Electromagnetic interference shielding effectiveness of carbon materials

Thermal stability and flame retardancy of polyurethanes

The ability to pattern materials in three dimensions is critical for several technological applications, including composites, microfluidics, photonics, and tissue engineering. Direct-write assembly allows one to design and rapidly fabricate materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in direct ink writing are reviewed with an emphasis on the push towards finer feature sizes. Opportunities and challenges associated with direct ink writing are also highlighted.

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Direct ink writing of 3D functional materials

A constitutive equation for computing particle concentration and velocity fields in concentrated monomodal suspensions is proposed that consists of two parts: a Newtonian constitutive equation in which the viscosity depends on the local particle volume fraction and a diffusion equation that accounts for shear‐induced particle migration. Particle flux expressions used to obtain the diffusion equation are derived by simple scaling arguments. Predictions are made for the particle volume fraction and velocity fields for steady Couette and Poiseuille flow, and for transient start‐up of steady shear flow in a Couette apparatus. Particle concentrations for a monomodal suspension of polymethyl methacrylate spheres in a Newtonian solvent are measured by nuclear magnetic resonance (NMR) imaging in the Couette geometry for two particle sizes and volume fractions. The predictions agree remarkably well with the measurements for both transient and steady‐state experiments as well as for different particle sizes.

A constitutive equation for concentrated suspensions that accounts for shear‐induced particle migration

Slip occurs in the flow of two-phase systems because of the displacement of the disperse phase away from solid boundaries. This arises from steric, hydrodynamic, viscoelastic and chemical forces and constraints acting on the disperse phase immediately adjacent to the walls. The enrichment of the boundary near the wall with the continuous (and usually low-viscosity) phase means that any flow of the fluid over the boundary is easier because of the lubrication effect. Because this effect is usually confined to a very narrow layer — with typical thickness of 0.1–10 μm—it so resembles the slip of solids over surfaces that it has historically been given the same terminology. The restoring force for all the forces that cause an increase in concentration is usually osmotic, and this will always limit the effective slip. In dilute systems, concentration gradients can be present over relatively large distances out from walls, giving what might be interpreted on an overall basis as a thick solvent-only layer. However, as the concentration of the system increases, the layer gets thinner and thinner because it is more difficult to create with the large reverse osmotic force present. However, the enormous increase in the bulk viscosity with increase in concentration means that although thinner, the layer becomes, paradoxically, even more important. Slip manifests itself in such a way that viscosity measured in different size geometries gives different answers if calculated the normal way — in particular the apparent viscosity decreases with decrease in geometry size (e.g. tube radius). Also, in single flow curves unexpected lower Newtonian plateaus are sometimes seen, with an apparent yield stress at even lower stresses. Sudden breaks in the flow curve can also be seen. Large particles as the disperse phase (remember flocs are large particles), with a large dependence of viscosity on the concentration of the dispersed phase are the circumstances which can give slip, especially if coupled with smooth walls and small flow dimensions. The effect is usually greatest at low speeds/flow rates. When the viscometer walls and particles carry like electrostatic charges and the continuous phase is electrically conducted, slip can be assumed. In many cases we need to characterise the slip effects seen in viscometers because they will also be seen in flow in smooth pipes and condults in manufacturing plants. This is usually done by relating the wall shear stress to a slip velocity using a power-law relationship. When the bulk flow has also been characterized, the flow in real situations can be calculated. To characterise slip, it is necessary to change the size of the geometry, and the results extrapolated to very large size to extract unambigouos bulk-flow and slip data respectively. A number of mathematical manipulations are necessary to retrieve these data. We can make attempts to eliminate slip by altering the physical or chemical character of the walls. This is usually done physically by roughening or profiling, but in the extreme, a vane can be used. This latter geometry has the advantage of being easy to make and clean. In either case—by extrapolation or elimination—we end up with the bulk flow properties. This is important in situations where we are trying to understand the microstructure/flow interactions.

A review of the slip (wall depletion) of polymer solutions, emulsions and particle suspensions in viscometers: its cause, character, and cure

Impact modified epoxy/montmorillonite nanocomposites: synthesis and characterization

The shear viscosity material function of a highly filled suspension consisting of a Newtonian poly(butadiene acrylonitrile acrylic acid terpolymer) matrix, PBAN, mixed with an ammonium sulfate filler at 60% by volume was studied. Both capillary and parallel disk torsional flows were employed. The rheological characterization revealed strong slip of the suspension at the walls over a broad range of shear stresses in both types of flows. The slip velocity increased approximately linearly with the shear stress. In capillary flows, above a critical shear stress, flow took place in a pluglike manner, owing to slip at the wall. The experimental findings were further elucidated to determine the slip layer thickness and the apparent shear viscosity behavior of highly filled suspensions at high shear stress at the wall values. It was concluded that the slip effects dominate the flow of highly filled suspensions and the true flow and deformation characteristics of the highly filled suspensions may be overshadowed by slip at the walls.

http://www.hfmi.stevens-tech.edu/publications/040.PDF

Slip Effects in Capillary and Parallel Disk Torsional Flows of Highly Filled Suspensions

The rheological behavior of a very concentrated suspension (76.5 vol %), which serves as a widely used solid rocket fuel simulant, was characterized employing both torsional and capillary flows. No comprehensive studies of the rheology of concentrated suspensions have been carried out previously at such a high solids content. The suspension exhibited shear thinning over the apparent shear rate range of 30–3000 s−1. Significant slip at the wall was observed in both torsional and capillary flows with the slip velocity increasing from about 0.001 mm/s at a shear stress of 4 Pa to as high as 60 mm/s at 100 kPa. A flow visualization technique was applied for the first time to determine the wall slip velocities in torsional flow directly, to also provide the true deformation rate and feedback on yielding. The contribution of the slip of the suspension at the wall to the volumetric flow rate in capillary flow was found to increase with decreasing shear stress, giving rise to plug flow at sufficiently low shear s...

Rheological behavior of a concentrated suspension: A solid rocket fuel simulant

The rheological characterization of highly filled suspensions consisting of a Newtonian matrix (hydroxyl-terminated polybutadiene), mixed with two different sizes of aluminum powder (30% and above by volume) and two different sizes of glass beads (50% and above by volume), was performed using a parallel disk rheometer with emphasis on the wall slip phenomenon. The effects of the solid content, particle size, type of solid particle material, and temperature on slip velocity and slip layer thickness were investigated. Suspensions of small particles of aluminum (mean diameter of 5.03 μm) did not show slip at any concentration up to the maximum packing fraction. However, suspensions of the other particles exhibited slip at the wall, at concentrations close to their maximum packing fraction. In these suspensions, the slip velocity increased linearly with the shear stress, and at constant shear stress, the slip velocity increased with increasing temperature. The slip layer thickness increased proportionally with increasing size of the particles for the glass beads. Up to a certain value of (filler content/maximum packing fraction), ϕ/ϕm, the slip layer thickness divided by the particle diameter, δ/DP, was 0, but it suddenly increased and reached a value that was independent of ϕ/ϕm and the temperature. On average, the ratio of δ/DP was 0.071 for aluminum and 0.037 for glass beads. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 515–522, 1998

Slip velocity and slip layer thickness in flow of concentrated suspensions

Composite film production based on cotton stalk xylan was studied, and the mechanical and physical properties of the films formed were investigated. Xylan and lignin were separated from cellulose by alkali extraction and, then, lignin was removed using ethanol washing. Self-supporting continuous films could not be produced using pure cotton stalk xylan. However, film formation was achieved using 8-14% (w/w) xylan without complete removal of lignin during xylan isolation. Keeping about 1% lignin in xylan (w/w) was determined to be sufficient for film formation. Films were produced by casting the film-forming solutions, followed by solvent evaporation in a temperature (20 degrees C) and relative humidity (40%) controlled environment. The elastic modulus and hypothetical coating strength of the films obtained by using 8% xylan were significantly different from the ones containing 10-14% xylan. The water vapor transfer rates (WVTR) decreased with increasing xylan concentration, which made the films thicker. The glycerol addition as an additional plasticizer resulting in more stretchable films having higher WVTR and lower water solubility values. As a result, film production was successfully achieved from xylan, which was extracted from an agricultural waste (cotton stalk), and the film-forming effect of lignin on pure xylan has been demonstrated.

Ulku Yilmazer

Papers

Impact modified epoxy/montmorillonite nanocomposites: synthesis and characterization

Slip Effects in Capillary and Parallel Disk Torsional Flows of Highly Filled Suspensions

Rheological behavior of a concentrated suspension: A solid rocket fuel simulant

Slip velocity and slip layer thickness in flow of concentrated suspensions

Production and characterization of films from cotton stalk xylan.