Simon Kuhn

Size-Controlled Flow Synthesis of Gold Nanoparticles Using a Segmented Flow Microfluidic Platform

Segmented flow is often used in the synthesis of nanomaterials to achieve narrow particle size distribution. The narrowness of the distribution is commonly attributed to the reduced dispersion associated with segmented flows. On the basis of the analysis of flow fields and the resulting particle size distribution, we demonstrate that it is the slip velocity between the two fluids and internal mixing in the continuous-phase slugs that govern the nature of the particle size distribution. The reduction in the axial dispersion has less impact on particle growth and hence on the particle size distribution. Synthesis of gold nanoparticles from HAuCl(4) with rapid reduction by NaBH(4) serves as a model system. Rapid reduction yields gold nuclei, which grow by agglomeration, and it is controlled by the interaction of the nuclei with local flow. Thus, the difference in the physical properties of the two phases and the inlet flow rates ultimately control the particle growth. Hence, a careful choice of continuous and dispersed phases is necessary to control the nanoparticle size and size distribution.

Diamond colour centres as a nanoscopic light source for scanning near‐field optical microscopy

Recently it was shown that a single molecule at cryogenic temperatures could be used as a local light source for illumination of a sample in the near field. Conventional light‐emitting systems such as dye molecules and semiconductor quantum dots could also be used for this purpose, but they suffer from lack of photostability. However, colour centres in diamond have been found to be remarkably stable against bleaching and blinking effects. Here we present the first SNOM images taken with nanoscopic diamond crystals as a light source.

Modification of single molecule fluorescence close to a nanostructure: radiation pattern, spontaneous emission and quenching

The coupling of nanostructures with emitters opens up ways for the realization of man-made subwavelength light emitting elements. In this article we present an overview of our recent progress in the modification of fluorescence when an emitter is placed close to a nanostructure. In order to control the wealth of parameters that contribute to this process, we have combined scanning probe technology with single molecule microscopy and spectroscopy. We discuss the enhancement and reduction of molecular excitation and emission rates in the presence of a dielectric or metallic nanoparticle and emphasize the role of plasmon resonances in the latter. Furthermore, we examine the spectral and angular emission characteristics of the molecule–particle system. The experimental findings are in excellent semi-quantitative agreement with the outcome of theoretical calculations. We show that the interaction of an emitter with a nanoparticle can be expressed in the framework of an optical nanoantennae and propose arrangements that could lead to the modification of spontaneous emission by more than 1000 times.

Controlled photon transfer between two individual nanoemitters via shared high-Q modes of a microsphere resonator

We realize controlled cavity-mediated photon transfer between two single nanoparticles over a distance of several tens of micrometers. First, we show how a single nanoscopic emitter attached to a near-field probe can be coupled to high-Q whispering-gallery modes of a silica microsphere at will. Then we demonstrate transfer of energy between this and a second nanoparticle deposited on the sphere surface. We estimate the photon transfer efficiency to be about 6 orders of magnitude higher than that via free-space propagation at comparable separations.

Teflon-Coated Silicon Microreactors: Impact on Segmented Liquid−Liquid Multiphase Flows

We describe fluoropolymer modification of silicon microreactors for control of wetting properties in chemical synthesis applications and characterize the impact of the coating on liquid-liquid multiphase flows of solvents and water. Annular flow of nitrogen gas and a Teflon AF (DuPont) dispersion enable controlled evaporation of fluoropolymer solvent, which in turn brings about three-dimensional polymer deposition on microchannel walls. Consequently, the wetting behavior is switched from hydrophilic to hydrophobic. Analysis of microreactors reveals that the polymer layer thickness increases down the length of the reactor from ∼1 to ∼13 μm with an average thickness of ∼7 μm. Similarly, we show that microreactor surfaces can be modified with poly(tetrafluoroethylene) (PTFE). These PTFE-coated microreactors are further characterized by measuring residence time distributions in segmented liquid-liquid multiphase flows, which display reduced axial dispersion for the coated microreactors. Applying particle image velocimetry, changes in segment shape and velocity fluctuations are observed resulting in reduced axial dispersion. Furthermore, the segment size distribution is narrowed for the hydrophobic microreactors, enabling further control of residence distributions for synthesis and screening applications.

Aerobic oxidations in flow: opportunities for the fine chemicals and pharmaceuticals industries

Molecular oxygen is without doubt the greenest oxidant for redox reactions, yet aerobic oxidation is one of the most challenging to perform with good chemoselectivity, particularly on an industrial scale. This collaborative review (between teams of chemists and chemical engineers) describes the current scientific and operational hurdles that prevent the utilisation of aerobic oxidation reactions for the production of speciality chemicals and active pharmaceutical ingredients (APIs). The safety aspects of these reactions are discussed, followed by an overview of (continuous flow) reactors suitable for aerobic oxidation reactions that can be applied on scale. Some examples of how these reactions are currently performed in the industrial laboratory (in batch and in flow) are presented, with particular focus on the scale-up strategy. Last but not least, further challenges and future perspectives are presented in the concluding remarks.

Wavy wall effects on turbulence production and large-scale modes

We describe how outer flow turbulence phenomena depend on the interaction with the wall. We quantify the influence of highly rough surfaces on turbulence statistics, eigenmodes and spectra of eigenvalues in the outer part of the wall shear layer of an isothermal turbulent flow. Surface geometries with different amplitude-to-wavelength ratios can be regarded as surfaces of defined wall roughness. The wavelength, Λ, and the wave amplitude, 2a, of a water channel are varied by changing the bottom wall in the test section. We consider three sinusoidal wall profiles defined by the amplitude-to-wavelength ratio α = 2a/Λ = 0.2 (Λ = 30 mm), α = 0.2 (Λ = 15 mm), and α = 0.1 (Λ = 30 mm). Digital particle image velocimetry is performed to examine the spatial variation of the streamwise, spanwise and wall-normal velocity components. Statistical quantities that are determined are the Reynolds stresses, turbulent kinetic energy, and turbulent energy production. Measurements are performed at a Reynolds number of 11200, defined by the half channel height and the bulk velocity. A characteristic length scale in the most dominant eigenmodes can be found that agrees for all wall profiles and is independent of the amplitude-to-wavelength ratio. The turbulence quantities compared at defined streamwise coordinates in wall-normal direction are independent of the wavy wall profile in the outer region when scaled with the friction velocity, u* w .

Cell Clarification and Size Separation Using Continuous Countercurrent Magnetophoresis

Nonmagnetic microparticles (e.g., cells, polymer beads) immersed in a magnetic fluid (ferrofluid) under a nonuniform magnetic field experience a magnetophoretic force in the direction of decreasing magnetic field strength. This phenomenon was exploited in the development of a continuous magnetophoretic countercurrent separation for the removal and concentration of micron‐sized particles from aqueous suspensions, and in particular as a viable approach for cell clarification of raw fermentation broth. A magnetic fluid is added to the cell suspension, the mixture is introduced to the magnetic separator, which consists of an open flow tube passing between pairs of magnets that move in a direction counter to the flow of the suspension. The cells are pushed ahead of the magnet pairs owing to the magnetophoretic forces acting on them, collected in a tube upstream of the feed injection point, and removed as a concentrated suspension for further treatment.

Synergy of Microfluidics and Ultrasound: Process Intensification Challenges and Opportunities

A compact snapshot of the current convergence of novel developments relevant to chemical engineering is given. Process intensification concepts are analysed through the lens of microfluidics and sonochemistry. Economical drivers and their influence on scientific activities are mentioned, including innovation opportunities towards deployment into society. We focus on the control of cavitation as a means to improve the energy efficiency of sonochemical reactors, as well as in the solids handling with ultrasound; both are considered the most difficult hurdles for its adoption in a practical and industrial sense. Particular examples for microfluidic clogging prevention, numbering-up and scaling-up strategies are given. To conclude, an outlook of possible new directions of this active and promising combination of technologies is hinted.

Coupling of plasmonic nanoparticles to their environments in the context of van der Waals-Casimir interactions

We present experiments in which the interaction of a single gold nanoparticle with glass substrates or with another gold particle can be tuned by in situ control of their separations using scanning probe technology. We record the plasmon resonances of the coupled systems as a function of the polarization of the incident field and the particle position. The distinct spectral changes of the scattered light from the particle pair are in good agreement with the outcome of finite-difference time-domain calculations. We believe that our experimental technique holds promise for the investigation of the van der Waals-Casimir-type interactions between nanoscopic neutral bodies.