A coin vibrational motor swimming at low Reynolds number

Low-cost coin vibrational motors, used in haptic feedback, exhibit rotational internal motion inside a rigid case. Because the motor case motion exhibits rotational symmetry, when placed into a fluid such as glycerin, the motor does not swim even though its oscillatory motions induce steady streaming in the fluid. However, a piece of rubber foam stuck to the curved case and giving the motor neutral buoyancy also breaks the rotational symmetry allowing it to swim. We measured a 1 cm diameter coin vibrational motor swimming in glycerin at a speed of a body length in 3 seconds or at 3 mm/s. The swim speed puts the vibrational motor in a low Reynolds number regime similar to bacterial motility, but because of the oscillations of the motor it is not analogous to biological organisms. Rather the swimming vibrational motor may inspire small inexpensive robotic swimmers that are robust as they contain no external moving parts. A time dependent Stokes equation planar sheet model suggests that the swim speed depends on a steady streaming velocity V _stream ~ Re _s ^1/2 U ₀ where U ₀ is the velocity of surface oscillations, and streaming Reynolds number Re _s = U ₀ ² /(ων) for motor angular frequency ω and fluid kinematic viscosity ν.     

World First for Diamond in Synchrotron-Based IR Photothermal Nanospectroscopy

For the first time, infrared spectra on the sub-wavelength scale have been delivered by a synchrotron-radiation-induced thermal expansion technique [1 P.M. Donaldson, Optics Express 24(3), 1852–1864 (2016).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]]. The novel experimental result was achieved by coupling an atomic force microscope (AFM) to an infrared (IR) beamline at the UK's national synchrotron facility, Diamond Light Source. Via broadband synchrotron illumination and an AFM sub-micron tip, molecular IR spectra were obtained by detecting a resonance-enhanced (RE) photothermal signal with spatial resolution beyond the diffraction limit. Together with results on synchrotron IR nanoscopy in scattering mode from the IR beamline at the Advanced Light Source two years ago, the Diamond photothermal nanoprobe approach moves vibrational analysis beyond the diffraction limit and into nanoscale absorption spectroscopy.     

Desorption-limited mechanism of release from polymer nanofibers

This work examines the release of a model water-soluble compound from electrospun polymer nanofiber assemblies. Such release attracts attention in relation to biomedical applications, such as controlled drug delivery. It is also important for stem cell attachment and differentiation on biocompatible electrospun nanofiber scaffolds containing growth factors, which have been encapsulated by means of electrospinning. Typically, the release mechanism has been attributed to solid-state diffusion of the encapsulated compound from the fibers into the surrounding aqueous bath. Under this assumption, a 100% release of the encapsulated compound is expected in a certain (long) time. The present work focuses on certain cases where complete release does not happen, which suggests that solid-state diffusion may not be the primary mechanism at play. We show that in such cases the release rate can be explained by desorption of the embedded compound from nanopores in the fibers or from the outer surface of the fibers in contact with the water bath. After release, the water-soluble compound rapidly diffuses in water, whereas the release rate is determined by the limiting desorption stage. A model system of Rhodamine 610 chloride fluorescent dye embedded in electrospun monolithic poly(methylmethacrylate) (PMMA) or poly(caprolactone) (PCL) nanofibers, in nanofibers electrospun from PMMA/PCL blends, or in core-shell PMMA/PCL nanofibers is studied. Both the experimental results and theory point at the above mentioned desorption-related mechanism, and the predicted characteristic time, release rate, and effective diffusion coefficient agree fairly well with the experimental data. A practically important outcome of this surface release mechanism is that only the compound on the fiber and pore surfaces can be released, whereas the material encapsulated in the bulk cannot be freed within the time scales characteristic of the present experiments (days to months). Consequently, in such cases, complete release is impossible. We also demonstrate how the release rate can be manipulated by the polymer content and molecular weight affecting nanoporosity and the desorption enthalpy, as well as by the nanofiber structure (monolithic fibers, fibers from polymer blends, and core-shell fibers). In particular, it is shown that, by manipulating the above parameters, release times from tens of hours to months can be attained.     

FTIR imaging coupled with multivariate analysis for study of initial diffusion of different solvents in cellulose acetate butyrate films

Fourier transform infrared (FTIR) spectroscopic imaging was used to study the initial diffusion of different solvents in cellulose acetate butyrate (CAB) films containing different amounts of acetyl and butyryl substituents. Different solvents and solvent/non-solvent mixtures were also studied. The FTIR imaging system allowed acquisition of sequential images of the CAB films as solvent penetration proceeded without disturbing the system. The interface between the non-swollen polymer and the initial swelling front could be identified using multivariate data analysis tools. For a series of ketone solvents the initial diffusion coefficients and diffusion rates could be quantified and were found to be related to the polar and hydrogen interaction parameters in the Hansen solubility parameters of the solvents. For the solvent/non-solvent system the initial diffusion rate decreased less than linearly with the weight-percent of non-solvent present in the solution, which probably was due to the swelling characteristic of the non-solvent. For a given solvent, increasing the butyryl content of the CAB increased the initial diffusion rate. Increasing the butyryl content from 17 wt.% butyryl to 37 wt.% butyryl produced a considerably larger increase in initial diffusion rate compared to an increase in butyryl content from 37 wt.% to 50 wt.% butyryl.     

Fast energy transfer within a self-assembled cyclic porphyrin tetramer

The structure of a cyclic self-assembled tetramer of an asymmetric meso-ethynylpyridyl-functionalized Zn(II)-porphyrin was established by solution-phase X-ray scattering and diffraction; femtosecond transient absorption and anisotropy spectroscopies were used to (a) observe rapid energy transfer (3.8 ps(-1)) between porphyrin subunits and (b) establish that the transfer occurs between adjacent units.     

Resonance hyper-Raman spectra of zinc phthalocyanine

Hyper-Raman spectra were obtained for zinc phthalocyanine in a dilute pyridine solution at excitation wavelengths that are two-photon resonant with the one-photon-allowed B band (360-380 nm) as well as with the two-photon absorption near 440 nm reported by Drobizhev et al. ( J. Chem. Phys. 2006, 124, 224701 ). In both regions, the hyper-Raman spectra were very different from the linear resonance Raman spectra at the corresponding excitation frequencies. While the resonance Raman spectra show only g symmetry modes, almost all of the hyper-Raman frequencies can be assigned as fundamentals of E u symmetry that also are observed in the infrared absorption spectrum or E u symmetry combination bands. These results contrast sharply with previous observations of highly noncentrosymmetric push-pull conjugated molecules and are consistent with a structure for phthalocyanine in solution that is centrosymmetric or nearly so. The hyper-Raman spectra show different intensity patterns in the two excitation regions, consistent with different Franck-Condon and/or vibronic coupling matrix elements for the different resonant states.     

Evidence for a reactive (alkene)peroxoiridium(III) intermediate in the oxidation of an alkene complex with O2

An (alkene)peroxoiridium(III) complex, [Ir(L)(cod)(O(2))] [where LH = PhN=C(NMe(2))NHPh and cod = 1,5-cyclooctadiene], was identified as an intermediate in the reaction of the Ir(I) precursor [Ir(L)(cod)] with O(2) and characterized by spectroscopic methods. Decay of the intermediate and further reaction with 1,5-cyclooctadiene produced 4-cycloocten-1-one.     

Controlled, simultaneous assembly of polyethylenimine onto nanoparticle silica colloids

A novel precision-assembly methodology is described on the basis of the controlled, simultaneous assembly (CSA) of a core nanoparticle substrate and polyelectrolyte solutions. The method is capable of assembly rates at least as fast as 10(16) core particles s(-1) L(-1) and affords concentrated suspensions of stable colloids with an adsorbed polyelectrolyte. The resulting dispersions are highly homogeneous, have a low viscosity and narrow particle-size distribution, and are stable colloids, even at solid concentrations of at least 33 wt %. The adsorption isotherm and the saturation adsorption for polyethylenimine (PEI) assemblies onto a 15 nm silica colloid have been evaluated with 1H NMR spectroscopy. The saturation adsorption is highly dependent upon the pH at assembly and is given by the equation PEIa (micromol m(-2)) = 1.73pH - 1.89, R2 = 0.986, where micromoles refers to the concentration of the EI monomer. The saturation concentration increases from 6.8 micromol m(-2) at pH 5.0 to 13.7 micromol m(-2) at pH 9.0. The adsorbed polyelectrolyte may be cross-linked and thereby permanently fixed to the colloid surface to prepare nanoparticle-polyelectrolyte colloidal assemblies having enhanced colloid stability, high homogeneity, and a high fraction (>80%) of permanently adsorbed polyelectrolyte. These assemblies are stable at physiological pH and ionic strength and may represent ideal substrates for bioconjugation and, ultimately, the design of nanocarriers for in vivo applications.     

Combining Trust-Region Techniques and Rosenbrock Methods to Compute Stationary Points

Rosenbrock methods are popular for solving a stiff initial-value problem of ordinary differential equations. One advantage is that there is no need to solve a nonlinear equation at every iteration, as compared with other implicit methods such as backward difference formulas or implicit Runge–Kutta methods. In this article, we introduce a trust-region technique to select the time steps of a second-order Rosenbrock method for a special initial-value problem, namely, a gradient system obtained from an unconstrained optimization problem. The technique is different from the local error approach. Both local and global convergence properties of the new method for solving an equilibrium point of the gradient system are addressed. Finally, some promising numerical results are also presented. This research was supported in part by Grant 2007CB310604 from National Basic Research Program of China, and #DMS-0404537 from the United States National Science Foundation, and Grant #W911NF-05-1-0171 from the United States Army Research Office, and the Research Grant Council of Hong Kong.