61 research outputs found
Capillary suspensions: Particle networks formed through the capillary force
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.Comment: Review article published in Current Opinion in Colloid & Interface
Scienc
Rheological Measurements in Liquid-Solid Flows
The behavior of liquid-solid flows varies greatly depending on fluid viscosity, particle and liquid inertia,
and collisions between particles. While particle collisions in inviscid fluids can be understood statistically,
liquid-solid flows are complicated by the fluid viscosity and forces acting on the particles (e.g. lift, drag,
added mass). These flows were first studied by Bagnold, whose investigation found two different flow
regimes: a macro-viscous regime where the shear and pressure forces are proportional to the shear rate, and
a grain-inertia regime defined by a dependance on the square of the shear rate [1, 2]. The scaling relations
he developed have been used to model and understand natural phenomena since
Structure of capillary suspensions and their versatile applications in the creation of smart materials
Rheological measurements of large particles in high shear rate flows
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
Shear Stress Measurements of Non-Spherical Particles in High Shear Rate Flows
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
- …