Single-wall carbon nanotubes (SWCNTs) exhibit versatile optoelectronic properties closely linked to their structural characteristics, such as chiral angles and diameters. Given this, they are promising materials for biosensors. However, in studies investigating SWCNT-based electrochemical biosensors, raw soot has been mostly used. Soot typically contains a mixture of different chiralities, metallic compounds, and various impurities from the synthesis process. As a result, this mixture significantly limits the reproducibility and precision of SWCNT-based sensors. To ensure consistent sensor performance, we employed an aqueous two-phase extraction (ATPE) technique to purify and sort single-chirality SWCNTs-specifically, semiconducting (6,5) SWCNTs and metallic (6,6) SWCNTs. In addition, we used multiple fabrication methods to ensure that only pure-chirality SWCNTs were deposited onto the electrodes. Our findings emphasise the importance of using surfactant-free systems when investigating the influence of chirality on the electrochemical behaviour of SWCNTs. By using monochiral SWCNTs, we achieved precise control over their concentration and density, allowing us to assess their electrochemical properties accurately. Our results reveal that the adsorption-controlled process of the inner sphere redox probe occurs on (6,5) SWCNTs, while a diffusion-controlled process is observed on (6,6) SWCNTs. These findings provide valuable insights that will enhance the performance of SWCNT-based electrochemical biosensors.

Biochar synthesized from corn starch with three different amylose/amylopectin ratios were characterized and their reinforcing performance in styrene-butadiene rubber (SBR) rubber was compared to carbon black N772. Biochar samples were produced via slow pyrolysis. Additionally, 3 different activated biochar samples were created, notably (1) activation under CO2, (2) activation with steam and N2 (Steam), and (3) steam activation via pyrolysis of a biochar-water slurry (SteamT2). The properties of the fillers were examined, and all biochar samples were blended with SBR to form composites, which were tested to evaluate their cure profiles and tensile properties. The results indicate that the amylose and amylopectin ratios of the feedstock had little effect on the reinforcing performance of the composites, but composites with biochar from high amylose corn starch had longer scorch periods during curing. Physical activation treatments, however, had a significant impact on the physicochemical properties of the biochar, specifically on the porosity, carbon, oxygen, and ash content. In terms of performance in SBR, composites with biochar pyrolyzed under normal N2 conditions and biochar activated with Steam had the most consistent tensile performance with the least brittle characteristics. Conversely, composites made with biochar activated with SteamT2 were exceptionally brittle, with limited reinforcement capabilities, despite having a comparatively higher carbon content. Thus, though physical activation can increase the carbon concentration of biochar, this does not necessarily correlate to improvements in its mechanical performance in SBR. As such, other optimization techniques may be more useful to tailor biochar for application in the rubber filler industry.

Optical image of the interface formed when two slightly misaligned pillar arrays are pressed into contact. Sliding is accommodated by motion of this pattern.

This study presents a characterization of corn-based biochar materials and examines their suitability as rein-forcing fillers in styrene-butadiene rubber (SBR) composites. The specific objectives were to determine whether there were significant differences in performance, depending on (1) the part of the plant they are sourced from and (2) how refined the feedstock was prior to processing. Corn cob, corn stover and the more highly refined corn starch feedstocks were pyrolyzed at 700 degrees C for 1 h, at a heating rate of 50 degrees C/min under nitrogen gas. Fillers were evaluated in terms of size, structure, and surface chemistry, which are the 3 primary areas contributing to filler reinforcement mechanisms. The cure, tensile and dynamic mechanical properties of the biochar SBR composites were also evaluated and compared to SBR composites filled with N772 grade carbon black. The results indicate that corn stover biochar has the highest reinforcing index in SBR, only 17.5 % lower than that of N772 in SBR. Biochar fillers from corn cob and corn stover had relatively similar characteristics, suggesting that they could be combined with minimal loss in performance. Starch biochar SBR composites were highly crosslinked but also significantly more brittle. Though all show promising reinforcing characteristics, corn-based biochar fillers are not yet on par with N772 carbon black, having poorer filler dispersion and lower reinforcement indexes. Further investigation and optimization of these alternative fillers is required before they can be considered for wide-spread use.

Lubricated contacts in soft materials are common in various engineering and natural settings, such as tires, haptic applications, contact lenses, and the fabrication of soft electronic devices. Two major regimes are elasto-hydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, and mixed lubrication (ML), in which there is partial solid-to-solid contact. The transition between these regimes governs the minimum sliding friction achievable and is thus very important. Generally, the transition from EHL to ML regimes is believed to occur when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. Here, it is reported that in lubricated sliding experiments on smooth, soft, poly(dimethylsiloxane) substrates, the transition can occur when the thickness of the liquid layer is much larger than the height of the asperities. Direct visualization of the "contact" region shows that the transition corresponds to the formation of wave-like surface wrinkles at the leading contact edge and associated instabilities at the trailing contact edge, which are believed to trigger the transition to the mixed regime. These results change the understanding of what governs the important EHL-ML transition in the lubricated sliding of soft solids.

A common way to increase friction during lubricated sliding is to increase energy dissipation in the slider or the substrate, for example, by bulk viscoelasticity. It has recently been shown that lubricated friction can also be enhanced by surface architecture, specifically by periodic variation of near-surface stiffness. We study this phenomenon by considering a rigid cylinder undergoing lubricated sliding on a substrate with a periodic variation in mechanical stiffness. We model the process using the Reynolds theory for transient elasto-hydrodynamic lubrication (t-EHL). We developed a numerical scheme to solve this t-EHL problem and used it to study how surface variation in stiffness affects the motion of the cylinder, the distribution of hydrodynamic pressure, the liquid film thickness, and the friction force. Our results indicate that increasing the variation of the near-surface stiffness can significantly increase the friction force. The numerical method developed in this work can be applied to other transient EHL problems with slight modification. Our results provide insight into the mechanism of friction enhancement and can guide design of surfaces to control friction during lubricated sliding.

In this study, chicken feather meal (CFM) and canola protein (CP) were converted into biochar and their suit-ability as reinforcing fillers in styrene-butadiene rubber (SBR) composites was evaluated. The protein-based feedstocks were pyrolyzed at 700 C for 1 h, at a heating rate of 50 C/min under different pyrolysis atmo-spheric conditions (N2, CO2, and steam). The flow rate of N2 and CO2 was 700 ml/min, while that of steam was 31 ml/min. Also, biochar was produced under N2 and CO2 gas flow and subsequently activated using steam. The physicochemical properties of the resulting CFM and CP biochars were characterized based on elemental and proximate analyses, surface area, Fourier-transform infrared spectroscopy (FTIR), and thermal field emission scanning electron microscopy. Results showed that activated CP, pyrolyzed under nitrogen and subsequently steam cooled (CP N2 + SC), had enhanced physicochemical properties such as lower ash content, higher fixed carbon content, and reduced polar surface functional groups compared to the other studied biochars in this study. Results also showed that CFM and CP pyrolyzed under nitrogen and subsequently steam cooled displayed higher final moduli and better filler dispersion in rubber than the other biochar samples produced in this study. Given that results showed favorable physicochemical properties and higher final moduli for CP N2 + SC, this sample was further tested using dynamic mechanical analysis (DMA) and results showed slight differences with carbon black regarding the temperature dependence of the shear storage modulus (G & PRIME;) and the loss tangent (tan delta). These differences were attributed to increased filler-filler interactions, reduced filler-rubber interactions, and a lower dispersion in rubber than carbon black. The observed differences were not large enough to explain the inability of the biochar to adequately reinforce the rubber. However, the biochar filled rubber sample had a comparable viscoelastic behavior to carbon black over the studied temperature range.

Controlling the lubricated sliding friction of compliant contacts is important for many mechanical and biological systems. Multiphase materials have been shown to exhibit varied lubricated friction responses when compared to controls of just one phase of the material. In this work, we describe a structured two-phase material composed of a plastic mesh embedded in a compliant elastomer matrix. This embedded mesh structure (EMS) exhibits increased lubricated sliding friction for a number of load, velocity, and lubricant viscosity conditions. The observed friction enhancement appears to be a result of the EMS sample transitioning to the mixed lubrication regime under conditions in which the control is in the elastohydrodynamic lubrication regime. Simulations suggest that the difference in lubrication regimes for the EMS sample compared to the unstructured control comes from areas of high contact pressure induced by the increased local contact stiffness of the material near the embedded mesh. We hypothesize that these areas of high pressure can lead to the destabilization of lubricant films under conditions where the control films are stable, leading to the difference in lubrication regime behaviors observed.

We study the friction force during lubricated sliding of a rigid cylindrical indenter against a viscoelastic substrate in the iso-viscous visco-elasto-hydrodynamic lubrication (VEHL) regime. The substrate is represented by a foundation model. The solution is controlled by three dimensionless parameters. The first of these, λ, measures the time for the indenter to move one contact zone relative to the viscoelastic relaxation time; the second is the ratio of the long time to short time compliance of the substrate, $$c_{\infty } /c_{0}$$ ; the third parameter, β, is the ratio of average fluid flow rate to the sliding velocity. Although our solution works well for the full range of parameters, we focus on the “Hertz” regime (β >>1) where practically all the fluid in the contact region is squeezed out. This regime is quite common in soft contact lubrication problems and presents significant numerical difficulties. Our analysis gives insight into why these numerical difficulties arise. The friction force can be decomposed into two parts, one due to viscoelastic dissipation and the other from hydrodynamics. Although these two are generally coupled, in the Hertz limit, an important result is that the viscoelastic portion of the friction force can be well approximated by the solution of the corresponding “dry” sliding problem, in which there is no lubricating fluid layer. This provides a simple way to decouple the hydrodynamic portion of the friction force from the viscoelasticity of the substrate. We study how hydrodynamic pressure and film thickness vary with the controlling dimensionless parameters. Scaling laws for these relationships are given in closed form.

Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur-sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain-induced scission of the coumarin dimer within the matrix of a particle-reinforced polybutadiene-based co-polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are molecular "weak links" within the matrix, and, by analogy, sulfur bridges are likely to be the same in vulcanized rubbers. The mechanophore is embedded in polymer main chains, grafting agent, and cross-linker positions in a polymer composite in order to generate experimental data to understand how macroscopic mechanical stress is transferred at the molecular scale especially in highly entangled cross-linked polymer nanocomposite. Finally, the extent of activation is enhanced by approximately an order of magnitude by changing the regiochemistry and stereochemistry of the coumarin dimer and embedding the mechanophore at the heterointerface of the particle-reinforced elastomer.

Lubricated contacts are present in many engineering and biological systems involving soft solids. Typical mechanisms considered for controlling the sliding friction in such lubricated conditions involve bulk material compliance, fluid viscosity, viscoelastic response of the material (hysteretic friction), and breaking of the fluid film where dry contact occurs (adhesive friction). In this work we show that a two-phase periodic structure (TPPS), with a varying modulus across the sliding surface, provides significant enhancement of lubricated sliding friction when the system is in the elastohydrodynamic lubrication (EHL) regime. We propose that the enhanced friction is due to extra energy loss during periodic transitions of the sliding indenter between the compliant and stiff regions during which excess energy is dissipated through the fluid layer. This is a form of elastic hysteresis that provides a novel mechanism for friction enhancement in soft solids under lubricated conditions.

Carbon black has been a key ingredient in high-performance composites, such as tire rubber, for over a hundred years. This reinforcing filler increases rubber rigidity and reduces tire wear, among many other useful effects. New nanomaterials, such as graphene and carbon nanotubes, may bring new performance improvements. However, their usefulness cannot be evaluated unless worker safety is assured by demonstrating that the nanoparticles are not released at harmful concentrations during manufacture and testing. Here, we present a flexible, general method for the quantitative evaluation of nanoparticle release from rubber nanocomposites. We evaluate manufacturing steps such as powder handling, uncured rubber milling, and curing. We also evaluate particle emission during cured rubber abrasion as an aggressive example of the testing rubber goods are subjected to. We quantify released nanoparticle concentrations for clay nanoparticles, graphene-like materials, and carbon nanotubes. We also describe a mechanistic framework based on the balance of adhesive and kinetic energies, which helps understand when nanoparticles are or are not released. This method contributes to the assessment of workers' exposure to nanoparticles during the various stages of the industrial process, which is an essential step in managing the risk associated with the use of nanomaterials in manufacturing.

Recent experiments on the rolling of a cylinder with a poly-dimethylsiloxane (PDMS) film-terminated ridge-channel surface structure against a flat rigid plate have shown significant enhancement in rolling resistance. For rolling perpendicular to the ridges, the rolling resistance initially increases with the ridge spacing. Treating the trailing edge of the rolling interface as an opening crack, this increase has been explained quantitatively by a crack-trapping model. However, beyond a critical value of spacing, the rolling resistance reaches a maximum value and observations suggest that its subsequent decrease is due to two factors. First is the nucleation of cavities ahead of the opening interfacial crack. Second is the growth of these defects parallel to the ridge. These two phenomena limit and eventually attenuate the effect of crack trapping, the primary mechanism of rolling resistance enhancement. Cavitation of the interface is a critical phenomenon limiting rolling resistance, and its mechanism is modeled in this work. We have developed a finite element method (FEM) to simulate the rolling process. Specifically, we developed a special cohesive element and numerical scheme to study how cavities nucleate and grow during rolling. Our simulation captures qualitatively the key experimental observation that cavitation is controlled by ridge spacing. However, our numerical model under-predicts the rolling resistance enhancement due to finite cohesive zone size effects.

Semiconducting single-wall carbon nanotubes (SWCNTs) with long lengths are highly desirable for many applications such as thin-film transistors and circuits. Previously reported length sorting techniques usually require sophisticated instrumentation and are hard to scale up. In this paper, we report for the first time a general phenomenon of a length-dependent precipitation of surfactant-dispersed carbon nanotubes by polymers, salts, and their combinations. Polyelectrolytes such as polymethacrylate (PMAA) and polystyrene sulfonate (PSS) are found to be especially effective on cholate and deoxycholate dispersed SWCNTs. By adding PMAA to these nanotube dispersions in a stepwise fashion, we have achieved nanotube precipitation in a length-dependent order: first nanotubes with an average length of 650 nm, and then successively of 450 nm, 350 nm, and 250 nm. A similar effect of nanotube length sorting has also been observed for PSS. To demonstrate the utility of the length fractionation, the 650 nm-long nanotube fraction was subjected to an aqueous two-phase separation to obtain semiconducting enriched nanotubes. Thin-film transistors fabricated with the resulting semiconducting SWCNTs showed a carrier mobility up to 18 cm(2) (V s)(-1) and an on/off ratio up to 10(7). Our result sheds new light on the phase behavior of aqueous nanotube dispersions under high concentrations of polymers and salts, and offers a facile, low-cost, and scalable method to produce length sorted semiconducting nanotubes for macroelectronics applications.

DNA carbon nanotube (DNA-CNT) hybrids are nanometer-sized, highly charged, rodlike molecules with complex surface chemistry, and their behaviors in aqueous solutions are governed by multifactorial interactions with both solvent and cosolutes. We have previously measured the force between DNA-CNTs as a function of their interaxial distance in low monovalent salts where interhybrid electrostatic repulsion dominates. The characteristics of DNA-CNT forces were further shown to closely resemble that of double-stranded DNA (dsDNA) in low salts. However, contrasting behaviors emerge at elevated monovalent salts: DNA-CNT condenses spontaneously, whereas dsDNA remains soluble. Here we report force distance dependencies of DNA-CNTs across wide-ranging monovalent salt concentrations. DNA-CNT force curves are observed to deviate from dsDNA curves above 300 mmol/L NaCl, and the deviation grows with increasing salts. Most notably, DNA-CNT forces become net attractive above 1 mol/L NaCl, whereas dsDNA forces are repulsive at all salt concentrations. We further discuss possible physical origins for the observed DNA-CNT attraction in monovalent salts, in consideration of the complex surface chemistry and unique polyelectrolyte properties of DNA-CNT hybrids.

We performed tunable resonance Raman spectroscopy on samples highly enriched in the (5,5), (6,6), (7,7), and (8,8) armchair structures of metallic single-wall carbon nanotubes. We present Raman excitation profiles (REPs) for both the radial breathing mode and $G$-band phonons of these species. $G$-band excitation profiles are shown to resolve the expected incoming and outgoing resonances of the scattering process. Notably, the profiles are highly asymmetric, with the higher-energy outgoing resonance weaker than the incoming resonance. These results are comparable to the asymmetric excitation profiles observed previously in semiconducting nanotubes, introduce a different electronic type, and broaden the structural range over which the asymmetry is found to exist. Modeling of the behavior with a third-order quantum model that accounts for the $k$ dependence in energies and matrix elements, without including excitonic effects, is found to be insufficient for reproducing the observed asymmetry. We introduce an alternative fifth-order model in which the REP asymmetry arises from quantum interference introduced by phonon-mediated state mixing between the ${E}_{11}^{M}$ and $K$-momentum excitons. Such state mixing effectively introduces a nuclear coordinate dependence in the transition dipole moment and thus may be viewed as a non-Condon effect from a molecular perspective. This result unifies a molecularlike picture of nanotube transitions (introduced by their excitonic nature) with a condensed matter approach for describing their behavior.

Because of their repetitive chemical structure, extreme rigidity, and the separability of populations with varying aspect ratio, SWCNTs are excellent candidates for use as model rodlike colloids. In this contribution, the sedimentation velocities of length and density sorted single-wall carbon nanotubes (SWCNTs) are compared to predictions from rod hydrodynamic theories of increasing complexity over a range of aspect ratios from <50 to >400. Independently measuring all contributions to the sedimentation velocity besides the shape factor, excellent agreement is found between the experimental findings and theoretical predictions for numerically calculated hydrodynamic radius values and for multiterm analytical expansion approximations; values for the hydrodynamic radii in these cases are additionally found to be consistent with the apparent hydrated particle radius determined independently by buoyancy measurements. Lastly, we utilize this equivalency to calculate the apparent distribution of nanotube lengths in each population from their sedimentation coefficient distribution without adjustable parameters, achieving excellent agreement with distributions from atomic force microscopy. The method developed herein provides an alternative for the ensemble measurement of SWCNT length distributions and others rodlike particles.