Distinct Yield Stress Research Articles

Wood flour and kraft lignin enable air-drying of the nanocellulose-based 3D-printed structures

The predominant technique for producing 3D-printed structures of nanocellulose involves freeze-drying despite its drawbacks in terms of energy consumption and carbon footprint. This study explores the less-energy-intensive drying approach by leveraging the valorization of forest residual streams. We utilized wood flour and Kraft lignin as fillers to facilitate room-temperature drying of the nanocellulose-based 3D printed structures. Various ink formulations, integrating cellulose nanofibers, wood flour, and lignin, were tested for direct ink writing (DIW). The formulations exhibited shear-thinning behavior and distinct yield stress with rising stress levels, ensuring the effective flow of the ink during DIW. Consequently, multilayered objects were printed with high shape fidelity and precise dimensions. Lignin and wood flour prevented structural collapse upon room-temperature drying. A reduced shrinkage was observed with the addition of lignin in freeze and room temperature drying. Moreover, the room-temperature dried samples were denser and demonstrated significantly higher resistance to applied compressive force, surpassing those reported for cellulose-based 3D composites in the existing literature. Remarkably, the trade-off effects of lignin are highlighted in terms of efficient stress-distributing and micro-scale sliding, enabling better strength. Along with wood flour, it further increases thermal stability. However, lignin hinders the hierarchical porous structure, the main ion transportation channels, reducing the double-layer capacitance of the carbonized structures. Overall, the results underscore the potential of all-biobased formulations for DIW for practical applications, highlighting their enhanced mechanical properties and structural integrity via the more sustainable drying method.

Thermo-Mechanical Behavior and Strain Rate Sensitivity of 3D-Printed Polylactic Acid (PLA) below Glass Transition Temperature (Tg).

This study investigated the thermomechanical behavior of 4D-printed polylactic acid (PLA), focusing on its response to varying temperatures and strain rates in a wide range below the glass transition temperature (Tg). The material was characterized using tension, compression, and dynamic mechanical thermal analysis (DMTA), confirming PLA's strong dependency on strain rate and temperature. The glass transition temperature of 4D-printed PLA was determined to be 65 °C using a thermal analysis (DMTA). The elastic modulus changed from 1045.7 MPa in the glassy phase to 1.2 MPa in the rubber phase, showing the great shape memory potential of 4D-printed PLA. The filament tension tests revealed that the material's yield stress strongly depended on the strain rate at room temperature, with values ranging from 56 MPa to 43 MPA as the strain rate decreased. Using a commercial FDM Ultimaker printer, cylindrical compression samples were 3D-printed and then characterized under thermo-mechanical conditions. Thermo-mechanical compression tests were conducted at strain rates ranging from 0.0001 s-1 to 0.1 s-1 and at temperatures below the glass transition temperature (Tg) at 25, 37, and 50 °C. The conducted experimental tests showed that the material had distinct yield stress, strain softening, and strain hardening at very large deformations. Clear strain rate dependence was observed, particularly at quasi-static rates, with the temperature and strain rate significantly influencing PLA's mechanical properties, including yield stress. Yield stress values varied from 110 MPa at room temperature with a strain rate of 0.1 s-1 to 42 MPa at 50 °C with a strain rate of 0.0001 s-1. This study also included thermo-mechanical adiabatic tests, which revealed that higher strain rates of 0.01 s-1 and 0.1 s-1 led to self-heating due to non-dissipated generated heat. This internal heating caused additional softening at higher strain rates and lower stress values. Thermal imaging revealed temperature increases of 15 °C and 18 °C for strain rates of 0.01 s-1 and 0.1 s-1, respectively.

Consolidation characteristics of a thermally cured sand–bentonite mixture

Consolidation behavior of clays and sand–clay mixtures is an important topic in environmental geotechnics, thermal geostructures and disposal of hazardous wastes. The purpose of this paper is investigation of the changes in consolidation behavior of a sand–bentonite mixture due to the pre-curing at elevated temperatures. For this reason, 1:1 mixture of sand–bentonite was used to prepare samples 100 mm in diameter and 200 mm in height. The specimens were cured at 40, 60 and 80 °C for 1, 3 and 5 days under a constant confining pressure in a modified thermal triaxial cell. Then, consolidation behavior of the prepared samples which have been named as cured samples was investigated. The results show that the cured samples shifted from a reconstituted state to a quasi-structured one. A distinct yield stress was observed in compression curves of cured samples which was not apparent for reconstituted ones. The preconsolidation pressure of cured samples increased with increase in curing temperature and curing time. Also, the coefficient of consolidation and permeability increased as temperature increased, while both values decreased with an increase in curing time. Moreover, a number of specimens were tested under cyclic thermal paths which clearly proved change in behavior of cured samples to a structured one compared to the samples without thermal curing.

The Effect of Strain Rate and Temperature on the Mechanical Behavior of Al/Fe Interface Under Compressive Loading

Molecular dynamics (MD) is employed to simulate the mechanical response of Al/Fe interface under compression at extreme conditions of seven temperatures and four strain rates ranging between 150 K and 900 K and 5.0 × 107 s−1 and 1.0 × 1010 s−1, respectively. Stress–strain histories show two distinct yield stress points for simulations at temperatures below 500 K, which tend to merge into one as the temperature increases. Microstructural simulations show nucleation of dislocations, which occur in the bulk of the aluminum region, is associated with the first yield point. In the iron region, dislocations nucleate at the Al/Fe interface and are associated with the second yield point. The incoherent interface employed in these simulations contributes to the heterogeneous nucleation in iron by creating a defected area favorable for this nucleation from the aluminum side. MD generated data show that the two yield stresses and the consequent flow stress decrease with increasing temperature for all strain rates and fit a thermally activated model function of strain rate. The competing mechanisms between dislocation motion and phonon drag driven deformation are also simulated and modeled. The flow stress of the interface was found to fall midway between Zerilli–Armstrong models of the two materials constituting it, whereas the relaxation in Fe followed similar trend to what is reported in the literature.

Chiral crystal-like droplets displaying unidirectional rotational sliding.

The self-assembly of organic molecules into supramolecular materials with structural ordering beyond the nanometre scale is challenging. Here, we report the spontaneous self-assembly of a chiral discotic triphenylene derivative into millimetre-sized droplets. The structure of the droplets is characterized by high positional and orientational ordering and a three-dimensional integrity similar to that of single crystals. Notwithstanding, these assemblies slide when placed on a vertical substrate demonstrating their fluid nature. X-ray imaging shows that during the sliding process the internal crystal-like structure is maintained and that the droplets undergo clockwise or counterclockwise unidirectional rotation, depending on the chirality of their molecular components. Rheological measurements suggest that this rotational behaviour might result from the distinct yield stress between the (R)- and (S)-enantiomers. Overall, our findings demonstrate that molecular chirality can determine the movement direction of a supramolecular structure, thus expanding the fundamental understanding of the structure and dynamics of soft materials.

Development of multifunctional plaster using nano-TiO2 and distinct particle size cellulose fibers

The present study reports on the development of multifunctional plaster showing superior thermal insulation, ability to control the indoor RH and NOx photocatalytic degradation. The plaster contains TiO2 nanoparticles (1wt.%) and 0–4wt.% fibers of two distinct sizes: 1mm<d<2mm (Fb1-2mm) and 2mm<d<4mm (Fb2-4mm). The formulations were adjusted based on flow table and rheometer tests, aiming to assure a desirable workability. Plasters with similar workability (spread on table) showed distinct yield stress values over time and Fb(1-2mm) plays an important role on the workability control. Rheological behaviour was governed by Fb(2-4mm). Binary mixture 2.0Fb1-2mm + 2.0Fb2-4mm2·0Fb2-4mm required less water amount to show the same workability of the single formulation 4.0Fb1-2mm. However, the control of the rheological behavior over time was found difficult. The apparent porosity, water absorption and capillary index were strongly incremented with the rise of cellulose fiber concentration. Singular and binary samples present similar results and no significant differences were observed by changing the fibers size. Plaster’s buffering capacity and NOx uptake was considerably improved by using cellulose fibers, being this increment primarily governed by the fiber concentration. Formulation containing 4.0wt.% Fb(2-4mm) revealed the optimal performance.

Modelling of pastes as viscous soils – Lubricated squeeze flow

Highly filled suspensions (or pastes) present complex rheological behaviour and squeeze flow testing is used frequently for rheological characterisation. The extent to which liquid phase migration (LPM) occurs in such tests, and the influence of material extruded from between the plates, was investigated in experiments supported by detailed modelling based on soil mechanics approaches. Lubricated squeeze flow (LSF) tests were conducted on a model saturated ballotini paste prepared with a viscous Newtonian binder, at plate speeds spanning two decades. The tests were simulated using a two-dimensional (2-D) axisymmetric finite element model with adaptive remeshing to circumvent mesh distortion. The paste was modelled as a viscoplastic soil (Drucker-Prager) to capture both rate-dependent effects at high shear rates and LPM at low shear rates. Capillary pressure was applied at the evolving free surface and the plate surfaces were modelled as frictionless for simplicity. Reasonable agreement was obtained between the measured and predicted squeezing pressure profiles at the highest solids volume fraction tested (ϕs=60%). Agreement was poor at the lowest ϕs (52.5%), which was due to this paste formulation behaving as a suspension/slurry without a distinct yield stress. For the first time, the predicted squeezing pressure was resolved into components using an energy analysis. The squeezing pressure was dominated by the work required to deform the paste in the gap. This result is specific to highly viscoplastic pastes and persisted to small plate separations when most of the sample lay outside the plates. Characterisation of the yield stress from the ‘shoulder’ in the squeezing pressure profile was reasonably accurate at h/h0≥96% (9% estimated error). LPM was neither observed nor predicted at the plate speeds tested, despite the favourable pore pressure driving force, due to the high binder viscosity and the zero dilation angle in the simulations. The flow field was characterised using a novel flow mode parameter derived from the shear rate tensor. The paste was predicted to undergo pure biaxial extension between the smooth plates, and for the first time was predicted to undergo pure uniaxial extension external to the plates and (briefly) pure shear at the boundary.

Formulation and rheological evaluation of ethosome-loaded carbopol hydrogel for transdermal application

Objective: To select a suitable ethosome-loaded Carbopol hydrogel formulation, specifically tailored for transdermal application that exhibits (i) plastic flow with yield stress of approximately 50–80 Pa at low polymer concentration, (ii) relatively frequency independent elastic (G′) and viscous (G″) properties and (iii) thermal stability.Methods: Carbopol (C71, C934, C941, C971 or C974) hydrogels were prepared by dispersing Carbopol in distilled water followed neutralization by sodium hydroxide. The effects of Carbopol grade, Carbopol concentration, ethosome addition and temperature on flow (yield stress and viscosity) and viscoelastic (G′ and G″) properties of Carbopol hydrogel were evaluated. Based on the aforementioned rheological properties evaluated, suitable ethosome-loaded Carbopol hydrogel was selected. In-vitro permeation studies of diclofenac using rat skin were further conducted on ethosome-loaded Carbopol hydrogel along with diclofenac-loaded ethosomal formulation as control.Results: Based on preliminary screening, C934, C971 and C974 grades were selected and further evaluated for flow and viscoelastic properties. It was observed that ethosome-loaded C974 hydrogel at concentration of 0.50 and 0.75% w/w, respectively, demonstrated acceptable plastic flow with distinct yield stress and a frequency independent G′ and G″. Furthermore, the flow and viscoelastic properties were maintained at the 4, 25 and 32 °C. The results from in vitro skin permeation studies indicate that ethosome-loaded C974 hydrogel at 0.5% w/w polymer concentration exhibited similar skin permeation as that of ethosomal formulation.Conclusion: The results indicate that suitable rheological properties of C974 could facilitate in achieving desired skin permeation of diclofenac while acting as an efficient carrier system for ethosomal vesicles.

Rheological behavior of fiber-filled polymer melts at low shear rate. Part. I. Modeling of rheological properties

In this review the models applied for the description of non-Newtonian liquids were presented. Highly filled polymer melts showing distinct yield stress are such liquids. The effects of forces of geometrical and physical interactions among the filler particles on the properties were discussed. These interactions forces depend on the type, size and shape of filler particles and in case of fibers also on their orientation. The deformations and flow of fibers assemblies in the melt under shear stress were discussed. The equations used for description of three- or two-dimensionally oriented fibers sets are presented.

Spray and combustion characteristics of aluminized gelled fuels with an impinging jet injector

The influence of the aluminium (Al) particle content of gelled Jet A-1 based fuels on spray and combustion characteristics as well as on rheological properties was investigated. Decreasing dynamic shear viscosity values were found with increasing shear rates and increasing viscosity values with increasing Al content. Also a distinct yield stress occurs for all investigated metallized gels. The atomization behavior of the gels with a doublet like-on-like impinging jet injector under ambient conditions concerning pressure and temperature was investigated, whereas different break-up modes were observed. Also the combustion behavior of gel sprays in air under both elevated pressure and air temperature conditions is presented here.

Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG)

Uniaxial and plane strain compression experiments are conducted on amorphous poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG) over a wide range of temperatures (25–110 °C) and strain rates (.005–1.0 s −1). The stress–strain behavior of each material is presented and the results for the two materials are found to be remarkably similar over the investigated range of rates, temperatures, and strain levels. Below the glass transition temperature ( θ g=80 °C), the materials exhibit a distinct yield stress, followed by strain softening then moderate strain hardening at moderate strain levels and dramatic strain hardening at large strains. Above the glass transition temperature, the stress–strain curves exhibit the classic trends of a rubbery material during loading, albeit with a strong temperature and time dependence. Instead of a distinct yield stress, the curve transitions gradually, or rolls over, to flow. As in the sub- θ g range, this is followed by moderate strain hardening and stiffening, and subsequent dramatic hardening. The exhibition of dramatic hardening in PETG, a copolymer of PET which does not undergo strain-induced crystallization, indicates that crystallization may not be the source of the dramatic hardening and stiffening in PET and, instead molecular orientation is the primary hardening and stiffening mechanism in both PET and PETG. Indeed, it is only in cases of deformation which result in highly uniaxial network orientation that the stress–strain behavior of PET differs significantly from that of PETG, suggesting the influence of a meso-ordered structure or crystallization in these instances. During unloading, PETG exhibits extensive elastic recovery, whereas PET exhibits relatively little recovery, suggesting that crystallization occurs (or continues to develop) after active loading ceases and unloading has commenced, locking in much of the deformation in PET.

The use of Ultrasound to Enhance the Thermophilic Digestion of Waste Activated Sludge

Thermophilic anaerobic digestion at 55°C is a process which can stabilise sewage sludge chemically and achieve pasteurisation. Treatment with ultrasound has been shown to enhance the gas yields of mesophilic digestion, but there is no information about its affect on thermophilic digestion. Treatment of thickened waste activated sludge by ultrasound showed that, based on the residual turbidity and the release of soluble carbohydrate, the optimum dose was 1.5-3.0kJ g TS−1. Five-hundred ml digesters, which were fed daily, were used to compare the gas yields obtained from sonicated and unsonicated waste activated sludges. The results showed that, with a 10 day hydraulic retention time, sonication increased the gas yield by 15%. Both digested sludge and waste activated sludges which had been thickened to > 15g TSl−1, had a distinct yield stress. Sonication reduced this significantly. This suggests that sonication could be used as a pre-treatment to enhance thermophilic digestion and as a post-treatment to improve the pumping characteristics.

Microphase separation and rheology of a semicrystalline poly(ether-ester) multiblock copolymer

A microphase separation transition (MST) of a thermoplastic elastomer based on soft segments of poly(tetra methylene oxide) and hard crystalline segments of poly(tetra methylene terephthalate) has been studied by means of rheological measurements, differential scanning calorimetry (DSC), and wide-angle X-ray scattering (WAXS), showing that the MST is entirely caused by melting/crystallization, and that no separate segmental mixing/demixing transition is involved. DSC and WAXS measurements show that melting starts at 190°C, leading to crystal reorganization effects up to above 200°C, and that a gradual decrease in crystallinity occurs from below 210°C up to 224°C, above which temperature no crystals are left. Rheological measurements reveal a wide MST (207–224°C) upon heating, which coincides perfectly with the melting range. From this coincidence together with the Maxwell fluid behavior directly following the MST, it is concluded that melting leads to a one-phase liquid, and that no separate segmental mixing transition occurs. Similar results are obtained upon cooling, indicating that crystallization is the driving force for phase separation and that no separate segmental demixing step precedes crystallization. The wide MST implies a large processing window over which the rheological properties change from highly elastic, with a distinct yield stress, to normal pseudoplastic, enabling application in preparation of structured blends. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1795–1804, 1998