This thesis presents experimental measurements of the shear stresses of a fluid-particulate flow ... more This thesis presents experimental measurements of the shear stresses of a fluid-particulate flow at high Reynolds numbers as a function of the volume fraction of solids. From the shear stress measurements an effective viscosity, where the fluid-particulate flow is treated as a single fluid, is determined. This viscosity varies from the fluid viscosity when no solids are present to several orders of magnitude greater than fluid viscosity when the particles near their maximum packing state. It is the primary goal of this thesis to determine how the effective viscosity varies with the volume fraction of solids. A variety of particle sizes, shapes, and densities were obtained through the use of polystyrene, nylon, polyester, styrene acrylonitrile, and glass particles, used in configurations where the fluid density was matched and where the particles were non-neutrally buoyant. The particle sizes and shapes ranged from 3 mm round glass beads to 6.4 mm nylon to polystyrene elliptical cyli...
Current opinion in colloid & interface science, 2014
The addition of small amounts of a secondary fluid to a suspension can, through the attractive ca... more The addition of small amounts of a secondary fluid to a suspension can, through the attractive capillary force, lead to particle bridging and network formation. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. This effect can even occur when the secondary fluid wets the particles less well than the bulk fluid. These materials, so-called capillary suspensions, have been the subject of recent research studying the mechanism for network formation, the properties of these suspensions, and how the material properties can be modified. Recent work in colloidal clusters is summarized and the relationship to capillary suspensions is discussed. Capillary suspensions can also be used as a pathway for new material design and some of these applications are highlighted. Results obtained to date are summarized and central questions that remain to be answered are proposed in this review.
The behavior of liquid-solid flows varies greatly depending on fluid viscosity; particle and liqu... more The behavior of liquid-solid flows varies greatly depending on fluid viscosity; particle and liquid inertia; and collisions and near-collisions between particles. Shear stress measurements were made in a coaxial rheometer with a height to gap ratio (b∕r0) of 11.7 and gap to outer radius ratio (h∕b) of 0.166 that was specially designed to minimize the effects of secondary flows. Experiments were performed for a range of Reynolds numbers, solid fractions and ratio of particle to fluid densities. With neutrally buoyant particles, the dimensional shear stress exhibits a linear dependence on Reynolds number: the slope is monotonic but a non-linear function of the solid fraction. Though non-neutrally buoyant particles exhibit a similar linear dependence at higher Reynolds numbers, at lower values the shear stress exhibits a non-linear behavior in which the stress increases with decreasing Reynolds number due to particle settling.
ABSTRACT This paper presents experimental measurements of the rheological behavior of liquid-soli... more ABSTRACT This paper presents experimental measurements of the rheological behavior of liquid-solid mixtures at moderate Stokes and Reynolds numbers. The experiments were performed in a coaxial rheometer that was designed to minimize the effects of secondary flows. By changing the shear rate, particle size, and liquid viscosity, the Reynolds numbers based on shear rate and particle diameter ranged from 20 to 800 (Stokes numbers from 3 to 90), which is higher than examined in earlier rheometric studies. Prior studies have suggested that as the shear rate is increased, particle-particle collisions also increase resulting in a shear stress that depends non-linearly on the shear rate. However, over the range of conditions that were examined in this study, the shear stress showed a linear dependence on the shear rate. Hence, the effective relative viscosity is independent of the Reynolds and Stokes numbers and a non-linear function of the solid fraction. The present work also includes a series of rough-wall experiments that show the relative effective viscosity is also independent of the shear rate and larger than in the smooth wall experiments. In addition, measurements were made of the near-wall particle velocities, which demonstrate the presence of slip at the wall for the smooth-walled experiments. The depletion layer thickness, a region next to the walls where the solid fraction decreases, was calculated based on these measurements. The relative effective viscosities in the current work are larger than found in low-Reynolds number suspension studies but are comparable with a few granular suspension studies from which the relative effective viscosities can be inferred.
ABSTRACT We introduce a novel method to produce macroporous ceramics by capillary suspensions. Ad... more ABSTRACT We introduce a novel method to produce macroporous ceramics by capillary suspensions. Adding a small amount (~1 vol%) of an immiscible secondary phase to a low concentration (~20 vol%) suspension can increase the yield stress by several orders of magnitude. This drastic change in flow behavior is induced by the creation of a sample‐spanning particle network in the suspension controlled by capillary forces. This strong network may persist even if the primary bulk phase is removed. Accordingly, capillary suspensions can be used as a precursor for manufacturing porous materials. Here, we focus on the specific features of this universal, low‐cost processing route for porous ceramics. An Al2O3 model system is used to demonstrate how to adjust porosity and pore size. With this system, we were able to achieve open porosities higher than 60% with an average pore size below 10 μm.
ABSTRACT Adding a small amount of an immiscible fluid to a particle suspension can lead to partic... more ABSTRACT Adding a small amount of an immiscible fluid to a particle suspension can lead to particle bridging and network formation. This effect occurs both if the secondary fluid wets the particles better or worse than the bulk fluid. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. This allows stable food products to be created as shown here for starch and cocoa model suspensions. Adding small fractions of water to suspensions of starch or cocoa particles in oil increases the yield stress by several orders of magnitude. The yield stress and viscosity can be tuned in a wide range by changing the fraction of the secondary liquid or the wetting properties of the ternary particle/ fluid/fluid system. The presence of aqueous capillary bridges between cocoa particles improves the heat stability of model chocolate systems. In suspensions of starch granules that have been conditioned over water, the network induced by capillary bridges forms spontaneously and results in the same yield stress as when the water is added to the suspension of dry particles. This demonstrates, that in contrast to Pickering emulsions, the formation of capillary suspensions is an energetically driven phenomenon. Water continuous suspensions can potentially be used to design novel low fat food products. We have modified suspensions of cocoa particles in water with trace amounts of appropriate oil to achieve texture and flow properties of regular fat continuous cocoa spreads.
ABSTRACT Es hat sich gezeigt, dass in Suspensionen Kapillarkräfte zwischen der dispersen und der ... more ABSTRACT Es hat sich gezeigt, dass in Suspensionen Kapillarkräfte zwischen der dispersen und der kontinuierlichen Phase eine tragende Rolle bzgl. der rheologischen Eigenschaften spielen. Im Rahmen dieses Beitrages wird aufgezeigt, dass das rheologische Verhalten einer in der kontinuierlichen Phase dispergierten dispersen Phase deutlich durch die Zugabe einer geringen Menge (< 1 %) nicht mit der Hauptphase mischbaren Zweitphase modifiziert werden kann, was auf Kapillarkräfte zurückzuführen ist. Diesen Effekt kann man sich nun zum gezielten Einstellen von Produkteigenschaften zu Nutze machen. It has been found that in suspensions capillary forces play an essential role between the disperse phase and the continuous phase in respect of their rheological behavior. In this review it is shown, that the rheological behavior of a disperse phase, which is dispersed in a continuous phase, can be changed significantly by adding a small amount (< 1 %) of a secondary liquid that is not mixable with the bulk phase. This effect now can be used to modify product properties specifically.
This thesis presents experimental measurements of the shear stresses of a fluid-particulate flow ... more This thesis presents experimental measurements of the shear stresses of a fluid-particulate flow at high Reynolds numbers as a function of the volume fraction of solids. From the shear stress measurements an effective viscosity, where the fluid-particulate flow is treated as a single fluid, is determined. This viscosity varies from the fluid viscosity when no solids are present to several orders of magnitude greater than fluid viscosity when the particles near their maximum packing state. It is the primary goal of this thesis to determine how the effective viscosity varies with the volume fraction of solids. A variety of particle sizes, shapes, and densities were obtained through the use of polystyrene, nylon, polyester, styrene acrylonitrile, and glass particles, used in configurations where the fluid density was matched and where the particles were non-neutrally buoyant. The particle sizes and shapes ranged from 3 mm round glass beads to 6.4 mm nylon to polystyrene elliptical cyli...
Current opinion in colloid & interface science, 2014
The addition of small amounts of a secondary fluid to a suspension can, through the attractive ca... more The addition of small amounts of a secondary fluid to a suspension can, through the attractive capillary force, lead to particle bridging and network formation. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. This effect can even occur when the secondary fluid wets the particles less well than the bulk fluid. These materials, so-called capillary suspensions, have been the subject of recent research studying the mechanism for network formation, the properties of these suspensions, and how the material properties can be modified. Recent work in colloidal clusters is summarized and the relationship to capillary suspensions is discussed. Capillary suspensions can also be used as a pathway for new material design and some of these applications are highlighted. Results obtained to date are summarized and central questions that remain to be answered are proposed in this review.
The behavior of liquid-solid flows varies greatly depending on fluid viscosity; particle and liqu... more The behavior of liquid-solid flows varies greatly depending on fluid viscosity; particle and liquid inertia; and collisions and near-collisions between particles. Shear stress measurements were made in a coaxial rheometer with a height to gap ratio (b∕r0) of 11.7 and gap to outer radius ratio (h∕b) of 0.166 that was specially designed to minimize the effects of secondary flows. Experiments were performed for a range of Reynolds numbers, solid fractions and ratio of particle to fluid densities. With neutrally buoyant particles, the dimensional shear stress exhibits a linear dependence on Reynolds number: the slope is monotonic but a non-linear function of the solid fraction. Though non-neutrally buoyant particles exhibit a similar linear dependence at higher Reynolds numbers, at lower values the shear stress exhibits a non-linear behavior in which the stress increases with decreasing Reynolds number due to particle settling.
ABSTRACT This paper presents experimental measurements of the rheological behavior of liquid-soli... more ABSTRACT This paper presents experimental measurements of the rheological behavior of liquid-solid mixtures at moderate Stokes and Reynolds numbers. The experiments were performed in a coaxial rheometer that was designed to minimize the effects of secondary flows. By changing the shear rate, particle size, and liquid viscosity, the Reynolds numbers based on shear rate and particle diameter ranged from 20 to 800 (Stokes numbers from 3 to 90), which is higher than examined in earlier rheometric studies. Prior studies have suggested that as the shear rate is increased, particle-particle collisions also increase resulting in a shear stress that depends non-linearly on the shear rate. However, over the range of conditions that were examined in this study, the shear stress showed a linear dependence on the shear rate. Hence, the effective relative viscosity is independent of the Reynolds and Stokes numbers and a non-linear function of the solid fraction. The present work also includes a series of rough-wall experiments that show the relative effective viscosity is also independent of the shear rate and larger than in the smooth wall experiments. In addition, measurements were made of the near-wall particle velocities, which demonstrate the presence of slip at the wall for the smooth-walled experiments. The depletion layer thickness, a region next to the walls where the solid fraction decreases, was calculated based on these measurements. The relative effective viscosities in the current work are larger than found in low-Reynolds number suspension studies but are comparable with a few granular suspension studies from which the relative effective viscosities can be inferred.
ABSTRACT We introduce a novel method to produce macroporous ceramics by capillary suspensions. Ad... more ABSTRACT We introduce a novel method to produce macroporous ceramics by capillary suspensions. Adding a small amount (~1 vol%) of an immiscible secondary phase to a low concentration (~20 vol%) suspension can increase the yield stress by several orders of magnitude. This drastic change in flow behavior is induced by the creation of a sample‐spanning particle network in the suspension controlled by capillary forces. This strong network may persist even if the primary bulk phase is removed. Accordingly, capillary suspensions can be used as a precursor for manufacturing porous materials. Here, we focus on the specific features of this universal, low‐cost processing route for porous ceramics. An Al2O3 model system is used to demonstrate how to adjust porosity and pore size. With this system, we were able to achieve open porosities higher than 60% with an average pore size below 10 μm.
ABSTRACT Adding a small amount of an immiscible fluid to a particle suspension can lead to partic... more ABSTRACT Adding a small amount of an immiscible fluid to a particle suspension can lead to particle bridging and network formation. This effect occurs both if the secondary fluid wets the particles better or worse than the bulk fluid. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. This allows stable food products to be created as shown here for starch and cocoa model suspensions. Adding small fractions of water to suspensions of starch or cocoa particles in oil increases the yield stress by several orders of magnitude. The yield stress and viscosity can be tuned in a wide range by changing the fraction of the secondary liquid or the wetting properties of the ternary particle/ fluid/fluid system. The presence of aqueous capillary bridges between cocoa particles improves the heat stability of model chocolate systems. In suspensions of starch granules that have been conditioned over water, the network induced by capillary bridges forms spontaneously and results in the same yield stress as when the water is added to the suspension of dry particles. This demonstrates, that in contrast to Pickering emulsions, the formation of capillary suspensions is an energetically driven phenomenon. Water continuous suspensions can potentially be used to design novel low fat food products. We have modified suspensions of cocoa particles in water with trace amounts of appropriate oil to achieve texture and flow properties of regular fat continuous cocoa spreads.
ABSTRACT Es hat sich gezeigt, dass in Suspensionen Kapillarkräfte zwischen der dispersen und der ... more ABSTRACT Es hat sich gezeigt, dass in Suspensionen Kapillarkräfte zwischen der dispersen und der kontinuierlichen Phase eine tragende Rolle bzgl. der rheologischen Eigenschaften spielen. Im Rahmen dieses Beitrages wird aufgezeigt, dass das rheologische Verhalten einer in der kontinuierlichen Phase dispergierten dispersen Phase deutlich durch die Zugabe einer geringen Menge (< 1 %) nicht mit der Hauptphase mischbaren Zweitphase modifiziert werden kann, was auf Kapillarkräfte zurückzuführen ist. Diesen Effekt kann man sich nun zum gezielten Einstellen von Produkteigenschaften zu Nutze machen. It has been found that in suspensions capillary forces play an essential role between the disperse phase and the continuous phase in respect of their rheological behavior. In this review it is shown, that the rheological behavior of a disperse phase, which is dispersed in a continuous phase, can be changed significantly by adding a small amount (< 1 %) of a secondary liquid that is not mixable with the bulk phase. This effect now can be used to modify product properties specifically.
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