tension is a rich area of both fluid and solid mechanics. For simple fluids, the breakup of a liquid as it is pulled apart generally exhibits an instability driven, pinch-off type behavior. In contrast, solid materials typically exhibit various forms of fracture under tension. The interaction of these two distinct failure modes is of particular interest for complex fluids, such as foams, pastes, slurries, etc. The rheological properties of complex fluids are well-known to combine features of solid and fluid behaviors, and it is unclear how this translates to their failure under tension. In this paper, we present experimental results for a model complex fluid, a bubble raft. As expected, the system exhibits both pinch-off and fracture when subjected to elongation under constant velocity. We report on the critical velocity below which pinch-off occurs and above which fracture occurs as a function of initial system width W, length L, bubble size R, and fluid viscosity for both monodisperse and polydisperse systems. Though both exhibit a transition from pinch-off to fracture, the behavior as a function of L/W is qualitatively different for the two systems. For the polydisperse systems, the results for the critical velocity are consistent with a simple scaling law , where the fluid viscosity sets the typical time for bubble rearrangements . We show that this scaling can be understood in terms of the dynamics of local bubble rearrangements (T1 events). For the monodisperse systems, we observe a critical value for L/W below which the system only exhibits fracture.
56(2012); http://dx.doi.org/10.1122/1.3695029View Description Hide Description
We formulate a large-scale network model for the dynamics and stability of a planar (gas–liquid) foam with low liquid fraction composed of approximately polygonal gas bubbles. We restrict attention to clean liquids (free of surfactants and colloidal particles) as a model for molten metallic foams produced using batch-processing techniques for manufacturing porous-metal solids. The liquid is primarily confined within highly curved regions around the bubble vertices, Plateau borders, interconnected by thin liquid films that form the edges of the bubbles and drain due to capillary-viscous suction. The model incorporates direct coupling between the pressure and area of the bubbles, surface-tension forces on the gas–liquid interfaces and draining and elongational flows in the films. The model also explicitly accounts for van der Waals instabilities that grow to rupture a liquid film once it becomes sufficiently thin, leading to bubble coalescence and hence coarsening of the foam. We consider foams confined within a rectangular box which is prewetted with a thin film of liquid. Initially, the foam is composed of regular polygonal bubbles with equal pressure, but the first film breakage triggers a dynamiccoalescence process where the mean bubble area increases quickly; numerical simulations elucidate the large-scale topological rearrangement as the foam coarsens.
56(2012); http://dx.doi.org/10.1122/1.3696875View Description Hide Description
We study the rheology of cornstarch suspensions, a non-Brownian particle system that exhibits discontinuous shear thickening. Using magnetic resonance imaging(MRI), the local properties of the flow are obtained by the determination of local velocity profiles and concentrations in a Couette cell. For low rotational rates, we observe shear localization characteristic of yield stress fluids. When the overall shear rate is increased, the width of the sheared region increases. The discontinuous shear thickening is found to set in at the end of this shear localization regime when all of the fluid is sheared: the existence of a nonflowing region, thus, seems to prevent or delay shear thickening. Macroscopic observations using different measurement geometries show that the smaller the gap of the shear cell, the lower the shear rate at which shear thickening sets in. We, thus, propose that the discontinuous shear thickening of cornstarch suspensions is a consequence of dilatancy: the system under flow attempts to dilate but instead undergoes a jamming transition, because it is confined. This proposition is confirmed by an independent measurement of the dilation of the suspension as a function of the shear rate. It is also explains the MRI observations: when flow is localized, the nonflowing region plays the role of a “dilatancy reservoir” which allows the material to be sheared without jamming.
Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests56(2012); http://dx.doi.org/10.1122/1.3698185View Description Hide Description
The non-Newtonian rheology of concentrated fiber suspensions, such as short fiber reinforced polymer composites during their processing, depends on both the microstructure of their fibrous network and the deformation micromechanisms arising at fiber–fiber contacts. In this work, these two aspects are investigated using model concentrated fiber suspensions made up of short glass fiber bundles impregnated in a transparent polymer. For that purpose, multiresolution x-ray microtomography was used to analyze the fibrous microstructures, showing that the studied suspensions exhibit a planar fiber orientation with fairly straight fiber bundles, the connectivity of which can be modeled by the geometrical statistical tube model. Besides, bundle–bundle contact forces together with the interaction between the bundles and the suspending fluid are analyzed by using pull-out experiments. These tests allow the influence of the pull-out velocity, the confining stress and the volume fraction of fiber bundles on the pull-out force to be quantified. Combined with the microstructure analysis, such results are then used to propose a bundle–bundle contact model which can be implemented into multiscale rheological models dedicated to concentrated fiber suspensions.
Rheological and morphological properties of reactively compatibilized thermoplastic olefin (TPO) blendsa)56(2012); http://dx.doi.org/10.1122/1.3700966View Description Hide Description
Thermoplastic olefin blends of polypropylene (PP) and ethylene octene copolymers (EC) were reactively compatibilized by means of functional reactive compounds capable of forming copolymers at the interface. For this purpose, amine functional groups were first incorporated into a PP in a solution reaction. The aminated PP was then used as the reactive compatibilizer during melt mixing. Linear viscoelasticmeasurements showed that the compatibilized blends feature the characteristics of materials in the sol–gel transition, with a power-law behavior for the dynamic moduli at low frequencies. The gel-like behavior was more pronounced in the blend with a high level of compatibilizer (10 wt. %). At high frequencies, however, the dynamic properties of all the blends investigated (compatibilized and noncompatibilized) were identical, suggesting that the bulk properties of the blends were not changed by the reactive compatibilization. The presence of a network structure was also confirmed by microscopic observations. A large transient viscosity with a significant and broad overshoot was observed for the compatibilized blends at low shear rate. In addition, the stress of the compatibilized blends relaxed over a much longer time as compared to the noncompatibilized system. The ability of different linear viscoelastic models in predicting the linear viscoelastic behavior of the compatibilized blends was also examined.
56(2012); http://dx.doi.org/10.1122/1.3702101View Description Hide Description
Additives were tested for their ability to modify the rheology of lignocellulosic biomass. Additive types included water-soluble polymers (WSPs), surfactants, and fine particles. WSPs were the most effective rheological modifiers, reducing yield stresses of concentrated biomass by 60–80% for additive concentrations of 1–2 wt. % (based on mass of dry biomass solids). Yield stress and plastic viscosity of rheologically modified biomass depended on WSP molecular weight and degree of substitution. The apparent shear stress-shear rate data are represented with the Bingham model. In the absence of WSP, the biomass exhibited a positive yield stress and a negative plastic viscosity, which suggests a nonmonotonic dependence of shear stress on shear rate. When WSP was added, the yield stress decreased and the plastic viscosity increased, becoming positive for sufficiently large WSP concentrations.