We introduce a new class of Pickering foams which can be manipulated using a magnetic field. These foams are stabilized by a mixture of magnetic and nonmagnetic particles. They exhibit excellent stability in the absence of a magnetic field, but can be rapidly destroyed on demand with the application of a threshold field. We characterize their stability in the absence of a magnetic field by measuring the rate of water drainage from the foam as a function of time. We also correlate their collapse behavior under a magnetic field to the foam liquid fraction, as well as the concentration of magnetic particles in the foam. This novel system can be used to study the properties of Pickering foams, and has potential applications in noncontact defoaming processes.

We present a phenomenological model for granular suspension rheology in which particle interactions enter as constraints to relative particle motion. By considering constraints that are formed and released by stress respectively, we derive a range of experimental flow curves in a single treatment and predict singularities in viscosity and yield stress consistent with literature data. Fundamentally, we offer a generic description of suspension flow that is independent of bespoke microphysics.Concentrated particulate dispersions are ubiquitous in industry. When the particle size is in the granular (i.e., non-Brownian) regime (radius R ≳ 1 µm), their flow is notoriously difficult to predict and control [1, 2]. Paradoxically, a suspension of non-Brownian hard particles has no intrinsic time or stress scale and so should have a viscosity η that is independent of shear stress σ and ratė γ [2, 3]. In reality, three classes of flow curve η(σ) are observed, none of which is Newtonian. Some granular suspensions shear thin (dη dσ < 0, class 1) [4,5], others shear thicken (dη dσ > 0, class 2) [6-8] while others show a varied combination of thinning and thickening (class 3): thinning then thickening (class 3a) [9, 10], thickening then thinning (class 3b) [11][12][13] or more complex behavior [10,14,15] (class 3c). In each class, the suspensions can become solid-like [16] or flow unstably [17,18].Such behavior likely stems from details of the particle interactions [2] set by, e.g., surface chemistry [19] or roughness [20]. Most models incorporate such interactions in a bespoke manner. Notably, a phenomenological model by Wyart and Cates (WC) [21] predicts thickening (class 2) due to a transition from frictionless (static friction coefficient µ ≈ 0) to frictional (µ > 0) particle contacts above a critical "onset stress". Atomic force microscopy confirms this picture for several systems [15,22] and the WC model fits a number of experimental flow curves [7,8,18]; although, quantitative discrepancies with microscopic simulations remain [23].To recast the WC model within a more general framework, recall that frictional contacts constrain interparticle sliding. Crucially, the WC model is agnostic to the exact mechanism by which sliding is constrained, so that disparate microphysics, e.g., stress-induced interlocking of asperities [20,24], hydrogen bonding [25] or 'traditional' Coulomb friction can all give rise to the same macroscopic, shear-thickening phenomenology.In this broader framework, the WC model deals with a single type of constraint: sliding. Rolling (rotations about axes perpendicular to the line of centres) and twisting (rotations about the line of centres) degrees of freedom remain unconstrained. By assuming that sliding constraints are formed at increasing stress, the WC model accounts for class 2 behavior, which, however, is rare in practice. Real systems are typically class 1 or 3, for which current explanations involve the ad hoc "bolting together" of different kinds of bespoke physics [10].Here, we generalize the ...

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Yielding behavior is well known in attractive colloidal suspensions. Adhesive non-Brownian suspensions, in which the interparticle bonds are due to finite-size contacts, also show yielding behavior. We use a combination of steady-state, oscillatory and shear-reversal rheology to probe the physical origins of yielding in the latter class of materials, and find that yielding is not simply a matter of breaking adhesive bonds, but involves unjamming from a shear-jammed state in which the micro-structure has adapted to the direction of the applied load. Comparison with a recent constraint-based rheology model shows the importance of friction in determining the yield stress, suggesting novel ways to tune the flow of such suspensions.

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.

The mixing of a powder of 10- to 50-μm primary particles into a liquid to form a dispersion with the highest possible solid content is a common industrial operation. Building on recent advances in the rheology of such “granular dispersions,” we study a paradigmatic example of such powder incorporation: the conching of chocolate, in which a homogeneous, flowing suspension is prepared from an inhomogeneous mixture of particulates, triglyceride oil, and dispersants. Studying the rheology of a simplified formulation, we find that the input of mechanical energy and staged addition of surfactants combine to effect a considerable shift in the jamming volume fraction of the system, thus increasing the maximum flowable solid content. We discuss the possible microscopic origins of this shift, and suggest that chocolate conching exemplifies a ubiquitous class of powder–liquid mixing.

The combined influence of surface topography and charge of a polymer surface on the adsorption of the protein avidin has been investigated. Atomic force microscopy contact mode imaging and charge writing were used to create defined topographical roughness and electrostatic charge patterns on the surface of polystyrene. Increased avidin adsorption was found on nanometer-size topographical patterns, but the adsorption remained unaffected by electrostatic patterns.

The complexations between human serum albumin (HSA) and the sodium perfluorooctanoate/sodium octanoate and sodium perfluorooctanoate/sodium dodecanoate systems have been studied by a combination of electrical conductivity, ion-selective electrode, electrophoresis, and spectroscopy measurements. The binary mixtures of the surfactants deviated slightly from ideality. Binding plots revealed the existence of two specific binding sites, the first site being more accessible than the second. Positive cooperative binding has been found, thus revealing the importance of the hydrophobic interactions in both kinds of surfactants. The Gibbs energies of binding per mole of surfactant (DeltaG(nu)) were calculated from the Wyman binding potential where, on the basis of the elevated number of binding sites, a statistical contribution has been included. Initially these energies are large and negative, gradually decreasing as saturation is approached. Changes in the slope of Gibbs energies have been identified with the saturation of the first binding set. These facts denote that the surfactants under study have different favorite adsorption sites along the protein and that the adsorption process of perfluorooctanoate is more closely followed by dodecanoate than by octanoate. Finally, electrophoresis and spectroscopy measurements suggest induced conformational changes on HSA depending on the surfactant mixture as well as the mixed ratio.