Microstructure Packing Research Articles

Yielding behaviour of chemically treated Pseudomonas fluorescens biofilms

The mechanics of biofilms are intrinsically shaped by their physicochemical environment. By understanding the influence of the extracellular matrix composition, pH and elevated levels of cationic species on the biofilm rheology, novel living materials with tuned properties can be formulated. In this study, we examine the role of a chaotropic agent (urea), two divalent cations and distilled deionized water on the nonlinear viscoelasticity of a model biofilm Pseudomonas fluorescens. The structural breakdown of each biofilm is quantified using tools of non-linear rheology. Our findings reveal that urea induced a softening response, and displayed strain overshoots comparable to distilled deionized water, without altering the microstructural packing fraction and macroscale morphology. The absorption of divalent ferrous and calcium cations into the biofilm matrix resulted in stiffening and a reduction in normalized elastic energy dissipation, accompanied by macroscale morphological wrinkling and moderate increases in the packing fraction. Notably, ferrous ions induced a predominance of rate dependent yielding, whereas the calcium ions resulted in equal contribution from both rate and strain dependent yielding and structural breakdown of the biofilms. Together, these results indicate that strain rate increasingly becomes an important factor controlling biofilm fluidity with cation-induced biofilm stiffening. The finding can help inform effective biofilm removal protocols and in development of bio-inks for additive manufacturing of biofilm derived materials.

Cavitation properties of rendering mortars with micro silica addition

Micro-silica is a highly efficient mineral additive whose role is reflected in improvements of microstructure packing, strength and durability of non-shaped composite building materials such as concrete and mortar. A comparative study of performances of rendering mortars with different quantities of micro silica was conducted. The experimental program included production of reference mortar based on Portland cement and quartz sand (CM) and three mortars with 5, 10, and 15 % addition of micro silica (SCM-5, SCM-10, and SCM-15). The effect that micro silica addition has on the thermal behavior and mechanical properties of mortars was discussed. Hydration mechanisms and thermally induced reactions were studied at temperatures ranging from ambient to 1100 ?C by differential thermal analysis. The results were supported by X-ray diffraction analysis. The cementing efficiency of micro silica was assessed by cavitation erosion test. The changes in the morphology of mortar samples prior and upon cavitation testing were monitored by means of the scanning electron microscope imagining. It was found that 5 % of superfine micro silica (SCM-5 mortar) has positive effects on mechanical strengths (15 % increase in compressive strength) due to microstructure densification arising from the successive filling of voids by the micro silica. Addition of micro silica also improved the cavitation erosion resistance in comparison with reference cement mortar (SCM-5 showed cavitation velocity as low as 0.09 mg/min). This qualifies mortars with micro silica addition as building materials which can be safely employed in potential hydro-demolition environment.

A critical examination of the influence of material characteristics and extruder geometry on 3D printing of cementitious binders

Extrusion pressure-displacement tests on cementitious pastes with multiple starting materials and differing particle packing, subjected to different extrusion geometries are reported. The steady state extrusion pressure (equivalent to extrusion yield stress) and the deadzone length (static zone of material buildup at die entry) are determined from the pressure-displacement response to characterize the extrudability and/or printability. A unique “geometric ratio” is used to account for the effects of barrel-die geometry on paste extrusion. The ratio of extrusion yield stress (25–75 kPa) to the measured shear yield stress (50–300 Pa), the deadzone length, and the measured-to-designed filament volume at a given flow rate (print speed) are found to remain relatively invariant with geometric ratio for mixtures with high degree of microstructural packing, and thus better extrudability and shape stability. This paper marries materials- and process-related issues and, could pave the way for test methods for 3D printable mixture qualification.

Dissecting the role of crosstalk between glioblastoma subpopulations in tumor cell spreading

Glioblastoma (GBM) is a highly infiltrative brain cancer, which is thus difficult to operate. GBM cells frequently harbor Epidermal Growth Factor Receptor amplification (EGFRwt) and/or activating mutation (EGFRvIII), generating at least two different cellular subpopulations within the tumor. We examined the relationship between the diffusive architectures of GBM tumors and the paracrine interactions between those subpopulations. Our aim was to shed light on what drives GBM cells to reach large cell–cell distances, and whether this characteristic can be manipulated. We established a methodology that quantifies the infiltration abilities of cancer cells through computation of cell–cell separation distance distributions in 3D. We found that aggressive EGFRvIII cells modulate the migration and infiltrative properties of EGFRwt cells. EGFRvIII cells secrete HGF and IL6, leading to enhanced activity of Src protein in EGFRwt cells, and rendering EGFRwt cells higher velocity and augmented ability to spread. Src inhibitor, dasatinib, at low non-toxic concentrations, reduced the infiltrative properties of EGFRvIII/EGFRwt neurospheres. Furthermore, dasatinib treatment induced compact multicellular microstructure packing of EGFRvIII/EGFRwt cells, impairing their ability to spread. Prevention of cellular infiltration or induction of compact microstructures may assist the detection of GBM tumors and tumor remnants in the brains and improve their surgical removal.

Synthesis and characterization of 3D-printable geopolymeric foams for thermally efficient building envelope materials

Synthesis and characterization of 3D-printable foamed fly ash-based geopolymer matrices for thermal insulation is the focus of this paper. A surfactant-based foaming process, multi-step mixing that ensures foam jamming transition and thus a dry foam, and microstructural packing to ensure adequate skeletal density are implemented to develop foamed suspensions amenable to 3D-printing. The foamed suspensions show lower yield stress with increasing surfactant contents, especially above the foam jamming transition. The mixtures demonstrate adequate extrudability, shape retention, and buildability. The geopolymeric foams show porosities ranging from 55 to 75% and bulk densities from 0.6 to 1.0 g/cm3, and these properties are similar irrespective of whether the mixtures are extruded or conventionally cast. The thermal conductivities of the foamed matrices range from 0.15 to 0.25 W/m-K. It is shown that designed architectures that minimize heat transfer can be printed using foamed matrices to obtain sandwich wall panels with thermal insulation properties comparable to or better than those of currently available insulated concrete wall panels. This positions 3D-printing as a strategy to develop composite systems with previously unattainable thermal performance.

Material design of economical ultra-high performance concrete (UHPC) and evaluation of their properties

Coupling of separate optimizations of binder and aggregate phases to produce sustainable and economical ultra high-performance concrete (UHPC) mixtures is reported. Binders containing commonly available cement replacement materials with a (mass-based) cement replacement level between 30% and 50% are selected using microstructural packing- and rheology-based criteria. These binders are used along with aggregate sizes/amounts selected based on maximum packing density predicted by a compressible packing model to produce UHPC mixtures having a compressive strength > 150 MPa. Three coarse aggregate sizes (maximum nominal size of 6.25 mm, 4.75 mm, and 2.36 mm) and two fine aggregate sizes (d50 of 0.6 mm and 0.2 mm) are used in the optimization process, which is implemented in a computer program. Packing considerations relevant for polydisperse mixtures and the presence of fibers are incorporated. The high compressive and flexural strengths of these mixtures coupled with very high resistance to moisture and ionic transport validates the material design procedure. A preliminary cost analysis is also presented to demonstrate that UHPC mixtures can be proportioned at a fraction of the cost of proprietary mixtures using a fundamental materials science-based design approach.

Effect of vacuum and Focused Ion Beam generated heat on fracture properties of hydrated cement paste

The paper focuses on quantifying of vacuum and heat influence as unavoidable effects that appear during preparation and in-situ monitoring of micromechanical performance of cement pastes. Experimental tests at micrometer scale employ microscopic techniques of Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB). High vacuum is applied to samples during SEM-FIB procedures causing dessication and densification of primary cement paste constituents, the CSH gels. Collapse of pores and microstructural packing leads to their local stiffening and change in their fracture properties compared to usual partially saturated conditions of atmospheric pressures. The effect of vacuum is quantified for individual paste constituents in terms of their elastic moduli, tensile strengths and fracture energies measured on 15–20 μm long cantilever micro-beams inside SEM chamber. It was found that application of vacuum in SEM increases elastic moduli of inner and outer products by ∼30%, tensile strength rises 2.3–2.5 times.The effect of local heating due to ionic interactions during FIB milling is studied by means of micromechanical measurements on micro-beams and finite element (FE) numerical model. It is shown that high energy milling (30 kV, 30 nA) causes substantial microstructural and subsequent mechanical changes leading to further stiffening and tensile strength increase in the micrometer scale. The effect originates from phase changes caused by elevated temperatures under the ion beam that can be locally very high (thousands of K according to the simplified FE model). The paper also reports micromechanical response received by low energy milling for which microstructural changes due to increased temperature are restricted to very small volumes and can be assumed to be negligible with respect to the micro-beam dimensions.

Insights into material design, extrusion rheology, and properties of 3D-printable alkali-activated fly ash-based binders

Material design of alkali activated fly ash-based binders for extrusion-based 3D printing, the rheological responses that are influential in ensuring printability, and the properties of such binders are discussed in this paper. Fly ash is supplemented with fine limestone, slag, or portland cement to provide adequate microstructural packing required for printability. The alkaline activators help reduce the yield stress and enhance the cohesiveness of the mixtures. Based on the measured shear yield stress at different times and concurrent printing of a filament, the printability window and yield stress bounds for printability, applicable for the chosen printing parameters, are established. This approach could be used for mixture qualification for extrusion-based printing. The Benbow-Bridgwater model is implemented on extrusion rheology results of pastes to determine the extrusion yield stress and wall slip shear stress, which are useful process-related parameters. It is shown that these parameters can also be related to shear and extensional rheological properties of alkali-activated pastes, thus ensuring a much-needed link between parameters related to material design and the process of extrusion. Mechanical properties and pore structure similar to those of conventionally cast mixtures are achieved.

Microstructural packing- and rheology-based binder selection and characterization for Ultra-high Performance Concrete (UHPC)

This paper reports strategies to design the binder phase of ultra-high performance concretes (UHPC) from commonly available cement replacement (fly ash, slag, microsilica, metakaolin) and fine filler (limestone) materials. A packing algorithm is used to extract the number density, mean centroidal distance, and coordination number of the microstructure. Similarly, rheological studies on the pastes provide yield stress, plastic viscosity, and min-slump spread. The selection criteria involves using the three microstructural and three rheological parameters individually or in combination to define packing and flow coefficients. The selection criteria is flexible enough to allow users modify the constraints depending on the application. In this study, only ternary and quaternary blends, with a total cement replacement of 30% by mass are selected for further characterization. The highly efficient microstructural packing in these mixtures and better workability that facilitated dispersion of particles to enhance the reactivity results in beneficial pore structure and mechanical properties.

Synthesis of CNT‐polyurethane nanocomposites using ester‐based polyols with different molecular structure: Mechanical, thermal, and electrical properties

ABSTRACTPolyurethanes (PUs) were prepared by in situ polymerization of three diisocyanate with three synthesized low cost ester‐based polyols. The effect of diisocyanate type, diol structure, and molar ratio of diisocyanate to polyol on the mechanical properties was examined and the optimum chemical structure was introduced regarding the superior mechanical properties. Also, in presence of well dispersed hydroxylated multiwalled carbon nanotubes (CNT), PU/CNT nanocomposites were synthesized and fully characterized. The results showed that PU synthesized based on 1,4‐butane diol (BDO) has the best mechanical properties and thermal stability. Also, the PU samples synthesized from 1,6‐hexamethylene diisocyanate (HDI) were more profitable than aromatic diisocyanate structures due to higher crystallinity and microstructure packing. The nanocomposite sample containing 1.5% CNT was the optimum composition for the maximum tensile strength and electrical conductivity. This result was related to the uniform dispersion and bonding of CNTs to PU chains at this composition, while aggregates were formed at higher concentration of CNTs which increased the defects and reduced the uniformity of the structure. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44567.

Favorable Molecular Orientation Enhancement in Semiconducting Polymer Assisted by Conjugated Organic Small Molecules

A bimodal texturing effect of semiconducting polymers is investigated by incorporating conjugated small molecules to significantly improve the charge transport characteristics via formation of 3D transport pathways. Solution blending of the electron‐transporting polymer, poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)), with small molecular crystals of tetrathiafulvalene and tetracyanoquinodimethane is used, and the thin film microstructures are studied using a combination of atomic force microscopy, transmission electron microscopy, 2D grazing incidence X‐ray diffraction, and surface‐sensitive near‐edge X‐ray absorption fine structure. Blended thin films show edge‐on and face‐on bimodal texture with long‐range order and microstructure packing orientation preferable for electron transport through the channel in organic field‐effect transistors, which is confirmed by high electron mobility 1.91 cm2 V−1 s−1, small contact resistance, and low energetic disorder according to temperature dependence of the field‐effect mobility. Structural changes suggest a 3D network charge transport model via lamella packing and bimodal orientation of the semiconducting polymers.

Stabilization of fluidized beds of particles magnetized by an external field: effects of particle size and field orientation

AbstractThis paper reports experimental measurements on the yield stress, the permeability to gas flow and the gas velocity at the jamming transition of gas-fluidized beds of magnetizable particles as affected by particle size and orientation and strength of an externally imposed magnetic field. Tested samples consisted of relatively monodisperse magnetite powders of $35$, $50$ and $65~\unicode[.5,0][STIXGeneral,Times]{x03BC} \mathrm{m} $ particle size. The permeability to gas flow and jamming transition velocity increase with particle size and in a specially marked way when the magnetic field is applied along the gas flow direction. The magnetic contribution to the yield stress is also particularly enhanced for co-flow magnetic fields. However, the effect of particle size on the yield stress shows a dependence on the microstructure packing as affected by particle size and orientation of the field. The magnetic yield stress increases with particle size for magnetic fields applied in the cross-flow configuration while the opposite trend is observed when the direction of the magnetic field is parallel to the gas flow. The observations reported in this paper are generally explained by the formation of chains of particles due to attractive magnetic forces between the magnetized particles and the orientation of these chains with respect to the magnetic field.

Influence of Sol-Gel Nanosilica on Hardening Processes and Physically-Mechanical Properties of Cement Paste

Active pozzolacic admixtures, such as silica fume (SF) and metakaolin, are used in modern concrete technology to obtain high performance properties. Nano-scale pozzolans helps to achieve more dense microstructural packing and more impermeable cement matrix. Nanosilica (NS) was synthesised in laboratory by Sol-gel method and added to cement/SF paste in amount 2wt.% from cement. Sol-gel nanosilica are characterised by very high specific surface (>30 m2/g). Thermal effect during cement paste hardening was investigated. Compressive strength and water absorption were tested for hardened cement paste (HCP) samples. Formation of microstructure was investigated using the method Fourier transmission infrared spectroscopy (FTIS). It may be concluded that adding Sol-gel nanosilica to cement/SF composition doesn’t give a considerable effect on final physical and mechanical properties of HCP, but significant effects on hardening processes in the initial stage of hydrating took place. The best intensification effect of hardening process refers to mix containing Sol-gel nanosilica calcined at 400°C.

Microstructural packing parameters on binary component tablets and correlation with their disintegrant profiles: [formula omitted], A. Yebra-Rodriguez 2 and A. López-Galindo 2 1 Dpto. Farmacia Tecnologia Farmacéutica, Fc. Farmacia, Univ. Granada. Campus Cartuja, s/n, 18071, Granada, Spain. 2 Instituto Andaluz de Ciencias de la Tierra. (CSIC-UGR), Spain

Microstructural packing parameters on binary component tablets and correlation with their disintegrant profiles: [formula omitted], A. Yebra-Rodriguez 2 and A. López-Galindo 2 1 Dpto. Farmacia Tecnologia Farmacéutica, Fc. Farmacia, Univ. Granada. Campus Cartuja, s/n, 18071, Granada, Spain. 2 Instituto Andaluz de Ciencias de la Tierra. (CSIC-UGR), Spain