Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z19: Invited Session: Shear Thickening of Dense Suspensions |
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Sponsoring Units: DFD GSOFT Chair: Heinrich Jaeger, University of Chicago Room: Mission Room 103B |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z19.00001: Dynamics of Discontinuous Shear Thickening suspensions Invited Speaker: Eric Brown Concentrated suspensions of hard particles such as cornstarch in water exhibit Discontinuous Shear Thickening, in which an increasing shear rate drives a transition from liquid- to solid-like mechanical behavior. In steady-state shear this phenomena is a result of a dynamic version of jamming in which forces are transmitted along particle contact networks that span to system boundaries and repeatedly form and break up. Several dynamic phenomena observed in such suspensions have long been assumed to be a consequence of this shear thickening, but cannot be explained as a direct result of shear thickening; for example a uniquely strong impact response which allows a person to run on the fluid surface. We perform experiments in which a concentrated suspension is subjected to transient impact. We find that the strong impact response is due a short-lived jammed contact network spanning to the boundaries and a delay time required for this dynamically jammed region to propagate to the boundary. The resulting ability of this system-spanning solid-like region to support loads can explain the ability of a person to run on the surface of these fluids. This delay before a solid-like response may also explain several other dynamic phenomena observed in these fluids. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:27PM |
Z19.00002: Disentangling the role of hydrodynamic and frictional forces in a shear-thickening suspension Invited Speaker: Itai Cohen Who among us has not spent countless hours squeezing, rubbing, and smushing gooey substances like, tooth paste, silly putty, corn starch, and even bodily fluids between our fingers? If we could magnify our view and look deep within the substances we are handling what structures would we find? How, do these structures lead to the fascinating mechanical properties that we experience on the scale of our fingers. In this talk I will address the phenomenon of shear thickening in which the viscosity of a suspension increases with increasing shear rate. I will describe recent measurements we have made using a newly developed confocal rheoscope that, for the first time, experimentally visualize the hydrodynamically induced particle clusters. Such clusters have been implicated in continuous shear thickening. It remains controversial as to whether thickening in such suspensions also arises from frictional interactions between particles. The distinct contributions of frictional and hydrodynamic forces are typically difficult to measure independently using conventional techniques. Here, I will describe our approach for using both bulk rheometry techniques and our confocal rheoscope to disentangle their contributions to the total stress response. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 1:03PM |
Z19.00003: Discontinuous Shear Thickening and Dilatancy: Frictional Effects in Viscous Suspensions Invited Speaker: Jeffrey Morris Shear thickening in concentrated suspensions has been well-known for quite a long time, yet a firm consensus on the basis for very abrupt or ``discontinuous'' shear thickening (DST) seen in suspensions of large solid fraction, $\phi$, has not been reached. This work addresses the DST phenomenon, and proposes a simulation method based in the Stokesian Dynamics algorithm to explore the role of various forces between the particles, including hydrodynamic, conservative potential, and frictional interactions. This work shows that allowance for friction between spherical particles suspended in a viscous liquid causes a significant reduction in the jamming solid fraction of the mixture, $\phi_{\rm max}$, taken as the maximum fraction at which the suspension will flow. A consequence of this is a shifting of the singularity in the effective viscosity, $\eta$, to smaller $\phi_{\rm max}$, and the frictional suspension has a larger viscosity than does the frictionless suspension of the same solid fraction, as is clear from the standard empirical modeling of $\eta(\phi) = (1-\phi/\phi_{\rm max} )^{-\alpha}$, $\alpha \approx 2$. When a counterbalancing repulsive force between the particles, representative for example of charge-induced repulsion, is incorporated in the dynamics, the mixture undergoes a transition from frictionless to frictional interactions, and from low to high effective viscosity, at a critical shear rate. Comparison with experimental data shows remarkable agreement in the features of DST captured by the method. The basic algorithm and results of both rate-controlled and stress-controlled simulations will be presented. Like the shear stress, the magnitude of the normal stress exerted by the suspended particles also increases abruptly at the critical shear rate, consistent with the long-standing notion that dilatancy and shear-thickening are synonymous. We will show that considering all shear thickening materials as dilatant is a misconception, but demonstrate the validity of the connection of dilatancy with DST in concentrated suspensions. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:39PM |
Z19.00004: Role of frictional particle interactions for the jamming of dense suspensions Invited Speaker: Claus Heussinger The jamming paradigm aims at providing a unified view for the elastic and rheological properties of materials as different as foams, emulsions, suspensions or granular media. The usefulness of such a unifying concept hinges on the presence or absence of phenomena that are in some sense ”universal”. One such question is the form of the jamming phase itself. It has long been known that certain suspensions can undergo arrest when driving is strong enough. By way of contrast, standard yield-stress fluids, like dense emulsions, yield when the driving exceeds a threshold. This inversion of the jamming phase diagram can now be linked to the action of frictional forces between the suspended particles. Without frictional forces the material yields and flows at high forces, with friction the material only flows for low forces. As a corollary of this inversion one finds a discontinuous and hysteretic jamming transition as well as continuous and discontinuous shear-thickening regimes. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 2:15PM |
Z19.00005: Settling of an object in a dense suspension Invited Speaker: Devaraj van der Meer Cornstarch suspensions exhibit remarkable behavior. Here, we present two surprising observations for an object settling in such a suspension: First, in the bulk of the liquid the velocity of the object oscillates around a terminal value, without damping. And secondly, near the bottom of the container the object comes to an expected full stop, but then accelerates again towards a second stop. This stop-go cycle is repeated up to seven times before the object comes to a final standstill close to the bottom. For the bulk oscillations we show that common shear thickening models cannot account for the observed phenomena and that the history of the suspension needs to be taken into account. A hysteretic model that goes beyond the traditional viscoelastic ones describes the experiments adequately, but due to its phenomenological origin lacks a solid physical interpretation. Subsequently, we propose a minimal jamming model to describe the behavior at the bottom. Finally, we will compare our experiments to other transient and steady state phenomena observed in dense suspensions and discuss them in the context of compressional and shear jamming. [Preview Abstract] |
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