Abstract: We present a comprehensive study of the slip and flow of concentrated colloidal suspensions using cone-plate rheometry and simultaneous confocal imaging. In the colloidal glass regime, for smooth, nonstick walls, the solid nature of the suspension causes a transition in the rheology from Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip behavior at low stress, which is suppressed for sufficient colloid-wall attraction or colloid-scale wall roughness. Visualization shows how the slip-shear transition depends on gap size and the boundary conditions at both walls and that partial slip persist well above the yield stress. A phenomenological model, incorporating the Bingham slip law and HB bulk flow, fully accounts for the behavior. Microscopically, the Bingham law is related to a thin (subcolloidal) lubrication layer at the wall, giving rise to a characteristic dependence of slip parameters on particle size and concentration. We relate this to the suspension’s osmotic pressure and yield stress and also analyze the influence of van der Waals interaction. For the largest concentrations, we observe nonuniform flow around the yield stress, in line with recent work on bulk shear banding of concentrated pastes. We also describe residual slip in concentrated liquid suspensions, where the vanishing yield stress causes coexistence of (weak) slip and bulk shear flow for all measured rates. (C) 2012 The Society of Rheology. [http://dx.doi.org/10.1122/1.4719775]
Abstract: We report experiments on hard-sphere colloidal glasses that show a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability due to shear-concentration coupling, a mechanism previously thought unimportant in these materials. Below a characteristic shear rate γ(c) we observe increasingly nonlinear and localized velocity profiles. We attribute this to very slight concentration gradients in the unstable flow regime. A simple model accounts for both the observed increase of γ(c) with concentration, and the fluctuations in the flow.
Abstract: Concentrated particulate suspensions, commonplace in the pharmaceutical, cosmetic and food industries, display intriguing rheology. In particular, the dramatic increase in viscosity with strain rate (shear thickening and jamming), which is often observed at high-volume fractions, is of practical and fundamental importance. Yet, manufacture of these products and their subsequent dispensing often involves flow geometries substantially different from that of simple shear flow experiments. In this study, we show that the elongation and breakage of a filament of a colloidal fluid under tensile loading is closely related to the jamming transition seen in its shear rheology. However, the modified flow geometry reveals important additional effects. Using a model system with nearly hard-core interactions, we provide evidence of surprisingly strong viscoelasticity in such a colloidal fluid under tension. With high-speed photography, we also directly observe dilatancy and granulation effects, which lead to fracture above a critical elongation rate.
Abstract: We study the pressure-driven flow of concentrated colloids confined in glass microchannels at the single-particle level using fast confocal microscopy. For channel to particle size ratios 2a/D[over ] less, similar30, the flow rate of the suspended particles shows fluctuations. These turn into regular oscillations for higher confinements (2a/D[over ] approximately 20). We present evidence to link these oscillations with the relative flow of solvent and particles (permeation) and the effect of confinement on shear thickening.
Abstract: Using fast confocal microscopy we image the three-dimensional dynamics of particles in a yielded hard-sphere colloidal glass under steady shear. The structural relaxation, observed in regions with uniform shear, is nearly isotropic but is distinctly different from that of quiescent metastable colloidal fluids. The inverse relaxation time tau(-1)(alpha) and diffusion constant D, as functions of the local shear rate (gamma) over dot, show marked shear thinning with tau(-1)(alpha)proportional to D proportional to (gamma) over dot(0.8) over more than two decades in (gamma) over dot. In contrast, the global rheology of the system displays Herschel-Bulkley behavior. We discuss the possible role of large scale shear localization and other mechanisms in generating this difference.
Abstract: We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a “pasteâ€) in a square capillary using confocal microscopy. The flow consists of a “plug†in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.
Abstract: Intriguing and novel physical aspects related to the vortex flow dynamics have been recently observed in mesoscopic channel devices of a-NbGe with NbN channel edges. In this work we have systematically studied the flow properties of vortices confined in such mesoscopic channels as a function of the magnetic field history, using dc-transport and mode-locking (ML) measurements. As opposed to the field-down situation, in the field-up case a kink anomaly in the dc I-V curves is detected. The mode-locking measurements reveal that this anomaly is, in fact, a flow induced vortex slip transition: by increasing the external drive (either dc or ac) a sudden change occurs from n to n+2 moving vortex rows in the channel. The observed features can be explained in terms of an interplay between field focusing due to screening currents and a change in the predominant pinning mechanism.
Abstract: Intriguing and novel physical aspects related to the vortex flow dynamics have been recently observed in mesoscopic channel devices of a-NbGe with NbN channel edges. In this work we have studied the flow properties of such confined vortices as a function of the magnetic field history, using dc-transport and mode-locking (ML) measurements. As opposed to the field down situation, in field up case a kink anomaly in the dc I-V curves is detected. The mode-locking measurements unveil the dynamic change in the flow configurations around this anomaly: n moving rows of vortex array at low velocity changes suddenly to n + 2 rows at high velocity around the anomaly. (c) 2005 Elsevier B.V. All rights reserved.
Notes: 4th International Conferene on Vortex Matter in Nanostructured Superconductors (VORTEX IV), Iraklion, GREECE, SEP 03-09, 2005
Abstract: We study numerically and analytically the behaviour of vortex matter in artificial flow channels confined by pinned vortices in the channel edges (CEs). The critical current density J(s) for channel flow is governed by the interaction with the static vortices in the CEs. Motivated by early experiments which showed oscillations of Js on changing (in) commensurability between the channel width w and the natural vortex row spacing b(0), we study structural changes associated with (in) commensurability and their effect on Js and the dynamics. The behaviour depends crucially on the presence of disorder in the arrays in the CEs. For ordered CEs, maxima in J(s) occur at commensurability w = nb(0) (n is an integer), while for w not equal nb(0) defects along the CEs cause a vanishing J(s). For weak disorder, the sharp peaks in J(s) are reduced in height and broadened via nucleation and pinning of defects. The corresponding structures in the channels (for zero or weak disorder) are quasi-1D n row configurations, which can be adequately described by a (disordered) sine-Gordon model. For larger disorder, matching between the longitudinal vortex spacings inside and outside the channel becomes irrelevant and, for w similar or equal to nb(0), the shear current J(s) levels at similar to 30% of the value J(s)(0) for the ideal commensurate lattice. Around ‘half filling’ (w/b(0) similar or equal to n +/- 1/2), the disorder leads to new phenomena, namely stabilization and pinning of misaligned dislocations and coexistence of n and n +/- 1 rows in the channel. At sufficient disorder, these quasi-2D structures cause a maximum in J(s) around mismatch, while J(s) smoothly decreases towards matching due to annealing of the misaligned regions. Near threshold, motion inside the channel is always plastic. We study the evolution of static and dynamic structures on changing w/b(0), the relation between the J(s) modulations and transverse fluctuations in the channels and find dynamic ordering of the arrays at a velocity with a matching dependence similar to J(s). We finally compare our numerical findings at strong disorder with recent mode-locking experiments, and find good qualitative agreement.
Abstract: The flow properties of confined vortex matter driven through disordered mesoscopic channels are investigated by mode locking (ML) experiments. The observed ML effects allow us to trace the evolution of both the structure and the number of confined rows and their match to the channel width as function of magnetic field. From a detailed analysis of the ML behavior for the case of three rows we obtain (i) the pinning frequency f(p), (ii) the onset frequency f(c) for ML (proportional to ordering velocity), and (iii) the fraction L-ML/L of coherently moving three-row regions in the channel. The field dependence of these quantities shows that, at matching, where L-ML is maximum, the pinning strength is small and the ordering velocity is low, while at mismatch, where L-ML is small, both the pinning force and the ordering velocity are enhanced. Further, we find that f(c)proportional tof(p)(2), consistent with the dynamic ordering theory of Koshelev and Vinokur. The microscopic nature of the flow and the ordering phenomena will also be discussed.
Abstract: The dynamics of vortex matter confined to mesoscopic channels has been investigated by means of mode locking experiments. When vortices are coherently driven through the potential provided by static vortices pinned in the channel edges, interference between the washboard frequency of the moving vortex lattice and the frequency of the superimposed rf-drive causes (Shapiro-like) steps in the dc-I-V curves. The position of the voltage steps uniquely determines the number of moving rows in each channel. It also shows how the frustration between row spacing and channel width behaves as a function of magnetic field. Maxima in flow stress (similar toI(c)) occur at mismatch conditions. They are related to the traffic-jam-like flow impedance caused by the disorder in the edges. At higher fields, near the 2D-melting line B-m(T), the mode-locking interference characteristic for crystalline motion, strongly depends on the velocity, i.e. the applied frequency at which the vortex motion is probed. The minimum velocity at which coherent motion could be observed, diverges when the melting line is approached from below. Above the melting line interference is absent for any frequency. These observations give the first direct evidence for a dynamic phase transition of vortex matter driven through a disorder potential as predicted by Koshelev and Vinokur. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 7th International Conference on Materials and Mechanisms of Superconductive and High Temperature Superconductors, Rio de Janeiro, BRAZIL, MAY 25-30, 2003
Abstract: We investigated vortex flow in mesoscopic channels and its field history dependence using mode locking (ML) experiments. In the channel system the critical current I-c oscillates with magnetic field due to a series of structural transitions from n to n+/-1 confined rows. In presence of a combined rf and dc current, ML is observed in the dc current-voltage curve. The field dependence of the ML voltage directly yields the evolution of n with field. We observed strong differences in the I-c oscillations and the evolution of n for different field history. The data show that matching effects in the channels are governed by the effective channel width which strongly depends on screening current flowing along the channel edges. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 16th International Symposium on Superconductivity (ISS 2003), Tsukuba, JAPAN, OCT 27-29, 2003
Abstract: We have studied flow properties of vortices driven through easy flow mesoscopic channels by means of mode locking (ML) technique. We observe ML jump with large voltage broadening in a real part of rf impedance. Upon approaching the pure dc flow by reducing rf amplitude, the NIL jump is smeared out by the divergence of the voltage width. This indicates a large spread of internal frequency of driven channel vortices and lack of coherence in dc state. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 7th International Conference on Materials and Mechanisms of Superconductive and High Temperature Superconductors, Rio de Janeiro, BRAZIL, MAY 25-30, 2003
Abstract: We observed mode-locking (ML) of rf-dc driven vortex arrays in a superconducting weak pinning a-NbGe film. The ML voltage shows the expected scaling V proportional to f rootB with f the rf-frequency and B the magnetic field. For large de-velocity (corresponding to a large ML frequency), the NIL current step width exhibits a squared Bessel function dependence on the rf-amplitude as predicted for NIL of a lattice moving elastically through a random potential. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 7th International Conference on Materials and Mechanisms of Superconductive and High Temperature Superconductors, Rio de Janeiro, BRAZIL, MAY 25-30, 2003
Abstract: We study depinning of vortex chains in channels formed by static, disordered vortex arrays. Depinning is governed either by the barrier for defect nucleation or for defect motion, depending on whether the chain periodicity is commensurate or incommensurate with the surrounding arrays. We analyze the reduction of the gap between these barriers as a function of disorder. At large disorder, commensurability becomes irrelevant and the pinning force is reduced to a small fraction of the ideal shear strength of ordered channels. Implications for experiments on channel devices are discussed.
Abstract: We study dynamic melting of confined vortex matter moving in disordered, mesoscopic channels by mode-locking experiments. The dynamic melting transition, characterized by a collapse of the mode-locking effect, strongly depends on the frequency, i.e., on the average velocity of the vortices. The associated dynamic ordering velocity diverges upon approaching the equilibrium melting line T-m,T-e(B) as v(c)similar to(T-m,T-e-T)(-1). The data provide the first direct evidence for velocity dependent melting and show that the phenomenon also takes place in a system under disordered confinement.
Abstract: We study the depinning transition of a driven-chain-like system in the presence of frustration and quenched disorder. The analysis is motivated by recent transport experiments on artificial vortex-flow channels in superconducting thin films. We start with a London description of the vortices and then map the problem onto a generalized Frenkel-Kontorova model and its continuous equivalent, the sine-Gordon model. In the absence of disorder, frustration reduces the depinning threshold in the commensurate phase, which nearly vanishes in the incommensurate regime. Depinning of the driven frustrated chain occurs via unstable configurations that are localized at the boundaries of the sample and evolve into topological defects which move freely through the entire sample. In the presence of disorder, topological defects can also be generated in the bulk. Further, disorder leads to pinning of topological defects. We find that weak disorder effectively reduces the depinning threshold in the commensurate phase, but increases the threshold in the incommensurate phase.
Abstract: We investigated the driven dynamics of vortices confined to mesoscopic flow channels by means of a dc-rf interference technique. The observed mode-locking steps in the IV curves provide detailed information on how both the number of vortex rows and the lattice structure in each flow channel change with magnetic field. Minima in flow stress occur when an integer number of rows is moving coherently, while maxima appear when the incoherent motion of mixed n and n +/- 1 row configurations is predominant. Simulations show that the enhanced pinning at mismatch originates from quasistatic fault zones with misoriented edge dislocations induced by disorder in the channel edges.
Abstract: We have used a scanning superconducting quantum interference device microscope to image individual vortices near lithographically patterned surface steps in weak-pinning superconducting thin films of amorphous MoGe. The field-cooled vortex distributions are strongly influenced by the surface steps, with an enhanced vortex density along the thin side of the steps and a wide vortex-free region along the thick side of the steps. The surface steps induce orientational order that persists for many intervortex spacings away from the steps. We study the effects of surface step pinning for different magnetic-field strengths and step heights by analyzing the intervortex spacing and computing vortex correlation functions. For certain sample configurations, we are able to apply a Lorentz force to the vortices and observe an asymmetric vortex response near the surface steps. We discuss the interaction between a vortex and a surface step and consider possible mechanisms for generating the vortex distributions which we observe upon field cooling.
Abstract: We investigated the dynamics of vortex matter confined to mesoscopic channels by means of mode locking experiments. When vortices move coherently through the pinning (shear) potential provided by static vortices in the channel edges, interference between the washboard frequency of the lattice and the frequency of superimposed rf-currents causes (Shapiro-like) steps in the dc-IV curves. These steps allow to trace directly how the number of moving rows in each channel and the frustration between row spacing and channel width, varies with magnetic field. The flow stress (similar to I-c) surprisingly exhibits maxima for mismatching (defective) structures, originating from traffic-jam-like flow due to disorder in the edges. We then focus on the behavior for higher fields, approaching the 2D melting field B-m. In this regime the presence of the interference phenomenon, characteristic for crystalline motion, strongly depends on the velocity (applied frequency) at which vortices are probed. The minimum velocity to observe coherent, solid-like motion is found to diverge when the field is increased towards B-m, above which the interference is absent for any frequency. This provides the first direct evidence for a velocity dependent, dynamic phase transition of vortex matter moving through disorder, as predicted by Koshelev and Vinokur.
Notes: International Workshop on Electronic Crystals (ECRYS-2002), ST FLOUR, FRANCE, SEP 02-07, 2002
Abstract: We introduce a type of vortex entry edge barrier which controls the critical current in a perpendicular magnetic field in thin-film weak-pinning superconducting strips. Measurements of the critical current in thin-film amorphous-MoGe strips show a linear decrease with increasing magnetic field strength at low magnetic fields, and a crossover at a well-defined threshold field to an inverse power-law decay that is independent of the strip width. This behavior has not been observed previously due to bulb pinning, which only becomes dominant in our MoGe samples at high magnetic fields. To describe our results, we present calculations of the current distribution in thin superconducting strips with a finite penetration depth and negligible bulk pinning, and show that the measured critical currents in our MoGe samples correspond to a current density at the strip edge which approaches the Ginzburg-Landau depairing limit. Shape variations and defects along the strip edges influence the vortex entry conditions, leading to deviations from the ideal behavior, including offsets in the critical current maximum with respect to zero field.
Abstract: We report on experiments in which ultrathin layers of vortex matter are sheared along nanofabricated Row channels in a NbN/Nb3Ge double layer. The structure of the vortex matter inside the channels can he tuned continuously, via the applied magnetic field, from ordered, dose-packed crystalline to highly disordered, amorphous configurations. We explore the evolution of the static and dynamic response as vortex matter inside the channels is built up row by row and analyze our results in terms of an effective static and dynamic friction. From the current and field dependence of this vortex friction, together with characteristic signatures in the measured velocity fluctuations, we find that ultrathin layers containing up to six commensurate vortex rows behave similar to brittle solids, while wider layers exhibit characteristics more akin to plastic flow.
Abstract: We have studied the flux dynamics in superconducting strips patterned from both Nb and weak-pinning amorphous MoGe films using a Scanning SQUID Microscope (SSM). The unparalleled flux sensitivity of the SSM allows us to image the vortices in the strip under a variety of field and cooling conditions with single vortex resolution for low flux density. We are able to apply transport currents while imaging the strip and observe the shift of the vortex distributions due to the Lorentz force. Surface steps etched into the strips significantly alter the flux patterns and introduce asymmetry in the vortex motion under applied transport currents. Both the change in vortex line energy across the step and the screening currents which flow along the step influence the vortex distributions and flux dynamics. We are studying the relationship between the vortex distributions from the SSM images and the transport characteristics of the strips.
Notes: International Conference on Materials and Mechanisms of Superconductivity High Temperature Superconductors VI, HOUSTON, TEXAS, FEB 20-25, 2000
Abstract: We investigate vortex flow confined to mesoscopic, weak pinning channels in a NbN/Nb3Ge double layer. In the limit where the channels are sufficiently narrow, the flow behavior is profoundly affected by commensurability between the channel width and the vortex spacing. We explore both static and dynamic features of the resulting ordered and disordered vortex configurations by measuring the mean flow velocity as well as the velocity fluctuations. For fixed drive, we find distinct minima in the flow velocity, which we can associate with vortex lattices consisting of an integer number of parallel vortex rows inside a channel. These flow minima correspond to peaks in the depinning current, i.e., the static yield strength. To compare the dynamic response of the commensurate and the incommensurate configurations, we measure the low-frequency voltage noise resulting from slow rearrangements in the pinning landscape along the channel walls. (C) 2000 Elsevier Science B.V. All rights reserved.
Notes: 1st Euroconference on Vortex Matter in Superconductors at Extreme Scales and Conditions, IRAKLION, GREECE, SEP 18-24, 1999
Abstract: We study a chain of interacting vortices in a randomly distorted periodic potential. For small disorder (In)Commensurability between the chain and the potential determines the nature of depinning. In the C-phase depinning occurs via periodic interstitial/vacancy nucleation thus lowering the critical force f(c) compared to the pure case. In the I-phase, the disordered Peierls landscape pins the defects, thereby increasing f(c) with respect to the pure case. The C-I transition is smeared when the pinning force for defects exceeds the nucleation force.
Notes: International Conference on Materials and Mechanisms of Superconductivity High Temperature Superconductors VI, HOUSTON, TEXAS, FEB 20-25, 2000
Abstract: A numerical and analytical study of vortex flow behavior in weak pinning channels has revealed that, for a channel in a perfect lattice, incommensurability between channel width and the vortex lattice constant causes (point) defects in the channel with a nearly vanishing barrier J(c). The transport characteristics exhibit a crossover from low mobility (defect motion) to full flux flow (coherent motion). J(c) versus commensurability parameter is a discontinuous function with sharp peaks at integer values. Disorder in the channel edges smears the sharp C-I transitions due to defect pinning at Incommensurability. Also it lowers the Commensurate J(c) due to defect generation. These observations agree with experiments we performed on weak pinning vortex channels in strong pinning films.
Notes: International Workshop on Electronic Crystals (ECRYS-99), LA COLLE-SUR-LOUP, FRANCE, MAY 31-JUN 05, 1999
Abstract: Incommensurate easy flow channels in an otherwise perfect vortex lattice are investigated. The associated (point) defects in the lattice inside the channel cause an almost vanishing critical current, as shown by molecular dynamics simulations and a comparison with the Frenkel-Kontorova model. In addition to the normal flux flow behavior, we find a low mobility regime at small drives associated with defect motion. We treat this situation analytically for the case of a single defective vortex row. We also briefly discuss the relation to existing experiments on artificial vortex channels.
Abstract: We review recent advances in imaging the flow of concentrated suspensions, focussing on the use of confocal microscopy to obtain time-resolved information on the single-particle level in these systems. After motivating the need for quantitative (confocal) imaging in suspension rheology, we briefly describe the particles, sample environments, microscopy tools and analysis algorithms needed to perform this kind of experiment. The second part of the review focusses on microscopic aspects of the flow of concentrated ‘model’ hard-sphere-like suspensions, and the relation to non-linear rheological phenomena such as yielding, shear localization, wall slip and shear-induced ordering. We describe both Brownian and non-Brownian systems and show how quantitative imaging can improve our understanding of the connection between microscopic dynamics and bulk flow.
Abstract: Interesting flow properties are observed when a concentrated suspension of colloidal particles flows into a geometrical constriction. We present here a description of two different experimental techniques used to study the pressure driven flow of dense suspensions of micron-sized hard spheres into glass capillaries. The first one involves the analysis of the driving pressure during the flow, the other one is based on fast confocal microscopy. Technical details are given, together with a selection of preliminary results.
Notes: Summer School on Aging and the Glass Transition, Luxembourg, LUXEMBOURG, SEP 18-24, 2005
Abstract: The length and time scales accessible to optical tweezers make them an ideal tool for the examination of colloidal systems. Embedded high-refractive-index tracer particles in an index-matched hard sphere suspension provide ‘handles’ within the system to investigate the mechanical behaviour. Passive observations of the motion of a single probe particle give information about the linear response behaviour of the system, which can be linked to the macroscopic frequrency-dependent viscous and elastic moduli of the suspension. Separate ‘dragging’ experiments allow observation of a sample’s nonlinear response to an applied stress on a particle-by particle basis. Optical force measurements have given new data about the dynamics of phase transitions and particle interactions; an example in this study is the transition from liquid-like to solid-like behaviour, and the emergence of a yield stress and other effects attributable to nearest-neighbour caging effects. The forces needed to break such cages and the frequency of these cage breakign events are investigated in detail for systems close to the glass transition.
Notes: Conference on Optical Trapping and Optical Micromanipulation III, San Diego, CA, AUG 13-17, 2006
