Rod Aspect Ratio Research Articles

Rapid Photothermal Lysis of the Pathogenic Bacteria, Escherichia Coli Using Synthesis of Gold Nanorods

A photon-to-thermal conversion nanosystem has been developed to rapidly elevate temperatures in poorly thermally conducting media using irradiation of gold nanorods. We first hypothesized that nanoparticles, especially gold nanorods, are capable of generating enough heat to lyse bacteria by heating sample solutions via laser irradiation. To test this, we synthesized Au nanorods (aspect ratio 3-4) and studied optothermal properties of these nanoparticles. The short Au nanorods were more efficient at absorbing 808 nm 450 mW laser irradiations resulting in more efficient temperature increase in glass vials compared to the long rods (aspect ratio -40). In bulk media, these nanoparticles could easily raise the temperature beyond 100 degrees C under continuous wave laser irradiation, enabling immediate cell lysis. Consequently, the pathogenic bacteria, Escherichia coli, within the sample solution are lysed by irradiating of the sample using a near infrared laser.

The effect of Brownian motion on the stability of sedimenting suspensions of polarizable rods in an electric field

We examine the collective dynamics of polarizable, Brownian, sedimenting rods of high aspect ratio. Previous work of Koch and Shaqfeh (J. Fluids Mech., vol. 209, 1989 pp. 521–542) has shown that in the absence of Brownian motion, sedimenting suspensions of rods are unstable to concentration fluctuations and form dense streamers via interparticle hydrodynamic interactions. Recently, Saintillan, Shaqfeh & Darve (Phys. Fluids, vol. 18 (121701), 2006b p. 1) demonstrated that electric fields can act to stabilize these non-Brownian suspensions of polarizable rods through induced-charge electrokinetic rotation, which forces particle alignment. In this paper, we employ a mean-field linear stability analysis as well as Brownian dynamics simulations to study the effect of thermal motion on the onset of instability. We find that in the absence of electric fields, Brownian motion consistently suppresses instability formation through randomization of particle orientation. However, when electric fields are applied, thermal motion can act to induce instability by counteracting the stabilizing effect of induced-charge orientation.

Verification of a method for measuring magnetostrictive parameters for use in transducer design modeling.

Previous studies have initiated an investigation into a new method of measuring the d33, μ33T, and s33H magnetostrictive constants of a given sample using a pair of Helmholtz coils, a laser vibrometer, and a sense coil. These parameters are calculated from measurements made as a function of dc magnetic bias and ac drive amplitude. This method contrasts with the traditional measurement technique which involves building the sample into a transducerlike device. The new method was demonstrated for laminated stress‐annealed Galfenol rods of high aspect ratio. Measurements obtained by this method appear credible and demonstrate good repeatability. For verification, the authors have returned to the established approach and designed a simple transducer using one of the measured Galfenol rods. The transducer was driven in air as a function of dc magnetic bias and ac drive amplitude. Data obtained from this transducer are compared to an analytical model that uses the material properties obtained experimentally with the new method. [This work was sponsored by the Office of Naval Research (Contract N00014‐06‐1‐0530) and by Etrema Products, Inc.]

Efficient utilization of plastic waste through product design and process adaptation: A case study on stiffness enhancement of beams produced from plastic lumber

AbstractThe aim of the present work is the development of a method for structural reinforcement of beams obtained by in‐mold extrusion of plastics from solid urban waste. The beams obtained by in‐mold extrusion are commonly used for outdoor furniture and structures. The material used for such applications is mainly composed of low‐density polyethylene derived from bags and films, with small amounts of high density polyethylene and polypropylene, as well as traces of polyethylene terephthalate. This material is usually referred to as “plastic lumber.” Plastic lumber products have a low stiffness, which results in high deflections under flexural loads, particularly under creep loading. In this study, reinforcing rods of high aspect ratio were incorporated into plastic lumber beams in specific positions with respect to the cross section of the beam. The reinforcement of the plastic lumber beams with fine rods is introduced in the typical intrusion process used for the production of unreinforced beams. Glass fiber reinforced pultruded rods were chosen for this purpose from a preliminary evaluation of different materials. Different diameter glass fiber reinforced pultruded rods, including surface‐abraded systems to increase the roughness, were used for the reinforcement of plastic lumber beams. The reinforced beams were tested in terms of flexural stiffness, creep resistance, and pullout resistance of the embedded rods. The results obtained from the mechanical tests showed a significant enhancement of flexural stiffness and creep resistance behavior. The performance at higher stress levels was shown to be significantly dependent on the interfacial adhesion between rods and polymer matrix. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 27:133–142, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20131

Controllable morphology evolution of electrodeposited ZnO nano/micro-scale structures in aqueous solution

Abstract This study introduces a cost-effective electrodeposition route for preparation of highly oriented and well-defined ZnO nano/micro-scale structures on indium tin oxide (ITO) glass substrates without a traditionally pre-prepared layer of ZnO seeds. It was demonstrated that the addition of KCl, NaCl, and ethylenediamine (EDA) and the hydrolysis of hexamethylenetetramine (HMT) pose the difference in growth rates between ZnO { 1 0 1 ¯ 0 } prismatic planes and (0 0 0 1) end plane and lead to the formation of highly oriented ZnO rods, flower-like ZnO bundles, nanowire arrays or hexagonal plates. The effect of different Cl− sources on the array manner of ZnO plates and the effect of concentration of EDA on the aspect ratio of ZnO rods were investigated, respectively. Electrodeposition potential plays an important role in the preferential growth orientation of ZnO crystals on the substrates without a traditionally pre-prepared layer of ZnO seeds. Room temperature photoluminescence (PL) properties of different ZnO structures were also investigated. In addition, different procreation manner on the (0 0 0 1) growing end planes for ZnO rods and nanowire arrays were observed. On the basis of observation, several growth mechanisms, such as layer-by-layer procreation manner for ZnO nanowires, are suggested.

The rheological modelling of carbon nanotube (CNT) suspensions in steady shear flows

This paper is concerned with the rheological modelling of both chemically treated and untreated carbon nanotube (CNT) suspended in a Newtonian epoxy resin. CNT suspensions generally exhibited shear-thinning characteristic—the apparent viscosity decreases as shear rate increases—when subject to steady shear flows. Chemically treated CNT suspensions with little optical microstructure were found to exhibit a less significant shear-thinning effect compared with untreated CNT suspensions where clear optical aggregates were observed. In the case of treated CNT suspensions, the shear-thinning characteristic could be described using a Fokker–Planck based orientation model. The model assumed that the treated CNTs behaved as high aspect ratio rods and that shear flow was able to align the CNTs in the flow direction, thereby resulting in a decrease in the shear viscosity. Despite the success in describing the rheological response of treated CNT in steady shear flows, the orientation model failed to explain the more pronounced shear-thinning effect observed in untreated CNT suspensions having a hierarchy of aggregate structures. A new model called the aggregation/orientation (AO) model was formulated by modifying the Fokker–Planck equation. The AO model considered elements of aggregation as well as CNT orientation and it was capable of capturing the steady shear response of untreated CNT suspensions.

Monte Carlo simulation of two-dimensional hard rectangles: Confinement effects

We use orientational-bias Monte Carlo simulations to examine the phase behavior of two-dimensional hard rectangles in the bulk and under confinement by hard walls. For all of the rod aspect ratios and area fractions studied, we find that confinement increases the degree of nematic ordering over the bulk, as confined rods tend to align their long axes parallel to the confining walls. The extent of nematic ordering increases as the separation between the confining walls decreases. If the aspect ratio of the rectangles is sufficiently large, they exhibit nematic ordering in both the bulk and under confinement, where the nematic director is set by the walls. Rods with a small aspect ratio are isotropic in the bulk and exhibit weak tetratic tendencies for sufficiently high densities. From studies of density profiles, angular distributions, and orientational correlation functions for confined, low-aspect-ratio rods, it is apparent that they align their long axes parallel to the wall in the near-wall region, where layering occurs for sufficiently high rod densities. However, confined rods with low aspect ratios still exhibit weak tetratic (isotropic) tendencies near the center of the confined region for all but the smallest wall separations. We note that although our studies probe the ordering of hard rectangles, the entropic tendencies that we observe here will be present for rods with energetic interactions. Thus, these studies serve as a general starting point for understanding and controlling the assembly of rods in two-dimensional confining geometries.

Density functional theory for molecular orientation of hard rod fluids in hard slits

A density functional theory (DFT) is used to investigate molecular orientation of hard rod fluids in a hard slit. The DFT approach combines a modified fundamental measure theory (MFMT) for excluded-volume effect with the first order thermodynamics perturbation theory for chain connectivity. In the DFT approach, the intra-molecular bonding orientation function is introduced. We consider the effects of molecular length (i.e. aspect ratio of rod) and packing fraction on the orientations of hard rod fluids and flexible chains. For the flexible chains, the chain length has no significant effect while the packing fraction shows slight effect on the molecular orientation distribution. In contrast, for the hard rod fluids, the chain length determines the molecular orientation distribution, while the packing fraction has no significant effect on the molecular orientation distribution. By making a comparison between molecular orientations of the flexible chain and the hard rod fluid, we find that the molecular stiffness distinctly affects the molecular orientation. In addition, partitioning coefficient indicates that the longer rodlike molecule is more difficult to enter the confined phase, especially at low bulk packing fractions.

Translational and rotational dynamics of colloidal rods by direct visualization with confocal microscopy

We report an experimental method to characterize the dynamics of colloidal rods by measuring their rotation and translation in three dimensions with confocal microscopy. The method relies on solvent viscosification to retard dynamics to time scales that are compatible with 3D confocal optical microscopy. Because the method yields a full three-dimensional characterization of rod displacement and orientation, it is applicable to situations in which complex, anisotropic dynamics emerge. Examples include behavior in liquid crystal phases with both orientational and positional order, suspensions subjected to applied fields such as shear flow or sedimentation, and the emerging area of anisotropic particle dynamics. We demonstrate the performance of the method by quantifying the Brownian motion of fluorescent poly(methyl methacrylate) rods (aspect ratio, L / D = 3.1 and 7.0) grafted with poly(dimethylsiloxane) stabilizer. The rods are dispersed at dilute concentration in a solvent mixture of viscosity 2.0 Pa s. Rod translational and rotational diffusivities are extracted from the measured translational mean square displacement of the centroid positions and of the rod unit vector u ( t ) , respectively. Rod orientational dynamics are characterized relative to both their azimuthal and polar angles. Probability distributions for the translation and rotation in the frame of rod are computed from the measurements. Experimental values obtained agree well with theory available for the dynamics of isolated rods.

Purely dipolar versus dipolar-exchange modes in cylindrical nanorods

Properties of purely dipolar and dipolar-exchange modes in finite circular rods are investigated numerically, with the sample static magnetization assumed to be uniform and parallel to the rod axis; we study magnetic excitations propagating along this direction. In particular, we demonstrate size-exchange effects induced by modifying the rod aspect ratio (the diameter to length ratio) and the strength of the exchange coupling. Untypical groups of purely dipolar modes are shown to occur in smooth rods: bulk-dead modes, characterized by a dead-amplitude region inside the rod, and comb modes, in which a nearly “dead” region is found at the sample borders, while numerous (“comb-like”) oscillations take place in the bulk. These two mode groups fade to disappear when the sample geometry evolves toward the thin-film one, or when the exchange becomes dominant. The existence of these untypical modes is related to local demagnetizing field profile nonhomogeneity, particularly to two edge wells found in the field profile. The bulk-dead modes are low-frequency modes confined to the well regions only (and therefore double-degenerated), while modes with higher frequency values (subjected to an interference within the barrier) become fast-oscillating comb modes. When the contribution of the exchange grows to prevail, the “dipolar” nonhomogeneity becomes negligible with respect to the total (exchange and dipolar) energy, and only the typically exchange-related bulk and surface modes are found to subsist.

Micromachining for Terahertz Artificial Materials

AbstractIn this paper we report on the development of micromachined filters for operation at terahertz frequencies. SU8, a negative photodefinable epoxy, is used to define arrays of high aspect ratio rods which are subsequently sputter coated in gold to form the filter. We fabricate and test a filter with a fixed period but variable diameter along the length of the array. By moving the array in the terahertz beam we demonstrate the ability to mechanically tune specific filter characteristics from a single device.

Simulation studies of self-assembly of end-tethered nanorods in solution and role of rod aspect ratio and tether length

We present temperature versus concentration phase diagrams for "shape amphiphiles" comprised of tethered moderate and low aspect ratio rods. Simulations of moderate aspect ratio rods (first reported by Horsch et al. [Phys. Rev. Lett. 95, 056105 (2005)]) predict their self-assembly into spherical micelles with bcc order, long micelles with nematic order, a racemic mixture of hexagonally ordered chiral cylinders, two perforated phases: one with tetragonal order and one with hexagonal order, and a smectic C lamellar phase. In contrast, we predict here that small aspect ratio tethered rods self-assemble into bcc ordered spherical micelles, hexagonally ordered cylinders, and a smectic C lamellar phase. We compare and contrast the phases obtained for the two aspect ratios and examine in further detail several unusual phases. Our simulations also reveal that for moderate aspect ratio rods there is a tendency toward phases with decreasing interfacial curvature with decreasing coil size, including a double gyroid phase. In addition, we investigate the role of tether length on the assembled structures. Our results are applicable to short rod-coil block copolymers and rodlike nanoparticles with polymer tethers, and to colloidal building blocks comprised of a flexible string of colloids tethered to a rigid string of colloids, with the interactions scaled appropriately.

Depletion effects in smectic phases of hard-rod-hard-sphere mixtures

It is known that when hard spheres are added to a pure system of hard rods the stability of the smectic phase may be greatly enhanced, and that this effect can be rationalised in terms of depletion forces. In the present paper we first study the effect of orientational order on depletion forces in this particular binary system, comparing our results with those obtained adopting the usual approximation of considering the rods parallel and their orientations frozen. We consider mixtures with rods of different aspect ratios and spheres of different diameters, and we treat them within Onsager theory. Our results indicate that depletion effects, and consequently smectic stability, decrease significantly as a result of orientational disorder in the smectic phase when compared with corresponding data based on the frozen-orientation approximation. These results are discussed in terms of the tau parameter, which has been proposed as a convenient measure of depletion strength. We present closed expressions for tau, and show that it is intimately connected with the depletion potential. We then analyse the effect of particle geometry by comparing results pertaining to systems of parallel rods of different shapes (spherocylinders, cylinders and parallelepipeds). We finally provide results based on the Zwanzig approximation of a fundamental-measure density-functional theory applied to mixtures of parallelepipeds and cubes of different sizes. In this case, we show that the tau parameter exhibits a linear asymptotic behaviour in the limit of large values of the hard-rod aspect ratio, in conformity with Onsager theory, as well as in the limit of large values of the ratio of rod breadth to cube side length, d, in contrast to Onsager approximation, which predicts tau approximately d (3). Based on both this result and the Percus-Yevick approximation for the direct correlation function for a hard-sphere binary mixture in the same limit of infinite asymmetry, we speculate that, for spherocylinders and spheres, the tau parameter should be of order unity as d tends to infinity.

On Weak Plane Couette and Poiseuille Flows of Rigid Rod and Platelet Ensembles

Films and molds of nematic polymer materials are notorious for heterogeneity in the orientational distribution of the rigid rod or platelet macromolecules. Predictive tools for structure length scales generated by shear-dominated processing are vitally important: both during processing because of flow feedback phenomena such as shear thinning or thickening, and postprocessing since gradients in the rod or platelet ensemble translate to nonuniform composite properties and to residual stresses in the material. These issues motivate our analysis of two prototypes for planar shear processing: drag-driven Couette and pressure-driven Poiseuille flows. Hydrodynamic theories for high aspect ratio rod and platelet macromolecules in viscous solvents are well developed, which we apply in this paper to model the coupling between short-range excluded volume interactions, anisotropic distortional elasticity (unequal elasticity constants), wall anchoring conditions, and hydrodynamics. The goal of this paper is to generalize scaling properties of steady flow molecular structures in slow Couette flows with equal elasticity constants (M. G. Forest et al., J. Rheol., 48 (2004), pp. 175-192) in several ways: to contrast isotropic and anisotropic elasticity; to compare Couette versus Poiseuille flow; and to consider dynamics and stability of these steady states within the asymptotic model equations.

Formation of Polymer Microrods in Shear Flow by Emulsification − Solvent Attrition Mechanism

Rodlike polymer particles could have interesting properties and could find many practical applications; however, few methods for the production of such particles are available. We report a systematic study of a droplet shearing process for the formation of polymer rods with micrometer or submicrometer diameter and a length of up to tens of micrometers. The process is based on emulsification of a polymer solution under shear, combined with solvent attrition in the surrounding organic medium. The droplets deform and elongate into cylinders, which solidify when the solvent transfers to the dispersion medium. Stopped flow experiments allow distinguishing all stages of the mechanism. The results are interpreted on the basis of the theory of droplet elongation and breakup under shear. The effects of the viscosity ratio and shear stress are matched against the theoretical expectations. The method is simple, efficient, and scalable, and we demonstrate how it can be controlled and modified. The experimental parameters that allow varying the rod size and aspect ratio include shear rate, medium viscosity, and polymer concentration. Examples of the specific properties of the polymer rods, including self-organization, alignment in external fields and in fluid flows, and stabilization of bubbles, droplets, and capsules, are presented.

Simulations of Concentrated Suspensions of Semirigid Fibers: Effect of Bending on the Rotational Diffusivity

Brownian dynamics simulations of the behavior of semirigid polymers demonstrate that small deviations from rigid models affect the dynamics of rod suspensions and that the dynamics are sensitive to the hydrodynamic model. Simulations were performed with two models having different short-time diffusivities. The first was a slender-body dimer model, where two rigid slender-bodies are joined at their ends, and the second was a three-bead trimer model where the beads are connected by rigid rods. A potential was applied to hinder the bending of the rods. An aspect ratio (rod length over diameter) of 50 was used for all simulations, and an excluded volume potential was used to prevent the rods from crossing one another. Simulations of systems of stiff rods which are not quite rigid show a delayed onset of semidilute behavior in comparison to simulations of rigid rod systems, with different hydrodynamic models providing different power-law scalings of the rotational diffusivities within the semidilute concentrat...

Synthesis of High Aspect Ratio Low-Silica Zeolite L Rods in Oil/Water/Surfactant Mixtures

Low-silica zeolite L (zeolite GL) syntheses in the presence of emulsions were investigated. High aspect ratio rods were obtained when zeolite LTL was crystallized in the presence of both cationic and nonionic two-phase emulsions. Conventional syntheses of zeolite L primarily led to low aspect ratio crystals possessing a length/width ratio of approximately 1.5. In the presence of the emulsion aspect ratios were increased with most samples possessing aspect ratios between 5 and 10, although crystals were observed with aspect ratios as high as 42. TEM analysis indicates the LTL pores are parallel to the long axis of the crystals. It is observed that anisotropic crystals are formed when the surfactant (CTAB) to cation ratio is below 3.5. By contrast when the CTAB/(Ba2++K+) is greater than four the LTL phase does not form. Igepal CO-720 was also used to determine if similar high aspect ratio crystals could be formed in nonionic emulsions. While the nonionic emulsions also led to LTL rods, the phase purity of the materials was lower based on SEM and PXRD. This report, to the best of knowledge, is the first of low-silica zeolite L crystals (Si/Al < 1.5) that possess such aspect ratios. Given the ability to exchange at least some of the extraframework cations, the materials reported here are possible candidates for use in applications including microelectronics and sensing.

Phase equilibria in solutions of platelike particles: Systems with steric and dispersive interactions between the platelets

Our statistical thermodynamics model of solution of stiff, platelike, biaxial particles interacting solely via repulsion on contact (athermal limit) [Phys. Rev. E 62, 5011 (2000)] is extended to incorporate dispersion interactions between the particles. Dispersion forces between anisotropic particles are accounted for using the Imura-Okano approach. Numerical calculations specialized to solutions of either rods or disks show that besides the isotropic-nematic biphasic coexistence range, inclusion of attractive forces resulted in the appearance of nematic-nematic coexistence in both, disks and rods, solutions. The critical divergence of the difference between the order parameters and concentrations of the two nematics is observed while approaching the critical temperature. The minimum aspect ratio of rods or disks for the formation of the nematic phase is also discussed.

Effect of monomer geometry on the fractal structure of colloidal rod aggregates.

The fractal structure of clusters formed by diffusion-limited aggregation of rodlike particles is characterized over three decades of the scattering vector q, and displays an unexpected dependence on the aspect ratio of the constituent monomers. Monte Carlo simulations of aggregating Brownian rods corroborate the experimental finding that the measured fractal dimension is an increasing function of the monomer aspect ratio. Moreover, increasing the rod aspect ratio eliminates the structural distinction between diffusion- and reaction-limited cluster aggregation that is observed for spheres.

Free-radical polymerization of styrene in worm-like micelles

The solubilization of styrene in wormlike micelles of the cationic surfactant, cetyltrimethylammonium tosilate (CTAT), and its polymerization is examined here by UV spectroscopy, oscillatory rheometry, small angle X-ray scattering, polarizing light microscopy, and transmission electron microscopy. At low CTAT concentrations, the polymerization of styrene yields small and rigid rods in coexistence with wormlike micelles that form from the excess surfactant after the polymerization process. At high CTAT concentrations, polymeric rods (of large aspect ratio), spheroid polymer particles, and wormlike micelles coexist. The polymerization rate is second order, indicating that polymerization reactions end mainly by bimolecular termination.