Ultra-high Shear Research Articles

Rheological Properties of Coating Colors Containing Biobased Latex

This paper focuses on the understanding of basic properties of water-swollen crosslinked starch nanoparticles as a function of crosslink density. The extent of their water swelling is decreased with increasing particle crosslink density and solid concentration and vice versa. This study elucidates the unique rheological properties of starch nanoparticle dispersions and paper coating formulations in comparison with water-soluble cooked starch and synthetic latex counterparts. These rheological studies extend over many decades of shear rates, using several different rheometers. Low shear viscosities were obtained using a Cannon-Fenske viscometer and a TA AR-2000 stress rheometer with double concentric cylinder geometry. Intermediate shear rate rheology was evaluated with a Hercules rheometer. High shear rates were studied with ACAV A2 ultra-high shear capillary and slit rheometers. Unlike conventional cooked and soluble starch solutions, starch nanoparticle latex dispersions are colloids that consist of internally crosslinked particles. With increasing intra-particle crosslink density these biobased colloids have been found to behave much like petroleum based synthetic latex colloids. However, at ultra-high shear their rheological properties are relatively more shear thinning compared to hard particles, including synthetic latex and pigment particles, which exhibit shear-thickening and dilatancy. The implications of the rheological data on high-speed coater runnability are discussed.

Gas-Enhanced Ultra-High Shear Mixing: A Concept and Applications

The processes of mixing, homogenizing, and deagglomeration are of paramount importance in many industries for modifying properties of liquids or liquid-based dispersions at room temperature and treatment of molten or semi-molten alloys at high temperatures, prior to their solidification. To implement treatments, a variety of technologies based on mechanical, electromagnetic, and ultrasonic principles are used commercially or tested at the laboratory scale. In a large number of techniques, especially those tailored toward metallurgical applications, the vital role is played by cavitation, generation of gas bubbles, and their interaction with the melt. This paper describes a novel concept exploring an integration of gas injection into the shear zone with ultra-high shear mixing. As revealed via experiments with a prototype of the cylindrical rotor–stator apparatus and transparent media, gases injected radially through the high-speed rotor generate highly refined bubbles of high concentration directly in the shear zone of the mixer. It is believed that an interaction of large volume of fine gas bubbles with the liquid, superimposed on ultra-high shear, will enhance mixing capabilities and cause superior refining and homogenizing of the liquids or solid–liquid slurries, thus allowing their effective property modification.

Fracture-resistant thin-film metallic glass: Ultra-high plasticity at room temperature

We report the first example of room-temperature rubber-like deformation in thin-film metallic glasses (TFMGs), 260-nm-thick Zr60Cu24Al11Ni5 layers, under ultra-high shear strain. The TFMGs were deposited, with no external heating, on Zr-based bulk metallic glass (BMG) and Si(001) substrates by rf magnetron sputtering in a 3 mTorr Ar plasma. Cross-sectional transmission electron microscopy (XTEM) analyses and nanoindentation results reveal that the TFMGs undergo an incredibly large shear strain, estimated to be ∼4000%, during fatigue tests, and thickness reductions of up to 61.5%, with no shear-banding or cracking, during extreme nanoindentation experiments extending through the film and into the substrate. TFMG/BMG samples also exhibit film/substrate diffusion bonding during deformation as shown by high-resolution XTEM.

Heat transfer modeling in asymmetrical sheet rolling of aluminium alloys with ultra high shear strain

Asymmetrical sheet rolling is a method of severe plastic deformation (SPD) for production of aluminium alloys with UFG structure. Prediction of sheet temperature during SPD is important. The temperature of sheet is changed due to the conversion of mechanical work into heat through sliding on contact surfaces and high shear strain. Paper presents the results of FEM simulation of the effect of contact friction, rolling speed and rolls speed ratio on the heating of aluminium sheets during asymmetrical rolling.

Toward Zero Micro/Macro-Scale Wear Using Periodic Nano-Layered Coatings.

Wear is an important phenomenon that affects the efficiency and life of all moving machines. In this regard, extensive efforts have been devoted to achieve the lowest possible wear in sliding systems. With the advent of novel materials in recent years, technology is moving toward realization of zero wear. Here, we report on the development of new functional coatings comprising periodically stacked nanolayers of amorphous carbon and cobalt that are extremely wear resistant at the micro and macro scale. Because of their unique structure, these coatings simultaneously provide high elasticity and ultrahigh shear strength. As a result, almost zero wear was observed even after one million sliding cycles without any lubrication. The wear rate was reduced by 8-10-fold compared with the best previously reported data on extremely low wear materials.

Highly oriented microstructures and surface mechanical properties of polypropylene (PP) molded by ultra-high shear rate

Raman spectroscopy and wide-angle X-ray diffraction (WAXD) were used to evaluate microstructures, i.e. molecular orientation, crystalline orientation and the relative amount of β-phase crystal (K), near the surface of injection molded polypropylene (PP). It was found that highly ordered structure near the surface (0–30 μm) was formed when ultra-high injection speed (1000 mm/s or γ˙ = 1670s−1) was utilized. Subsequently, increase of relative amount of β-phase crystal near the surface was achieved. The relationship between the microstructures and surface mechanical properties measured by micro-cutting method and progressive load scratch tests was investigated. The highly oriented PP showed higher surface mechanical properties due to the increase in degree of molecular and crystalline orientation. The increased scratch properties were considered to be also affected by higher β-phase crystal content.

Endothelial retention and phenotype on carbonized cardiovascular implant surfaces

Heart valve disease is an increasing clinical burden for which there is no effective treatment outside of prosthetic replacement. Over the last 20 years, clinicians have increasingly preferred the use of biological prosthetics to mechanical valves despite their superior durability because of the lifelong anticoagulation therapy that is required. Mechanical valve surface engineering has largely focused on being as non-thrombogenic as possible, but despite decades of iteration has had insufficient impact on the anticoagulation burden. In this study, we systematically evaluate the potential for endothelialization of the pyrolytic carbon surface used in mechanical valves. We compared adsorbed adhesion ligand type (collagen I, fibronectin, laminin, and purified adhesion domain fragments GFOGER and FN7-10) and concentration on endothelial adhesion rates and adhesion strength on Medtronic-Hall prosthetic valve surfaces. Regardless of ligand type or concentration, endothelial adhesion strengthening was insufficient for their intended ultra-high shear stress environment. We then hypothesized that microfabricated trenches would reduce shear stress to tolerable levels while maintaining endothelial access to the flow stream, thereby promoting a confluent and anticoagulant endothelial monolayer. Computational fluid dynamics simulations predicted an empirical relationship of channel width, depth, and spacing that would maintain interior surface shear stress within tolerable levels. Endothelial cells seeded to confluence in these channels retained a confluent monolayer when exposed to 600 dyn/cm2 shear stress for 48 h regardless of applied adhesive ligand. Furthermore, sheared EC expressed a mature anti-coagulant profile, including endothelial nitric oxide synthase (eNOS), VE-cadherin, and significantly downregulated plasminogen activator inhibitor-1 (PAI-1). As a final test, channeled pyrolytic carbon surfaces with confluent EC reduced human platelet adhesion 1000-fold over pyrolytic carbon alone. These results advance a promising biohybrid approach to enable active moderation of local coagulative response in mechanical heart valves, which could significantly extend the utility of this important treatment for heart valve disease.

Structural Basis for Unique Hierarchical Cylindrites Induced by Ultrahigh Shear Gradient in Single Natural Fiber Reinforced Poly(lactic acid) Green Composites

A local shear flow field was feasibly generated by pulling the ramie fiber in single fiber reinforced poly(lactic acid) (PLA) composites. This was featured by an ultrahigh shear gradient with a maximum shear rate up to 1500 s(-1), a level comparable to that frequently occurring during the practical polymer processing. To distinguish shear-induced self-nucleation and ramie fiber-induced heterogeneous nucleation, the shear history was classified by pulling the fiber for 5 s (pulled sample) and pulling out the fiber during 10 s (pulled-out sample), while the static fiber-induced crystallization was carried out as the counterpart. As a result of the ultrahigh shear gradient, the combination of primary shear-induced nucleation in the central region and secondary nucleation in the outer layer assembled the unique hierarchical superstructures. By comparing the architectural configurations of interphases formed in the static, pulled, and pulled-out samples, it was shown that the hierarchical cylindrites underwent the process of self-nucleation driven by the applied shear flow, very different from the formation of fiber-induced transcrystallinity (TC) triggered by the heterogeneous nucleating sites at the static fiber surface. The twisting of transcrystallized lamellae may take place due to the spatial hindrance induced by the incredibly dense nuclei under the intense shearing flow, as observed in the synchrotron X-ray diffraction patterns. The influence of chain characteristics on the crystalline morphology was further explored by adding a small amount of poly(ethylene glycol) (PEG) to enhance the molecular mobility of PLA. It was of interest to find that the existence of PEG not only facilitated the growth rates of TC and cylindrites but also improved the preferential orientation of PLA chains and thus expanded the ordered regions. We unearthed lamellar units that were composed of rich fibrillar extended chain crystals (diameter of 50-80 nm). These results are of importance to shed light on tailoring crystalline morphology for natural fibers reinforced green composite materials. Of immense practical significance, too, is the crystalline evolution that has been tracked in the simple model penetrated with an ultrahigh shear gradient, which researchers have so far been unable to replicate during the practical melt processing, such as extrusion and injection molding.

Tumbling and Deformation of Isolated Polymer Chains in Shearing Flow

Using Brownian dynamics simulations of polymer chains over a wide range of resolution, we find universal scaling laws for polymer coil dimensions and tumbling time. At high shear rates γ̇, the coil thickness in the gradient direction becomes independent of chain length, scaling as NK0γ̇–1/4, where NK denotes the numbers of Kuhn steps, and the tumbling time scales as NKγ̇–3/4, correcting scaling laws presented in prior studies. We find this to be a consequence of the formation of loops whose length is limited by the time required to stretch them and derive scaling laws from a balance of convection and diffusion of monomers. We find that, in the absence of hydrodynamic interaction (HI) and excluded volume (EV), for wormlike chains, the shrinkage in chain stretch observed at ultrahigh shear rates is pushed out to arbitrarily high shear rates if the chain is resolved increasingly finely below the persistence length. Finally, scaling laws in the presence of excluded volume and hydrodynamic interactions are derived that are expected to be valid for long chains at high shear rates.

Hydrodynamic Friction Reduction in a MAC–Hexadecane Lubricated MEMS Contact

Recent research has shown that hydrodynamic lubrication is an effective means of reducing friction in high sliding micro-electromechanical systems (MEMS). At high speeds, however, such lubrication can lead to increased friction due to viscous drag. This article describes a series of hydrodynamic tests on a silicon MEMS contact lubricated with a blend of hexadecane and a multiply-alkylated cyclopentane (MAC). Results show that the presence of the MAC reduces hydrodynamic friction compared with neat hexadecane. Such behaviour is contrary to conventional hydrodynamic theory, since the viscosity of the MAC—a mixture of di- and tri-(2-octyldodecyl)-cyclopentane—is significantly higher than that of neat hexadecane. This effect increases with MAC concentration up to an optimum value of 3 wt%, where the hydrodynamic friction coefficient at 15,000 rpm is reduced from 0.5 to 0.3. Above this concentration, friction begins to rise due to the overriding effect of increasing viscosity. The viscosity of the blended lubricant increased monotonically with MAC concentration, when measured using both a Stabinger and an ultra-high shear viscometer. In addition to this, no reduction in friction was observed when a squalane–hexadecane blend of equal viscosity was tested. This suggests that some property of the MAC–hexadecane lubricant, other than its viscosity, is influencing hydrodynamic lubrication. A tentative explanation for this behaviour is that the MAC induces the liquid to slip, rather than shear, close to the silicon surfaces. This hypothesis is supported by the fact that the friction reducing ability of the MAC blend was inhibited by the inclusion of octadecylamine—a substance known to form films on silicon surfaces. Furthermore, the MAC reduces friction in the mixed regime, in a manner suggesting that the formation of a viscous boundary layer. This unusual behaviour may have useful implications for reducing hydrodynamic friction in liquid-lubricated MEMS devices.

New Method of Measuring Permanent Viscosity Loss of Polymer-Containing Lubricants

An ultrahigh shear rate viscometer (USV) was used to measure the viscosity of polymer solutions. It was found that some polymer solutions in base oil, including those used as engine oil viscosity modifiers, show permanent viscosity loss when subjected to very high shear rates above 106 s−1. The USV was modified to automatically carry out a series of viscosity measurements on the same test lubricant sample. This enabled the accumulation of permanent viscosity loss to be measured over successive strain cycles. As expected, permanent viscosity loss increased with both strain rate and molecular weight. When carried out at 5 × 106 s−1 and 100°C, the test was more severe than the Kurt Orbahn test because samples of lubricants subjected to the latter underwent further shear thinning in the USV. The USV test appears to be a rapid and convenient way to quantify the permanent viscosity loss of polymer-containing lubricants for engine use, and a protocol to assess permanent viscosity loss (PVL) and permanent shear stability index (PSSI) based on viscosity measurements at 106 s−1 before and after shear thinning is outlined. The study also shows that it is important to take into account possible permanent viscosity loss when measuring the viscosity of polymer solutions in very high shear rate viscometers such as the USV. This can be done by minimizing the amount of shear to which the lubricant is subjected or by taking successive measurements and subtracting the permanent viscosity loss taking place in each of the first few strain rate cycles.

Application of urea-intercalated kaolinite for paper coating

Kaolinite was successfully intercalated by urea in a static and sealed condition at 95 °C for 3.5 h. The urea-intercalated kaolinite was applied for paper coating. The high shear rate rheologies were used to study the flowing properties of them and to stimulate the realistic coating paper at ultra high shear rate. The structure, morphology, chemical constitution, particle size distribution and rheological properties of the samples were measured by XRD, SEM, ICP-MS, particle size analyzer and by rheometer, respectively. The particle size of urea-intercalated kaolinite became finer with the growth of intercalation ratio. The rheological properties of slurries were changed from pseudo plastic behavior to complex flow and the solid content of slurries at the viscosity of 500 mPa·s increased from 70.84 % to 74.09 % due to intercalation of urea. The urea-intercalated kaolinite improved slightly the properties of papers, while the better rheology could enhance the process of coating paper with ultra high speed.

Rheological Studies of Microfibrillar Cellulose Water Dispersions

The rheological behaviour of microfibrillar cellulose (MFC) water dispersions has been investigated. For the first time a detailed study of shear dependent viscosity at low and high shear rate is presented. A peculiar time dependent behaviour measured in the area between 0 and 1000 s−1 was identified. The study shows a hysteresis loop in the shear rate–viscosity relationship at low shear rate. Additional time dependent shear rate measurements were performed, and a hypothesis for a mechanism of interaction and formation of a fibrils network was suggested. Higher temperature caused lower viscosity values, and this difference was increased at higher shear rate values. Oscillatory measurements showed how the closeness of the fibrils helps network creations. Data obtained from high shear measurements showed that 1% MFC water dispersions had a dilatant behaviour at ultra high shear rates with values increasing from 120 to 300 Pa s in the area between 180,000 and 330,000 s−1 .

Coesite eclogite facies metamorphic tectonite: Evidence for ultrahigh pressure deformation

An ultrahigh pressure ductile shear melange crops out on the beach of Yangkou Bay near Qingdao City. The melange is composed of weakly deformed blocks in a highly ductilly flow mylonites. Ultrahigh pressure metamorphic (UHPM) tectonite includes strongly deformed eclogite and mylonitized eclogite. Coesite occurs in the tectonite as both interstitial mineral and inclusions in garnet and omphacite, indicating that the deformation took place in the stability field of coesite (800–850°C, >30 GPa) in the upper mantle. Coesite is rounded or short prismatic grains with undulatory extinction, and often fractured, suggesting brittle deformation. Garnet is also characterized by brittle fractures and sometimes necked and slightly elongated. Omphacite is elongated, with long axis preferred orientation. Undulatory extinction, subgrains and dynamically recrystallized grains suggest plastic flow of omphacite. Ultrahigh pressure metamorphic tectonite was probably formed in the ductile shear zone during the early stage of exhumation of the ultrahigh pressure metamorphic rocks. Its kinematic indicators point to the transport direction of the UHPM slab during the early stage of exhumation.

Friction-transfer of polymeric materials using ultra-high shear rates

Frictional wear of polymeric materials is usually considered as a process damaging the polymeric material. It is known that friction at polymer surfaces increases the temperature of the polymeric material by several tens of degrees [1]. Tabor published some very fundamental considerations on the molecular and atomic processes in friction and wear [2, 3]. In most of the investigations until now wear is considered as a damaging process but in 1991 Wittmann et al. published a paper [4], in which they describe the formation of thin oriented films by a frictional process. The oriented ultrathin PTFE films resulting from a frictional wear process were used as a substrate for epitaxially orienting low and high moleculer mass linear organic molecules (liquid crystals etc) [5], and also for physical vapor deposited metals [6]. In their method they use a sliding speed of about 1 mm/s. As ultra-thin polymeric films play a major role in many modern technologies (polymeric semiconductors etc.), their preparation is very important. Considering that a relatively low sliding speed was used in the previous work of the frictional transfer processes [7–12], we have designed an experimental setup with about three orders of magnitude higher sliding speed. This may give us the opportunity to vary the experimental parameters like heat production caused by the friction, shear rates and so on over a much larger scale. As shown in Fig. 1, an experimental setup is described, in which shear rates (γ ) of about 108 s−1 in the polymer rods at the interface to the substrate have been performed. The apparatus consists of a piston which is accelerated

Numerical search for morphologies providing ultra high elastic stiffness in filled rubbers

We have numerically studied the transverse elastic behavior of a unidirectional composite comprising non-overlapping silica fibers dispersed in a rubber matrix. Some composite morphologies that provided an ultra high transverse shear modulus at rather moderate silica loadings were identified. For these morphologies, predicted elastic stiffening levels were in agreement with those measured at low strains in carbon black and silica filled rubbers, leading one to surmise that such elastic stiffening may also play an important role in the low strain mechanical responses of actual carbon black or silica filled rubbers.

The influence of impeller type on mean drop size and drop size distribution in an agitated vessel

In most work on agitated liquid–liquid dispersions, Rushton turbines have been used. Here, mean drop size and drop size distributions are reported for six different impellers covering 3 generic types over a range of mean specific energy dissipation rates. Both viscous and non-viscous dispersed phases have been used at concentrations by volume of 1 and 5%. It has been found that at the same mean specific energy dissipation rate, low power number impellers (whether of the so-called “ultra-high shear” or “high flow” type) all produced similar sized drops at equilibrium which were much smaller than those found with two “high shear”, high power number impellers, i.e., the standard Rushton turbine and another six-blade disc impeller. By considering the energy dissipation rate to be confined to the impeller swept volume, these equilibrium drop size could be approximately correlated. The low power number impellers also achieved that equilibrium more rapidly and the drop size distributions in the dispersions produced by them were narrower than those formed when agitated by the Rushton turbine and the other six-blade disc impeller. However, further analysis of the flow in the impeller region and the inclusion of advanced coalescence models would appear to be required in order to enhance the interpretation of these results.