Large Shear Rates Research Articles

Scaling Properties of Gelling Systems in Nonlinear Shear Experiments.

We study model near-critical polymer gelling systems made of gluten protein dispersions stabilized at different distances from the gel point. We impose different shear rates and follow the time evolution of the stress. For sufficiently large shear rates, an intermediate stress overshoot is measured before reaching the steady state. We evidence self-similarity of the stress overshoot as a function of the applied shear rate for samples with various distances from the gel point, which is related to the elastic energy stored by the samples, as for dense systems close to the jamming transition. In concordance with the findings for glassy and jammed systems, we also measure that the stress after flow cessation decreases as a power law with time, with a characteristic relaxation time that depends on the shear rate previously imposed. These features revealed in nonlinear rheology could be the signature of a mesoscopic dynamics, which would depend on the extent of gelation.

Seepage disturbance mechanism and interface force of cylindrical barrier in fracture

The fluid flow in the crack is disturbed by the contact area. The parallel-plate model with a circular cylindrical contact area is regarded as an idealization of the real contact fracture, and a computational fluid dynamics simulation is performed on it to analyze the interaction between the contact area and the flow around it. The size of the contact area controls the disturbance intensity to the flow velocity magnitude and the disturbance range to the velocity direction. Due to the existence of viscosity, the flow near the wall has a larger shear rate. The intermediate transition region is located between the contact area and the fracture lateral wall and does not contain the region with strong viscous friction caused by the wall. The flow velocity magnitude in the intermediate transition zone changes exponentially in space and has a peak value near the contact area. After the flow bypasses the contact area, the inertial effect caused by the increase in flow velocity magnitude in the local flow channel controls the generation and development of the low-velocity region and the asymmetric degree of the velocity direction distribution. Both the mechanical aperture and the inlet flow velocity affect the stress distribution on the surface of the contact area. The occurrence of the nonlinear flow behavior of fracture seepage and nonlinear change trend of logarithmic drag coefficient curve can be predicted with the same critical Reynolds number. These results provide a useful guide for further exploring the local flow and the surface stress distribution of the local interior geometric property in a single rough-walled fracture on the mesoscopic scale.

Decreased erythrocyte aggregation in Glenn and Fontan: univentricular circulation as a rheologic disease model.

In the Fontan palliation for single ventricle heart disease (SVHD), pulmonary blood flow is non-pulsatile/passive, low velocity, and low shear, making viscous power loss a critical determinant of cardiac output. The rheologic properties of blood in SVHD patients are essential for understanding and modulating their limited cardiac output and they have not been systematically studied. We hypothesize that viscosity is decreased in single ventricle circulation. We evaluated whole blood viscosity, red blood cell (RBC) aggregation, and RBC deformability to evaluate changes in healthy children and SVHD patients. We altered suspending media to understand cellular and plasma differences contributing to rheologic differences. Whole blood viscosity was similar between SVHD and healthy at their native hematocrits, while viscosity was lower at equivalent hematocrits for SVHD patients. RBC deformability is increased, and RBC aggregation is decreased in SVHD patients. Suspending SVHD RBCs in healthy plasma resulted in increased RBC aggregation and suspending healthy RBCs in SVHD plasma resulted in lower RBC aggregation. Hematocrit corrected blood viscosity is lower in SVHD vs. healthy due to decreased RBC aggregation and higher RBC deformability, a viscous adaptation of blood in patients whose cardiac output is dependent on minimizing viscous power loss. Patients with single ventricle circulation have decreased red blood cell aggregation and increased red blood cell deformability, both of which result in a decrease in blood viscosity across a large shear rate range. Since the unique Fontan circulation has very low-shear and low velocity flow in the pulmonary arteries, blood viscosity plays an increased role in vascular resistance, therefore this work is the first to describe a novel mechanism to target pulmonary vascular resistance as a modifiable risk factor. This is a novel, modifiable risk factor in this patient population.

Reduced‐Order Models Unravel the Joint Impact of Aperture Heterogeneity and Shear‐Thinning Rheology on Fracture‐Scale Flow Metrics

AbstractSubsurface industrial operations often make use of complex engineered fluids. In fractured media, the hydraulic behavior of a geological fracture is affected by the shear‐thinning (ST) rheology of such fluids, depending on the imposed pressure gradient. Besides, owing to the stochastic nature of heterogeneities in the fracture's local apertures, obtaining the generic behavior of a fracture under ST flow requires large statistics of fractures whose aperture fields have the same statistical properties, namely their mean value, standard deviation, correlation length Lc, and Hurst exponent (which controls the scale invariances at scales smaller than Lc). The first such Monte Carlo study was recently proposed by Lenci, Putti, et al. (2022) using a new lubrication‐based model relying on a non‐linear Reynolds equation. This model describes the flow of an Ellis fluid, with both a low shear rate quasi‐Newtonian viscosity plateau and a large shear rate power law ST behavior, as measured for engineered fluids such as biopolymer solutions (e.g., xanthan gum). Here we aim to obtain the dependence of the fracture transmissivity and its dispersion over the statistics, as well as of the flow correlation length, on fracture closure, geometry correlation length, and applied pressure gradient, over a vast volume of the parameter space, and in a simple mathematical form. Employing reduced‐order models based on the polynomial chaos expansion theory to this aim, we discuss the properties of the obtained topographies of interest in the parameter space.

Rheology of granular rafts.

Rheology of macroscopic particle-laden interfaces, called "granular rafts," has been experimentally studied in the simple shear configuration. The shear-stress relation obtained from a classical rheometer exhibits the same behavior as a Bingham fluid, and the viscosity diverges with the surface fraction according to evolutions similar to 2D suspensions. The velocity field of the particles that constitute the granular raft has been measured in the stationary state. These measurements reveal nonlocal rheology similar to dry granular materials. Close to the walls of the rheometer cell, one can observe regions of large local shear rate while in the middle of the cell a quasistatic zone exists. This flowing region, characteristic of granular matter, is described in the framework of an extended kinetic theory showing the evolution of the velocity profile with the imposed shear stress. Measuring the probability density functions of the instantaneous local shear rate, we provide evidence of a balance between positive and negative instantaneous local shear rate. This behavior is the signature of a quasistatic region inside the granular raft.

Effect of mechanical properties of red blood cells on their equilibrium states in microchannels

The equilibrium positions of red blood cells (RBCs) and their steady motions in microchannel affect the hemodynamics in vivo and microfluidic applications on a cellular scale. However, the dynamic behavior of a single RBC in three-dimensional cylindrical microchannels still needs to be classified systematically. Here, with an immersed boundary method, the phase diagrams of the profiles and positions of RBCs under equilibrium states are illustrated in a wide range of Capillary numbers. The effects of initial positions are explored as well. Numerical results present that the profiles of RBCs at equilibrium states transform from snaking, tumbling to slipper, or parachute with the increase in flow rates, and whether RBCs finally approach slipper or parachute motion under large shear rates is dependent on their initial positions. With the increase in tube diameters, the equilibrium positions of RBCs are closer to tube walls relatively. Although both the increase in membrane shear modulus and the viscosity ratio are regarded as the stiffening of RBCs, the change of membrane property does not affect the dependence of the profiles and positions of RBCs at equilibrium states on the shear rates of the flow obviously, but with the increase in viscosity ratio, RBCs move further away from the centerline of the tube associating with more asymmetric characteristics in their stable profiles. The present results not only contribute to a better understanding of the dynamic behavior and multiple profiles of single RBC in microcirculation, but also provide fundamentals in a large range of Capillary numbers for cell sorting with microfluidic devices.

Combining Molecular Dynamics and Machine Learning to Analyze Shear Thinning for Alkane and Globular Lubricants in the Low Shear Regime.

Lubricants with desirable frictional properties are important in achieving an energy-saving society. Lubricants at the interfaces of mechanical components are confined under high shear rates and pressures and behave quite differently from the bulk material. Computational approaches such as nonequilibrium molecular dynamics (NEMD) simulations have been performed to probe the molecular behavior of lubricants. However, the low-shear-velocity regions of the materials have rarely been simulated owing to the expensive calculations necessary to do so, and the molecular dynamics under shear velocities comparable with that in the experiments are not clearly understood. In this study, we performed NEMD simulations of extremely confined lubricants, i.e., two molecular layers for four types of lubricants confined in mica walls, under shear velocities from 0.001 to 1 m/s. While we confirmed shear thinning, the velocity profiles could not show the flow behavior when the shear velocity was much slower than thermal fluctuations. Therefore, we used an unsupervised machine learning approach to detect molecular movements that contribute to shear thinning. First, we extracted the simple features of molecular movements from large amounts of MD data, which were found to correlate with the effective viscosity. Subsequently, the extracted features were interpreted by examining the trajectories contributing to these features. The magnitude of diffusion corresponded to the viscosity, and the location of slips that varied depending on the spherical and chain lubricants was irrelevant. Finally, we attempted to apply a modified Stokes-Einstein relation at equilibrium to the nonequilibrium and confined systems. While systems with low shear rates obeyed the relation sufficiently, large deviations were observed under large shear rates.

Comparison of shear viscosity and normal stress measurements by rotational and on-line slit rheometers with tube model predictions

In-extruder measurements of shear viscosity and normal stresses are important as these measurement techniques allow determining the rheological state of the polymer melt at processing conditions up to high shear rates. However, validation of viscosity and normal stress data obtained by in-line slit rheometers at high shear rates is difficult due to a lack of overlap of the in-line data and the off-line measurements by rotational rheometers limited to lower shear rates. Here, shear viscosity and normal stress data measured in-line at large shear rates during extrusion and off-line at low shear rates are compared to predictions of the Doi-Edwards model and the Hierarchical Multi-Mode Molecular Stress Function (HMMSF) model using linear-viscoelastic off-line small amplitude oscillating shear data of two polystyrenes and a low-density polyethylene as input parameters. For polystyrene, the results of this investigation do not only validate the experimental data obtained by rotational as well as slit-die rheometry, but also demonstrate the agreement between experiments and models up to very high shear rates, which were not experimentally accessible earlier. The low-density polyethylene shows a more complex behaviour, which follows the HMMSF model at low shear rates, but approaches the Doi-Edwards model at high shear rates.

On the Use of a Non-Constant Non-Affine or Slip Parameter in Polymer Rheology Constitutive Modeling

Since its introduction in the late 1970s, the non-affine or slip parameter, ξ, has been routinely employed by numerous constitutive models as a constant parameter. However, the evidence seems to imply that it should be a function of polymer deformation. In the present work, we phenomenologically modify a constitutive model for the rheology of unentangled polymer melts [P. S. Stephanou et al. J. Rheol. 53, 309 (2009)] to account for a non-constant slip parameter. The revised model predictions are compared against newly accumulated rheological data for a C48 polyethylene melt obtained via direct non-equilibrium molecular dynamics simulations in shear. We find that the conformation tensor data are very well predicted; however, the predictions of the material functions are noted to deviate from the NEMD data, especially at large shear rates.

Rheological properties of konjac glucomannan composite colloids in strong shear flow affected by mesoscopic structures: Multi-scale simulation and experiment

The rheological properties of biomacromolecule colloids under large shear rates (> 103 s-1) have attracted considerable interest due to the fundamental role in spraying of macromolecular solutions, degradation of macromolecular by atomization, and high-speed transportation of colloids. However, it is difficult for common instruments to characterize the rheological property under large shear rates and investigate the effect of the dynamical cluster structure on the rheological properties. In this study, multi-scale simulations and experiments were implemented to investigate the rheological property of biomacromolecule colloids composed of konjac glucomannan and hydroxypropyl methylcellulose. The multi-scale method combined with molecular dynamics simulation and Brownian dynamics simulation was effective and accurate to analyze the dynamical behavior of clusters (~ 500 nm) for a long time (~ 2 ms). Moreover, for the first time, the cluster structure and rheological properties in the strong shear flows were predicted by multi-scale simulations, such as the breakage of polymers and decreased storage modulus at large shear rates (104 ~ 105 s−1) where the test accuracy of common instruments is limited. This study provided more understanding of the rheological property of biomacromolecule colloids, which will shed light on the application of the biomacromolecule colloids in strong shear flows.

Long-Ranged Order and Flow Alignment in Sheared p-atic Liquid Crystals.

We formulate a hydrodynamic theory of p-atic liquid crystals, namely, two-dimensional anisotropic fluids endowed with generic p-fold rotational symmetry. Our approach, based on an order parameter tensor that directly embodies the discrete rotational symmetry of p-atic phases, allows us to unveil several unknown aspects of flowing p-atics, that previous theories, characterized by O(2) rotational symmetry, could not account for. This includes the onset of long-ranged orientational order in the presence of a simple shear flow of arbitrary shear rate, as opposed to the standard quasi-long-ranged order of two-dimensional liquid crystals, and the possibility of flow alignment at large shear rates.

Non-Newtonian Rheology in Twist–Bend Nematic Liquid Crystals

A simple qualitative model has been presented to describe shear rheological behavior of the twist–bend nematic liquid crystals (NTB). It has been found that at relatively low shear rate (dot {gamma } leqslant {{dot {gamma }}_{{c1}}}) the stress tensor σ created by this shear strain, scales as sigma propto {{dot {gamma }}^{{1/2}}}. Thus, the effective viscosity decreases with the shear rate (eta propto {{dot {gamma }}^{{ - 1/2}}}) manifesting so-called shear-thinning phenomenon. At intermediate shear rate {{dot {gamma }}_{{c1}}} leqslant dot {gamma } leqslant {{dot {gamma }}_{{c2}}}, σ is almost independent of dot {gamma } (a sort of plateau), and at large shear rate (dot {gamma } geqslant {{dot {gamma }}_{{c2}}}), sigma propto dot {gamma }, and it looks like as Newtonian rheology. Within our theory the critical values of the shear rate scales as {{dot {gamma }}_{{c1}}} propto {{(tilde {eta }_{2}^{0}{text{/}}tilde {eta }_{3}^{0})}^{2}}, and {{dot {gamma }}_{{c2}}} propto {{(tilde {eta }_{2}^{0}{text{/}}tilde {eta }_{3}^{0})}^{4}}, respectively. Here tilde {eta }_{2}^{0} and tilde {eta }_{3}^{0} are bare coarse grained shear viscosity coefficients of the effective smectics equivalent to the NTB phase at large scales. The results of our work are in the agreement with recent experimental studies.

DSMC Study of Strong Shear Nonequilibrium Phenomenon in Hypersonic Knudsen-Layer Flows

A series of plane Couette flows of rarefied argon gas under different flowfield settings are simulated by the direct simulation Monte Carlo method. The relaxation process of strong shear nonequilibrium is systematically studied. The results show a qualitatively platform-shaped distribution of molecular streamwise velocity under the conditions of small wall spacing (roughly in the Knudsen layer), and large shear rate and wall speed. The analysis of this phenomenon will help understand the relaxation process of hypersonic Knudsen-layer flow and provide some references for future modelling research.

Two-dimensional hydrodynamics of a Janus particle vesicle

We develop a new model, to our knowledge, for the many-body hydrodynamics of amphiphilic Janus particles suspended in a viscous background flow. The Janus particles interact through a hydrophobic attraction potential that leads to self-assembly into bilayer structures. We adopt an efficient integral equation method for solving the screened Laplace equation for hydrophobic attraction and for solving the mobility problem for hydrodynamic interactions. The integral equation formulation accurately captures both interactions for near touched boundaries. Under a linear shear flow, we observe the tank-treading deformation in a two-dimensional vesicle made of Janus particles. The results yield measurements of intermonolayer friction, membrane permeability and, at large shear rates, membrane rupture. The simulation studies include a Janus particle vesicle in both linear and parabolic shear flows, and interactions between two Janus particle vesicles in shear and extensional flows. The hydrodynamics of the Janus particle vesicle is similar to the behaviour of an inextensible, elastic vesicle membrane with permeability.

Simulation of interacting elastic sheets in shear flow: Insights into buckling, sliding, and reassembly of graphene nanosheets in sheared liquids

In liquid-based material processing, hydrodynamic forces are known to produce severe bending deformations of two-dimensional (2D) materials such as graphene. The non-linear rotational and deformation dynamics of these atomically thin sheets is extremely sensitive to hydrodynamic particle-particle interactions. To investigate this problem, we developed a computational model of the flow dynamics of elastic sheets suspended in a linear shear flow, solving the full fluid-solid coupling problem in the two-dimensional, slender-body, Stokes flow regime. Both single and pairs of sheets in close proximity are analyzed. Despite the model being two-dimensional, the critical non-dimensional shear rate yielding single-particle buckling is comparable in order of magnitude to that reported for fully three-dimensional, disk-like sheets. For pairs of interacting sheets, hydrodynamic interactions lead either to parallel sliding or bending, depending on the value of an elasto-viscous number based on particle length. For sufficiently low bending rigidity or large shear rates, large deformations of initially stacked sheets lead to sheet reattachment after separation, unlike for the rigid case. A peeling-like dynamics where lubrication provides a viscous bonding force is observed for sheet pairs when one of the two sheets is more rigid than the other. Practical implications for graphene processing and exfoliation are discussed.

Frictional properties of the rupture surface of a carbonate rock avalanche

To investigate frictional properties of the rupture surface of the carbonate Jiweishan rock avalanche, we conducted high-velocity-friction tests on the limestone and shale samples at different shear rates. The samples showed shear- and rate-strengthening of friction at low shear rates, but transformed to shear- and rate-weakening of friction at large shear rates. The critical shear rate was deduced to be between 0.05 and 0.2 m/s in our tests. The friction coefficient reduced to an extremely low value of less than 0.1 at a shear rate of 2.1 m/s. The sliding rock mass therefore could slide out of the rupture surface at a high velocity and consequently traveled a long distance as granular debris. Two mechanisms may have reduced the frictional resistance. Recrystallized calcite nanograins and CaO nanograins produced through calcite decomposition covered and lubricated the shear surface. Furthermore, the CO2 emitted through calcite decomposition reduced the effective normal stress and therefore the friction on the surface. Our conclusions for Jiweishan rock avalanche are also useful for understanding the high mobility of other widely distributed carbonate rock avalanches.

The sliding mode and dissipative force of moving nanodroplets on smooth and striped hydrophobic surfaces

The motion of nanodroplets on hydrophobic surfaces is ubiquitous in a broad range of applications such as nanofluidic devices. The dissipative force and sliding mode are two vital factors for the utilization of moving nanodroplets. However, the kinetics of moving nanodroplets is different from millimeter-sized droplets due to their extremely small size, and the dissipative force and sliding mode of nanodroplets have not been clearly understood. In this work, the dissipative force and sliding mode of moving nanodroplets on smooth and striped hydrophobic surfaces are investigated via molecular dynamics simulation. We find that the dissipative force has a more significant effect on nanodroplets than millimeter-sized droplets, and the nanodroplets show a combination of rolling and slipping motion on the hydrophobic surfaces. We report that the dissipative force of moving nanodroplets can be divided into the viscous force and the hysteresis force, and the viscous force is the major resistance to the motion of nanodroplets. We demonstrate that the sliding mode of nanodroplets is jointly determined by the significant slippage and the enhanced viscous effect, and the large shear rate of nanodroplets can promote slipping by increasing the surface slip length. Furthermore, the effects of striped structure on the moving nanodroplets are studied. On the one hand, the striped structure increases the apparent contact angle of nanodroplets and thus reduces the dissipative force. On the other hand, the slip length of the striped surface can be lower than that of the smooth surface under the action of large Laplace pressure of nanodroplets, leading to the increment of the dissipative force and the enhancement of rolling.

Three-dimensional printing of transparent ceramics by lithography-based digital projection

Scalable, massive fabrication of complex three-dimensional (3D) structured highly transparent Nd: YAG (Y 3 Al 5 O 12 ) ceramics was achieved by a lithography-based (photopolymerization) digital light processing or digital light projection (DLP) 3D printing method. 3D-printed ceramic green bodies with a printing layer height of 25 µm and an xy resolution of 40 µm were fabricated by using a photocurable slurry with 39 vol% solid loading. The developed slurry exhibited a stable high viscosity of 80–340 Pa s over a large shear rate range. A specially designed pre-sintering treatment based on YAM (Y 4 Al 2 O 9 ) phase formation was applied to prevent cracking and bubbling formation in the DLP 3D printing approach for fabricating highly transparent Nd: YAG ceramics. This thermal treatment also reduced the average grain size, leading to a higher optical quality. Following a series of thermal treatments, the sintered transparent ceramics reached a relative density of 99.9% with a maximum in-line transmittance of 80% in the visible wavelength range. Moreover, the fabricated Nd-doped YAG exhibited 1064-nm emission under 808-nm laser excitation.

Quantification of shear viscosity and wall slip velocity of highly concentrated suspensions with non-Newtonian matrices in pressure driven flows

The rheological characterization of concentrated suspensions is complicated by the heterogeneous nature of their flow. In this contribution, the shear viscosity and wall slip velocity are quantified for highly concentrated suspensions (solid volume fractions of 0.55–0.60, D4,3 ~ 5 µm). The shear viscosity was determined using a high-pressure capillary rheometer equipped with a 3D-printed die that has a grooved surface of the internal flow channel. The wall slip velocity was then calculated from the difference between the apparent shear rates through a rough and smooth die, at identical wall shear stress. The influence of liquid phase rheology on the wall slip velocity was investigated by using different thickeners, resulting in different degrees of shear rate dependency, i.e. the flow indices varied between 0.20 and 1.00. The wall slip velocity scaled with the flow index of the liquid phase at a solid volume fraction of 0.60 and showed increasingly large deviations with decreasing solid volume fraction. It is hypothesized that these deviations are related to shear-induced migration of solids and macromolecules due to the large shear stress and shear rate gradients.

Effect of sedimentation on the rheological properties of cement pastes

Rheology has been widely used to describe the flow properties of fresh cement-based suspensions. Nevertheless, the characterization and understanding of the rheological behavior of cement-based materials have become essential with the recent introduction of novel applications like digital construction in the field of civil engineering, among others. It has been well documented that concrete and mortar may suffer stability problems (i.e. the ability of the mixture to maintain homogeneous). At the scale of cement paste, this phenomenon can still arise and affect the rheological properties of the measured paste. Therefore, inaccuracies during the measurements and incorrect interpretation may occur, resulting in erroneous conclusions. This study explores the impact of sedimentation of cement particles when characterizing the rheological properties of cement paste. The w/c employed in the pastes were 0.45 and 0.35 with the addition of superplasticizer to obtain a dispersed system. The rheological properties were measured using a coaxial cylinder configuration and followed up to 180 min after the contact between cement and water. Repeated measurements were executed with and without a homogenization procedure after each application of a measuring procedure. Comparing the results shows a clear influence of sedimentation on the evolution of rheological properties with time, and the effect is more pronounced for the mixtures with high w/c and for the procedure with the larger shear rate. Therefore, it is strongly recommended to keep a measuring procedure sufficiently short in duration.