Mechanism of adhesion between polymer fibers at nanoscale contacts

Adhesive force exists between polymer nano/microfibers. An elaborate experiment was performed to investigate the adhesion between polymer nano/microfibers using a nanoforce tensile tester. Electrospun polycaprolactone (PCL) fibers with diameters ranging from 0.4-2.2 μm were studied. The response of surface property of electrospun fiber to the environmental conditions was tracked by FTIR and atomic force microscopy (AFM) measurements. The effect of temperature on molecular orientation was examined by wide angle X-ray diffraction (WAXD). The adhesive force was found to increase with temperature and pull-off speed but insensitive to the change of relative humidity, and the abrupt increase of adhesion energy with temperature accompanied by a reduced molecular orientation in the amorphous part of fiber was observed. Results show that adhesion is mainly driven by van der Waals interactions between interdiffusion chain segments across the interface.     

Effective zones of hydrodynamical interaction of fluid particles at small separations

A system of two approaching fluid particles at small gap widths is investigated. The concept of an effective zone of interaction is used to analyse the stationary Navier-Stokes equations in the respective phases. Particular cases are distinguished in which it is possible to make significant simplifications in the governing differential equations. A validity criterion for the lubrication theory equations in the thin layer is proposed, which accounts for the tangential mobility of the interfaces.     

Shape and eccentricity effects in adhesive contacts of rodlike particles

The effects of shape and eccentricity on adhesion and detachment behavior of long, rodlike particles in contact with a half-space are analyzed using contact mechanics. The particles are considered to have cross sections that are squarish, oblate, or prolate rather than circular. Such cross sections are represented very generally by using superellipses. The contact mechanics model allows deduction of closed-form expressions for the contact pressure, load-contact size relation, detachment load, and detachment contact size. It is found that even relatively small deviations in shape from a cylinder have a significant influence on the detachment load. Eccentricity also affects the adhesive behavior, but to a lesser extent, with oblate shapes requiring larger separation loads than prolate shapes. The load-contact size solution reduces to that for a right-circular, cylindrical rod when the appropriate limit is taken. The detachment behavior of right-circular cylinders is also found to be mimicked by an entire family of rod shapes with different cross sections.     

Monitoring adsorption on small particles

A review on the application of photoelectron emission from small particles suspended in a gas to monitor the adsorption properties and to detect adsorbates on the particles is given. The relation between surface coverage with an adsorbate and the shift in photothreshold and photoelectric yield is demonstrated by two examples: The adsorption of O₂ on silver particles and of perylene on carbon particles. A third example concerns the detection of polyaromatic hydrocarbons (PAH) on particles arising from combustions.     

Capillary forces between spherical particles floating at a liquid-liquid interface

We study the capillary forces acting on sub-millimeter particles (0.02-0.6 mm) trapped at a liquid-liquid interface due to gravity-induced interface deformations. An analytical procedure is developed to solve the linearized capillary (Young-Laplace) equation and calculate the forces for an arbitrary number of particles, allowing also for a background curvature of the interface. The full solution is expressed in a series of Bessel functions with coefficients determined by the contact angle at the particle surface. For sub-millimeter spherical particles, it is shown that the forces calculated using the lowest order term of the full solution (linear superposition approximation; LSA) are accurate to within a few percents. Consequently the many particle capillary force is simply the sum of the isolated pair interactions. To test these theoretical results, we use video microscopy to follow the motion of individual particles and pairs of interacting particles at a liquid-liquid interface with a slight macroscopic background curvature. Particle velocities are determined by the balance of capillary forces and viscous drag. The measured velocities (and thus the capillary forces) are well described by the LSA solution with a single fitting parameter.     

Strain rate effects on the mechanical response of polypropylene-based composites deformed at small strains

The mechanical properties and response of two polypropylene (PP)-based composites have been determined for small strains and for a range of strain rates in the quasi-static domain. These two materials are talc-filled and unfilled high-impact PP. Uniaxial tensile tests were performed at different strain rates in order to characterize the mechanical response and the strain rate effect. The experimental results showed that both unfilled and talc-filled high-impact PP were sensitive to strain rate and exhibited nonlinear behavior even at relatively low strains. SEM analysis was conducted to obtain a better comprehension of deformation mechanisms involved during loading by observations of the microstructure evolution. For each of these two materials, two existing modeling approaches are proposed. The first one is a three-parameter nonlinear constitutive model based on the experimental results. The second is a micromechanically based approach for the elastic-viscoplastic behavior of the composite materials. The stress-strain curves predicted by these models are in fairly good agreement with our experimental results. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 6, pp. 1051–1059. This article was submitted by the authors in English.     

Movement of colloidal particles in two-dimensional electric fields

We characterize the movement of carbon black particles in inhomogeneous, two-dimensional dc electric fields. Motivated by display applications, the particles are suspended in a nonpolar solvent doped with a charge control agent. The two-dimensional fields are generated between strip electrodes on a glass slide spaced 120 microm apart with field strengths up to 10(4) V/m. Such fields are insufficient to drive either electrohydrodynamic instabilities or natural convection due to ohmic heating, but they move the particles between the electrodes in about 30 s. In the center region between the strip electrodes, the particles move by electrophoresis; that is, the particle velocity is proportional to the electric field. However, when imposing a constant-potential or constant-current boundary condition at the electrodes to derive the electrical field, the electrophoretic mobility calculated from the measured particle velocities is outside the range of mobilities predicted from the theory of O'Brien and White. Near the electrodes the particles either speed up or slow down, depending on the polarity of the electrode, and these changes in velocity cannot be explained simply by electrophoresis in a spatially varying electric field. We suggest that this anomalous motion arises from electrohydrodynamic flows originating from the interaction between the space charge of the polarized layers above the electrodes and the electric field. Approximate calculations indicate such flows could be sufficiently strong to explain the anomalous trajectories near the edges of the electrodes.     

Optical absorption of small metallic particles

In an extremely small metallic particle in which electron energies take discrete values, a light is absorbed by the surface plasmon which is not decayed by the surface scattering. The usually used relation between polarizability of a particle and electrical susceptibility does not hold.     

Topological segregation in small bimetallic particles

The spatial distribution of the two components at the surface of small RhM/Al₂O₃ (M=Sn, Pb, Ge) bimetallic aggregates has been investigated by using hydrogenolysis of model alkanes, IR spectroscopy of adsorbed CO and quantum chemical calculations. The addition to a Rh aggregate of a small amount of Sn or Pb inhibits selectively the sites of low coordination, and this demonstrates that there is a topological segregation to these sites; Ge atoms appear to be randomly distributed at the surface. Quantum chemical calculations of the relative stabilities of Rh₁₂M model clusters support these conclusions.     

Chemisorption and reactions of small molecules on small gold particles

The activity of supported gold particles for a number of oxidations and hydrogenations starts to increase dramatically as the size falls below ~3 nm. This is accompanied by an increased propensity to chemisorption, especially of oxygen and hydrogen. The explanation for these phenomena has to be sought in kinetic analysis that connects catalytic activity with the strength and extent of chemisorption of the reactants, the latter depending on the electronic structure of the gold atoms constituting the active centre. Examination of the changes to the utilisation of electrons as particle size is decreased points to loss of metallic character at about 3 nm, as energy bands are replaced by levels, and a band gap appears. Detailed consideration of the Arrhenius parameters (E and ln A) for CO oxidation points clearly to a step-change in activity at the point where metallic character is lost, as opposed to there being a monotonic dependence of rate on a physical property such as the fraction of atoms at corners or edges of particles. The deplorable scarcity of kinetic information on other reactions makes extension of this analysis difficult, but non-metallic behaviour is an unavoidable property of very small gold particles, and therefore cannot be ignored when seeking to explain their exceptional activity.     

Interface Fermi level pinning at contacts between PEDOT: PSS and molecular organic semiconductors.

Photoemission studies of interfaces between molecular organic semiconductors and the conducting polymer PEDOT:PSS [mixture of PEDOT (poly-3,4-ethylenedioxy-thiophene) and PSS (polystyrenesulfonate)] demonstrate that it is impossible to control the charge injection barriers at such contacts by either a systematic change of the work function of the conducting polymer or that of the organic semiconductor. Instead, these interfaces are, in all cases, characterized by a charge transfer across the interface and a resulting Fermi level pinning. Thus interfacial charge barriers do not explain observed changes in device parameters as a function of the work function of the polymer electrode.     

Ohmic contacts between sulfonated polyaniline and metals

We have compared the resistivity, ρ, of sulfonated polyaniline (SPAN) prepared via three different synthetic routes: electrochemical synthesis with a supporting electrolyte (camphorsulfonic acid), ρ =340 Ω·m; electrochemical synthesis without a supporting electrolyte, ρ =530 Ω·m, and; chemical synthesis, ρ =166 Ω·m. Independent of the metal contact (Al, Au, Cu), SPAN forms ohmic contacts with the metal and the contact resistance r _c ~5 Ω does not correlate with the metal's work function. Electronic Publication     

An in situ FTIR study of the electrochemical oxidation of methanol at small platinum particles

It is shown for the first time that it is possible to employ electrochemical in situ Fourier transform infrared (FTIR) spectroscopy to study the electro-oxidation of methanol at small platinum particles. A number of differences with respect to the behaviour at a bulk metal electrode are observed; these observations demonstrate beyond doubt the very significant difference between bulk and particle electrode mechanisms for the oxidation of methanol, and reveal the dangers of extrapolating from the former to the latter.     

Concentrating particles on drop surfaces using external electric fields

We propose to use an externally applied uniform electric field to alter the distribution of particles on the surface of a drop immersed in another immiscible liquid. Specifically, we seek to generate well-defined concentrated regions at the drop surface while leaving the rest of the surface particle free. Experiments show that when the dielectric constant of the drop is greater than that of the ambient liquid the particles for which the Clausius-Mossotti factor is positive move along the drop surface to the two poles of the drop. Particles with a negative Clausius-Mossotti factor, on the other hand, move along the drop surface to form a ring near the drop equator. The opposite takes place when the dielectric constant of the drop is smaller than that of the ambient liquid, namely particles for which the Clausius-Mossotti factor is positive form a ring near the equator while those for which such a factor is negative move to the poles. This motion is due to the dielectrophoretic force that acts upon particles because the electric field on the surface of the drop is nonuniform, despite the uniformity of the applied electric field. Experiments also show that when small particles collect at the poles of a deformed drop the electric field needed to break the drop is smaller than without particles. These phenomena could be useful to concentrate particles at a drop surface within well-defined regions (poles and equator), separate two types of particles at the surface of a drop or increase the drop deformation to accelerate drop breakup.     

The structure of small penta-twinned gold particles

The structural feathers of penta-twinned gold particles (size between 2 and 6 nm) generated by gas evaporation have been investigated by high resolution TEM. The structural characteristic of penta-twinned particles is different from that of quasi-crystals that the five coherent or incoherent twin boundaries separating the twin oriented segments do not join up along a common edge. The lattice parameter is reduced by 4–5% in comparison to that of bulk gold. The formation of the penta-twinned particles is proposed to occur by particle collision. The particles were observed to be crystalline at ambient temperature.     

Interactions between drug delivery particles and mucin in solution and at interfaces

Cubosome particles were produced by fragmenting a cubic crystalline phase of glycerol monooleate and water in the presence of a stabilizing poly(ethylene oxide)-based polymer. The aim of our investigation was to study the interaction between these particles and mucin to gain information on how they would perform as a vehicle for mucosal drug delivery. Particle electrophoresis was used to investigate the interactions between particles and mucin in solution, and ellipsometry was utilized to study the interactions between particles and mucin-coated silica surfaces. The interaction studies were performed at relevant physiological conditions, and the pH and ionic strength were varied to gain more information about the driving forces for the interaction. The results from electrophoretic measurements showed that mucin in solution adsorbed to the particles at pH 4, whereas at pH 6 no clear interaction was detected. From ellipsometric measurements it was evident that the particles adsorb reversibly to a mucin-coated silica surface at pH 4, while no adsorption of particles could be detected at pH 6. The overall conclusion is that the interaction between these particles and mucin is weak and pH-dependent. These findings are in agreement with other investigations of the interactions between mucin and poly(ethylene oxide) chains.     

Core-electron relaxation energy in small metallic particles

A general calculation method for screening in a finite electron gas proposed by Cini, is applied to study of core-hole relaxation energy in small metallic particles. In order to obtain quantitative results, a pseudopotential theory is developed for the core-hole perturbation which provides excellent agreement with the generally accepted excited-atom model in the bulk limit. The transition and noble metals treated by means of a semi-empirical extension of the method. The present calculation method of extra-atomic relaxation energies, involving an electron gas approximation for the conduction electrons allows straightforward applications of the method of Cini in the case of a finite-spherical metal particle. The relaxation energy is found to give an important contribution to core-hole binding energy shifts in small particles.     

Synthesis and characterisation of small ZnS particles

Small ZnS particles, prepared at room temperature in an alcoholic medium using a zinc salt and thioacetamide as sulphur source, have been characterised using a suite of techniques which includes XRD, TEM and Zn K-edge EXAFS. The investigation suggests that aggregates of small sphalerite particles (cubic lattice), with average size of 3.5 nm and well-defined morphology are obtained and the particle size appears not to change with increase in the reaction time from 2 to 24 h. Zn K-edge EXAFS experiments were performed at 10 K, in order to reduce thermal disorder and the refinement of the EXAFS data resulted in very small second shell coordination numbers with respect to the bulk samples. The result is in good agreement with SEM and XRD data about the presence of nanosized particles, having a large number of surface atoms with low second shell coordination number.