Meso-length Scales Research Articles

On-surface chemical dynamics of monolayer, bilayer, and many-layered graphene surfaces probed with supersonic beam scattering and STM imaging.

We have developed the capability to elucidate interfacial reaction dynamics using an arguably unique combination of supersonic molecular beams combined with in situ STM visualization. These capabilities have been implemented in order to reveal the complex spatiotemporal correlations that govern the oxidation of graphitic systems spanning atomic-, nano-, and meso-length scales. In this study, the 3 nm periodic moiré pattern of monolayer and bilayer graphene on Ru(0001) provides a diverse palette of potential scattering and binding sites at the interface for ground state atomic oxygen. We resolve the site-specificity of atomic oxygen placement on the moiré lattice for both monolayer and bilayer graphene on Ru(0001) with atomic resolution. Angle- and energy-controlled scattering of O(3P) on these interfaces reveals an incisive side-by-side comparison of preferential reactivity of the monolayer surface compared to a more free-standing bilayer graphene ruthenium interface. Morphologically dependent reactivity of many layered graphene (HOPG) and monolayer graphene on Ru(0001) reveal anisotropic on-surface reactivity dependent on the presence of proximal reacted sites or local regions. The kinetics of on-surface oxidation are additionally shown to influence the morphology of surface products by varying the temperature of the interface and flux of reactant species. Such correlations are important in chemisorption, catalysis, materials oxidation and erosion, and film processing-and tunable moiré templated adsorption is a route to well-ordered self-assembled 2D materials for use in next-generation platforms for quantum devices and catalysis. Taken together, these results highlight a new direction in the examination of interfacial reaction dynamics where incident beam kinetic energy and angle of incidence can be used as reaction control parameters, with outcomes such as site-specific reactivity, changes for overall time-evolving mechanisms, and the relative importance of non-adiabatic channels in adsorption all linked to the on-surface fate of chemisorbed species.

Micromechanical Models for FDM 3D-Printed Polymers: A Review.

Due to its large number of advantages compared to traditional subtractive manufacturing techniques, additive manufacturing (AM) has gained increasing attention and popularity. Among the most common AM techniques is fused filament fabrication (FFF), usually referred to by its trademarked name: fused deposition modeling (FDM). This is the most efficient technique for manufacturing physical three-dimensional thermoplastics, such that FDM machines are nowadays the most common. Regardless of the 3D-printing methodology, AM techniques involve layer-by-layer deposition. Generally, this layer-wise process introduces anisotropy into the produced parts. The manufacturing procedure creates parts possessing heterogeneities at the micro (usually up to 1 mm) and meso (mm to cm) length scales, such as voids and pores, whose size, shape, and spatial distribution are mainly influenced by the so-called printing process parameters. Therefore, it is crucial to investigate their influence on the mechanical properties of FDM 3D-printed parts. This review starts with the identification of the printing process parameters that are considered to affect the micromechanical composition of FDM 3D-printed polymers. In what follows, their (negative) influence is attributed to characteristic mechanical properties. The remainder of this work reviews the state of the art in geometrical, numerical, and experimental analyses of FDM-printed parts. Finally, conclusions are drawn for each of the aforementioned analyses in view of microstructural modeling.

Size dependent creep deformation of elastically constrained compliant metallic joints

The present study proposes a novel method to predict the size dependent creep behavior of bi-material joints formed by constraining a creep compliant metal, such as Sn, having varied thickness ranging from 1.4 mm to 170 µm, between stiff elastic substrates, such as Cu. A dramatic reduction in the secondary or minimum creep rate was observed with decrease in joint thickness. Finite element (FE) analysis using continuum formulations attributed this strengthening to the geometric constraints imposed by Cu, which increases the triaxiality in the joints and hence reduces the effective stress. While FE results were in close agreement with experiment in thick joints, it significantly overpredicted the creep rate of miniature joints. Further, orientation imaging revealed that microstructure varied with length scale, from bulk Sn with multiple grains to miniature joints having a few grains. The additional strengthening was captured using dislocation-based crystal plasticity (CP) creep modeling of Sn-Cu joints by incorporating this microstructural length scale, in addition to the geometric constraints. CP simulations revealed that orientation anisotropy of Sn and the constraints imposed by substrates on dislocation motion lead to higher strength in the miniature joints. Using the insights from FE and CP modeling a unified length scale sensitive model, incorporating both geometric or continuum and microstructural factors, was developed that can accurately predict the creep response of the joints of macro and meso length scale.

Creep Micromechanics in Meso-Length Scale Samples

This study focuses on the creep response of meso-length scale systems, whose smallest dimensions lie in the range of a few hundred micrometers to a few millimeters. Here, uniaxial creep experiments were performed on commercially pure Al polycrystals using samples having the smallest (or, characteristic sample) dimension of 500 μm to 7 mm in the “five”-power law regime. Compared to the interior, the near-surface region developed creep compliant dislocation substructures, marked by lower dislocation density and larger subgrain size. Furthermore, the extent of this surface-affected region (SAR) inversely varied with applied stress and was limited by the grain size. As SAR became comparable to the characteristic sample dimension (CSD), the creep response of the material was significantly affected by the “weaker” surface, thereby reducing the overall creep resistance. This also resulted in a decrease in stress exponent with a decrease in CSD. The differential creep resistance of the surface and the interior leads to load-shedding between the “weak” surface and the “strong” interior. A microstructure-sensitive iso-strain composite model quantifying this load-shedding effect was formulated to explain the observed creep response across various length scales. The critical insights into the creep micro-mechanics obtained in this study, therefore, seamlessly unify the power-law creep response at large and small length scales.

Probing interfacial dynamics of water in confined nanoporous systems by NMRD

The confined dynamics of water molecules inside a pore involves an intermittence between adsorption steps near the interface and surface diffusion and excursions in the pore network. Depending on the strength of the interaction in the layer(s) close to the surface and the dynamical confinement of the distal bulk liquid, exchange dynamics can vary significantly. The average time spent in the surface proximal region (also called the adsorption layer) between a first entry and a consecutive exit allows estimating the level of ‘nanowettablity’ of water. As shown in several seminal works, NMRD is an efficient experimental method to follow such intermittent dynamics close to an interface. In this paper, the intermittent dynamics of a confined fluid inside nanoporous materials is discussed. Special attention is devoted to the interplay between bulk diffusion, adsorption and surface diffusion on curved pore interfaces. Considering the nano or meso length scale confinement of the pore network, an analytical model for calculating the inter-dipolar spin–lattice relaxation dispersion curves is proposed. In the low-frequency regime (50 KHz–100 MHz), this model is successfully compared with numerical simulations performed using a 3D-off lattice reconstruction of Vycor glass. Comparison with experimental data available in the literature is finally discussed.

Integral properties of turbulent-kinetic-energy production and dissipation in turbulent wall-bounded flows

Turbulent-kinetic-energy (TKE) production $\mathscr{P}_{k}=R_{12}(\unicode[STIX]{x2202}U/\unicode[STIX]{x2202}y)$ and TKE dissipation $\mathscr{E}_{k}=\unicode[STIX]{x1D708}\langle (\unicode[STIX]{x2202}u_{i}/x_{k})(\unicode[STIX]{x2202}u_{i}/x_{k})\rangle$ are important quantities in the understanding and modelling of turbulent wall-bounded flows. Here $U$ is the mean velocity in the streamwise direction, $u_{i}$ or $u,v,w$ are the velocity fluctuation in the streamwise $x$- direction, wall-normal $y$- direction, and spanwise $z$-direction, respectively; $\unicode[STIX]{x1D708}$ is the kinematic viscosity; $R_{12}=-\langle uv\rangle$ is the kinematic Reynolds shear stress. Angle brackets denote Reynolds averaging. This paper investigates the integral properties of TKE production and dissipation in turbulent wall-bounded flows, including turbulent channel flows, turbulent pipe flows and zero-pressure-gradient turbulent boundary layer flows (ZPG TBL). The main findings of this work are as follows. (i) The global integral of TKE production is predicted by the RD identity derived by Renard & Deck (J. Fluid Mech., vol. 790, 2016, pp. 339–367) as $\int _{0}^{\unicode[STIX]{x1D6FF}}\mathscr{P}_{k}\,\text{d}y=U_{b}u_{\unicode[STIX]{x1D70F}}^{2}-\int _{0}^{\unicode[STIX]{x1D6FF}}\unicode[STIX]{x1D708}(\unicode[STIX]{x2202}U/\unicode[STIX]{x2202}y)^{2}\,\text{d}y$ for channel flows, where $U_{b}$ is the bulk mean velocity, $u_{\unicode[STIX]{x1D70F}}$ is the friction velocity and $\unicode[STIX]{x1D6FF}$ is the channel half-height. Using inner scaling, the identity for the global integral of the TKE production in channel flows is $\int _{0}^{\unicode[STIX]{x1D6FF}^{+}}\mathscr{P}_{k}^{+}\text{d}y^{+}=U_{b}^{+}-\int _{0}^{\unicode[STIX]{x1D6FF}^{+}}(\unicode[STIX]{x2202}U^{+}/\unicode[STIX]{x2202}y^{+})^{2}\,\text{d}y^{+}$. In the present work, superscript $+$ denotes inner scaling. At sufficiently high Reynolds number, the global integral of the TKE production in turbulent channel flows can be approximated as $\int _{0}^{\unicode[STIX]{x1D6FF}^{+}}\mathscr{P}_{k}^{+}\,\text{d}y^{+}\approx U_{b}^{+}-9.13$. (ii) At sufficiently high Reynolds number, the integrals of TKE production and dissipation are equally partitioned around the peak Reynolds shear stress location $y_{m}:\,\int _{0}^{y_{m}}\mathscr{P}_{k}\,\text{d}y\approx \int _{y_{m}}^{\unicode[STIX]{x1D6FF}}\mathscr{P}_{k}\,\text{d}y$ and $\int _{0}^{y_{m}}\mathscr{E}_{k}\,\text{d}y\approx \int _{y_{m}}^{\unicode[STIX]{x1D6FF}}\mathscr{E}_{k}\,\text{d}y$. (iii) The integral of the TKE production ${\mathcal{I}}_{\mathscr{P}_{k}}(y)=\int _{0}^{y}\mathscr{P}_{k}\,\text{d}y$ and the integral of the TKE dissipation ${\mathcal{I}}_{\mathscr{E}_{k}}(y)=\int _{0}^{y}\mathscr{E}_{k}\,\text{d}y$ exhibit a logarithmic-like layer similar to that of the mean streamwise velocity as, for example, ${\mathcal{I}}_{\mathscr{P}_{k}}^{+}(y^{+})\approx (1/\unicode[STIX]{x1D705})\ln (y^{+})+C_{\mathscr{P}}$ and ${\mathcal{I}}_{\mathscr{E}_{k}}^{+}(y^{+})\approx (1/\unicode[STIX]{x1D705})\ln (y^{+})+C_{\mathscr{E}}$, where $\unicode[STIX]{x1D705}$ is the von Kármán constant, $C_{\mathscr{P}}$ and $C_{\mathscr{E}}$ are addititve constants. The logarithmic-like scaling of the global integral of TKE production and dissipation, the equal partition of the integrals of TKE production and dissipation around the peak Reynolds shear stress location $y_{m}$ and the logarithmic-like layer in the integral of TKE production and dissipation are intimately related. It is known that the peak Reynolds shear stress location $y_{m}$ scales with a meso-length scale $l_{m}=\sqrt{\unicode[STIX]{x1D6FF}\unicode[STIX]{x1D708}/u_{\unicode[STIX]{x1D70F}}}$. The equal partition of the integral of the TKE production and dissipation around $y_{m}$ underlines the important role of the meso-length scale $l_{m}$ in the dynamics of turbulent wall-bounded flows.

Pulsed laser remelting of A384 aluminum, part I: Measuring homogeneity and wear resistance

The objective of this study is to understand how pulsed laser remelting of aluminum alloy A384 redistributes its surface compositions, and affects its hardness and wear resistance. As a result of each laser pulse, the material experiences rapid heating (melting) and cooling (solidification), which can change the microstructure and degree of chemical homogeneity significantly. These changes in microstructure would manifest themselves as changes in mechanical properties. The proeutectic silicon precipitates found in as-cast A384 melt and disperse into the aluminum matrix during pulsed laser remelting, and do not have sufficient time to grow into large precipitates due to the rapid cooling rate. The result is a more homogeneous surface. The homogeneity is investigated by scanning electron microscope/energy dispersive spectroscopy and two-dimensional power spectral density analysis. Nanoindentation and wear tests at micro- and meso-length scales are applied to identify the effect of element redistribution on hardness and wear resistance. Compared to the base material, the hardness of the pulsed laser remelted surface becomes more uniform, which is an expected consequence of elemental homogenization. The remelted surface exhibits a higher hardness compared with the matrix of the base material, and homogenization is found to improve the wear resistance at both the micro- and meso-length scale.

Polycrystalline micropillars by a novel 3-D printing method and their behavior under compressive loads

We present an entirely new method of bottoms-up fabrication of polycrystalline micropillars using direct printing and sintering of nanoparticles in 3D and study their behavior under compression for different microstructures. The pillars showed brittle behavior with higher effective modulus for small grain sizes with high porosity, while highly ductile behavior with a lower effective modulus and larger grain sizes but low porosity. These unusual trends are explained by a porosity model. The results point to a novel method of fabricating micropillars with different microstructures to study fundamental materials science of polycrystalline materials at micro to meso-length scales.

Nano- and Mesoscale Ion and Water Transport in Perfluorosulfonic-Acid Membranes

High performance of polymer-electrolyte fuel cells requires rapid cation conduction through perflurosulfonic-acid (PFSA) membranes. Consequently, understanding water and aqueous cation transport in aqueous nanoscale domains (nano length scale) and connectivity and tortuosity of the microscale domains (meso length scales) is critical to optimizing PFSA membrane design. Due to transport coupling, membrane ionic conduction depends not only on electrostatic potential gradients but also on water content gradients (i.e. electro-osmosis). In this work, we model coupled cation and water transport in PFSA membranes at the nano- and mesoscale and predict macroscopic properties. We adopt a mean-field local-density theory for cation and water chemical potentials to predict transport in the nanoscale PFSA aqueous domains. Cation-polymer electrostatic interactions and solvation energy are included. Concentrated solution transport theory (Stefan-Maxwell formulation) models diffusion and specifically accounts for dielectric friction induced by the strong electrostatic fields of charged polymer groups. To model the mesoscale, a resistor network that includes Stefan-Maxwell coupling quantifies transport along the segments of the collection of aqueous domains in PFSA membranes. The nanoscale model parameterizes the properties of each segment of the resistor network. With this methodology, we decouple the influences of nano- and mesoscale resistances on macroscopic water and cation transport and electro-osmosis. At the nanoscale there is coupling between water and cation transport from both diffusional drag and electrostatic-driven convection. Electrostatic and pressure forces on the membrane are countered by immobilization of the polymer matrix. The models show the nature of electro-osmosis depends on water content and the vapor and electrostatic potential boundary conditions of the membrane. This work provides a compelling approach to elucidate the multiscale resistances to water and cation transport in PFSA membranes.

Engineered coiled-coil protein microfibers.

The fabrication of de novo proteins able to self-assemble on the nano- to meso-length scales is critical in the development of protein-based biomaterials in nanotechnology and medicine. Here we report the design and characterization of a protein engineered coiled-coil that not only assembles into microfibers, but also can bind hydrophobic small molecules. Under ambient conditions, the protein forms fibers with nanoscale structure possessing large aspect ratios formed by bundles of α-helical homopentameric assemblies, which further assemble into mesoscale fibers in the presence of curcumin through aggregation. Surprisingly, these biosynthesized fibers are able to form in conditions of remarkably low concentrations. Unlike previously designed coiled-coil fibers, these engineered protein microfibers can bind the small molecule curcumin throughout the assembly, serving as a depot for encapsulation and delivery of other chemical agents within protein-based 3D microenvironments.

Fracture Energy of Graphite from Microstructure-informed Lattice Model

Graphite remains a key structural material in the nuclear industry, the integrity assessment of which in demanding reactor environments is critical for safe operation of plant. Fracture of graphite is preceded by growth and coalescence of distributed micro-cracks within a process zone, classifying it as a quasi-brittle material alongside cement-based and ceramic materials. The evolution of a micro-crack population to failure is well represented by discrete lattice models, e.g. (Wang and Mora 2008). Here, a recently developed 3D lattice (Jivkov and Yates 2012), with elastic spring elements and brittle-damage behaviour is used to generate microstructure representative models of two graphite grades at a representative meso length scale. Micro-cracks are represented by spring failures and the macroscopic damage results from their collective behaviour. Presented results capture a transition from graceful, plastic-like failure at lower porosities, with energy dissipation via micro-cracking, to glass-like behaviour with negligible energy dissipation at higher porosities. The results are in good agreement with experimental data. Thus, the proposed methodology can calculate fracture energy from the stress-strain curve, or formulate cohesive and damage evolution laws for continuum models, based exclusively on microstructural features.

Layer-by-Layer Enabled Nanomaterials for Chemical Sensing and Energy Conversion

The layer-by-layer (LbL) technique is a wet chemical method for the assembly of ultrathin films, with thicknesses up to 100 nm. This method is based on the successive transfer of molecular layers to a solid substrate that is dipped into cationic and anionic solutions in an alternating fashion. The adsorption is mainly driven by electrostatic interactions so that many molecular and nanomaterial systems can be engineered under this method. Moreover, it is inexpensive, can be easily performed, and does not demand sophisticated equipment or clean rooms. The most explored use of the LbL technique is to build up molecular devices for chemical sensing and energy conversion. Both applications require ultrathin films where specific elements must be organized with high control of thickness and spatial distribution, preferably in the nanolength and mesolength scales. In chemical sensors, the LbL technique is employed to assemble specific sensoactive materials such as conjugated polymers, enzymes, and immunological elements onto appropriated electrodes. Molecular recognition events are thus transduced by the assembled sensoactive layer. In energy-conversion devices, the LbL technique can be employed to fabricate different device’s parts including electrodes, active layers, and auxiliary layers. In both applications, the devices’ performance can be fully modulated and improved by simply varying film thickness and molecular architecture. The present review article highlights the main features of the LbL technique and provides a brief description of different (bio)chemical sensors, solar cells, and organic light-emitting diodes enabled by the LbL approach.

Recent advances in the synthesis of hierarchically nanoporous zeolites

Hierarchically nanoporous zeolites possessing mesopores as well as the inherent micropores of zeolite structures are garnering attention as a solution to the diffusion limitation problem of solely microporous classical zeolites. The hierarchical structure enables zeolites to have maximum structural functions in a limited space and volume owing to the high diffusion efficiency. Hierarchical zeolites are in high demand for the development of advanced materials for applications in adsorption, separation and catalysis. Herein, recent advances in synthesis routes to hierarchically nanoporous zeolites are reviewed with their catalytic contributions. Particular emphasis is given to the recently developed synthesis method which uses surfactants that are functionalized with a zeolite-structure-directing group. This type of surfactants can direct porous structures in micro- and meso-length scales simultaneously.

Imaging magnetic flux lines with iron oxide nanoparticles using a “fossilized liquid assembly”

Directed self-assembly of nanomaterials via external fields is an attractive processing tool for industry as it is inherently inexpensive and flexible. Direct observations of this process are however challenging due to the nano and meso length scales involved. The self-assembly of magnetic nanoparticles in particular has gained much recent interest for applications ranging from biomedical imaging and targeted cancer therapy to ferrofluid mechanical damping devices, that rely on the state of aggregation and alignment of the nanoparticles. We utilize an oil–water platform to directly observe directed self-assembly of magnetic nanoparticles that are field ordered into two-dimensional mesostructures through the fossilized liquid assembly (FLA) method. Our system consisted of polymer-coated iron-oxide nanoparticles (25 nm) which were assembled in the vicinity of the interface between a crosslinkable hydrophobic monomer (UV-polymerizable) oil, and water through the use of external magnetic fields, and then cured with UV light. This flash curing system effectively provides a snapshot of the assembly process and allows for direct visualization of assemblies through the use of both atomic force and optical microscopy. In this study, entire magnetic flux field lines in various geometrical configurations were successfully modelled and mapped out by the magnetic nanoparticles, both in-plane and in perpendicular orientations utilizing FLA. The assemblies showed strong directional selectivity and alignment with the flux field lines and provided evidence of strong dipole interactions which partially caused aggregate sedimentation.

In situ and time-resolved ultra small-angle neutron scattering observation on growing poly(methyl methacrylate)-block-polystyrene via reversible addition–fragmentation chain transfer living radical polymerization

Reversible addition–fragmentation chain transfer (RAFT) living radical polymerization of poly(methyl methacrylate)-block-polystyrene (PMMA-b-PS) was investigated by a combined method of gel permeation chromatography (GPC) and in situ and time-resolved ultra small-angle neutron scattering (tr-USANS) measurements. GPC enables us to examine a growing single molecule as a function of polymerization time, with respect to monomer conversion, molecular weight (Mn) and polydispersity index (Mw/Mn) of PMMA-b-PS. On the other hand, tr-USANS, observing in meso-length scales from nm to µm, reveals polymerization-induced molecular self-assembly, such as microphase separation by PMMA-b-PS or macrophase separation between PMMA-b-PS and homo-polystyrene (by-product). By combining these two experimental methods, we elucidated that RAFT living polymerization was retarded by micro- and macrophase separations.

Effects of Shearing Flow with Inertia on Monolayer Mesoscale Structure

In an experimental flow system capable of imparting a well-controlled shear-rate distribution with inertia to a monolayer consisting of coexisting phases, we have studied the resulting phase morphology and domain fragmentation. These evolve on distinct time scales: the viscous time associated with the viscosity in the bulk and the Marangoni stress and the fragmentation/relaxation time associated with the phase morphology. A relationship between the microstructure (line tension) and macroflow (shear rate) determining the meso length scale of the coexisting phase domains has been deduced from dimensional analysis and was found to correlate well with the quantitative experimental observations.

Quantifying transport within a porous medium over a hierarchy of length scales

Magnetic resonance techniques are used to probe transport within a porous medium over length scales of microns to centimeters. In particular, the apparent discrepancy between estimates of dispersion within porous media determined by pulsed field gradient magnetic resonance techniques and a conventional elution analysis is addressed. The model porous medium considered is a packed bed of height and internal diameter 22.5 and 16.8mm, respectively, packed with highly porous cross-linked dextran particles approximately 50μm in diameter. Experiments were performed for Peclet numbers in the range 1<Pe<31. First, a nonspatially resolved displacement encoding Alternating Pulsed Field Gradient Stimulated Echo Nuclear Magnetic Resonance (APGSTE NMR) measurement was used to yield estimates of bed porosity (0.898±0.004), mobile phase volume fraction (0.29±0.02), intraparticle diffusion coefficient [(2.8±0.2)×10−10m2s−1], and characteristic time, Te, for exchange between the intra- and interparticle pore space (∼300ms). The value of porosity was in excellent agreement with that obtained by elution analysis. However, values of the axial dispersion coefficient obtained using the two approaches did not agree well. For example, at Pe=1.1, the dispersion coefficients measured by APGTSE NMR and elution analysis were (1.6±0.1)×10−9m2s−1 and (1.8±0.2)×10−8m2s−1, respectively. These results suggest that whilst the micro-/mesolength scale properties of the porous medium are well characterized using the APGSTE NMR measurement, the technique is unable to probe the millimeter length scales in the bed over which heterogeneities in the flow may exist and therefore contribute significantly to the macroscopic dispersion characteristic of the bed, as determined by elution analysis. This is confirmed by demonstrating that the contribution of mechanical mixing to dispersion within the porous medium extends to the longest time scales studied (>6Te). To identify the dominant influences on the macroscopic dispersion characteristics of the porous medium, magnetic resonance flow velocity images within the packed bed were acquired. Numerical reconstructions of the residence time distribution of the fluid within the bed using these data yielded a value of the dispersion coefficient of (0.8±0.4)×10−8m2s−1, in far better agreement with the elution analysis, thereby demonstrating that it is the millimeter-scale heterogeneity in the flow field within the bed that is the dominant contribution to the macroscopic dispersion. Extension of the model to incorporate the effect of maldistribution of the input pulse further improves agreement with the elution analysis.

The challenge of biominerals to simulations

Biomaterials are hierarchical systems whose structure and properties represent a major challenge to simulation. We briefly discuss the remarkable experimental data now available both on the structure, formation and properties of biominerals. In the light of this we discuss current attempts to simulate biomaterials at atomic and meso length scales and timescales and the range of physical effects that are important in producing biomaterials. We emphasise the importance of obtaining robust forcefields for these systems and the sensitivity of simulations to the forcefields used. We conclude by suggesting future directions for the field and remaining problems to be solved.

Mesoscaling of Reynolds Shear Stress in Turbulent Channel and Pipe Flows.

Experimental and numerical data of the Reynolds shear stress in turbulent channel and pipe flows under a mesonormalization are presented. The mesolength scale associated with this normalization is intermediate to the traditional inner and outer lengths. Justification for the mesoscales is provided by a direct analysis of the mean momentum equation. Specifically, the mesonormalization is revealed through a rescaling that appropriately reflects the physics of an internal mesolayer within which a balance breaking, and subsequent balance exchange of terms in the mean momentum equation takes place. Direct numerical simulation and experimental data are examined and shown to be in good agreement with the new scaling, supporting the new theory. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Protein-Based Thermoplastic Elastomers

Investigations of high molecular weight recombinant protein triblock copolymers demonstrate unique opportunities to systematically modify material microstructure on both nano- and meso-length scales in a manner not been previously demonstrated for protein polymer systems. Significantly, through the biosynthesis of BAB-type copolymers containing flanking, plastic-like end blocks and an elastomeric midblock, virtually cross-linked protein-based materials were generated that exhibit tunable properties in a manner completely analogous to synthetic thermoplastic elastomers. Through the rational choice of processing conditions that control meso- and nanoscale structure, changes of greater than 3 orders of magnitude in Young's modulus (0.03−35 MPa) and 5-fold in elongation to break (250−1300%) were observed. Extensibility of this range or magnitude has not been previously reported for virtually cross-linked copolymers that have been produced by either chemical or biosynthetic approaches. We anticipate that these versatile protein-based thermoplastic elastomers will find applications as novel scaffolds for tissue engineering and as new biomaterials for controlled drug release and cell encapsulation.