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University of Twente Publications (50,998 recursos)
University of Twente Repository provides publications of UT scientific staff, in many cases full text and publicly available.

Group = KEUR

Mostrando recursos 1 - 20 de 6,908

1. The contribution of plasmon-enhanced photoluminescence to the SERS background - Le Thi Ngoc, Loan; Wiedemair, Justyna; Berg, Albert van den; Carlen, Edwin T.
We report single-photon plasmon-enhanced photoluminescence from nanostructured gold surfaces by controlling the extent of the spectral overlaps of the excitation energy hωo, the surface plasmon resonance band (with peak resonance energy hωSPR), and the critical points ΔX and ΔL, of the joint density of states of the d and sp bands, near the X- and Lsymmetry points of the electronic band structure, respectively. For hωo~ΔX (hωo<ΔX), the peak photoluminescence intensity is strongly enhanced for a strong spectral overlap of hωo~hωSPR, and weakly enhanced for a weak spectral overlap. We show that the background continuum accompanying surface-enhanced Raman scattering (SERS) spectra...

2. Frequency dependent AC electroosmotic flow in nanochannels - Beld, Wesley T.E. van den; Sparreboom, Wouter; Berg, Albert van den; Eijkel, Jan C.T.
We report frequency-dependent bidirectional AC electroosmotic flow (AC-EOF) in a nanochannel with double layer overlap. This work follows our report in μTas 2008 of unidirectional AC-EOF in nanochannels [1]. Observed is a bidirectional pumping behavior; simulations of the low frequency pumping confirm a direction opposite to that of AC-EOF in microchannels. By this frequency-dependent bidirectional pumping, nanochannel AC-EOF behaves in fundamentally different way than microchannel AC-EOF. Generally, the results are of importance for the understanding of ion and liquid transport in nanoconfinement.

3. Nanopore fabrication by heating au particles on ceramic substrates - Vreede, Lennart J. de; Berg, Albert van den; Eijkel, Jan C.T.
We found that gold nanoparticles, when heated to close to their melting point on substrates of amorphous SiO2 or amorphous Si3N4, move perpendicularly into the substrate. Dependent on applied temperatures, particles can become buried or leave nanopores of extreme aspect ratio (diameter congruent to 25 nm, length up to 800 nm). The process can be understood as driven by gold evaporation and controlled by capillary forces and can be controlled by temperature programming and substrate choice.

4. Temperature Dependence of the 1727 cm–1 Interstitial Oxygen Absorption Band Studied by Attenuated Total Internal Reflection Infrared Spectroscopy in a Newly Developed Microreactor - Susarrey-Arce, A.; Tiggelaar, R.M.; Sanders, R.G.P.; Geerdink, B.; Lefferts, L.; Gardeniers, J.G.E.; Houselt, A. van
In this work we study oxygen interstitial defects (Oi) in a silicon-technology-based microreactor with an integrated attenuated total internal reflection (ATR) crystal as a function of an externally applied potential by means of attenuated total internal reflection infrared spectroscopy (ATR-IR). A reduction of the 1727 cm–1 Oi IR absorption band is observed with increasing potential. We evidence that this band is related to the presence of Oi, which enables detection of Oi by ATR-IR spectroscopy. The observed reduction with increasing potential is ascribed to an increase in temperature, due to resistive heating of the silicon.

5. Spatial-spectral Hamiltonian Boussinesq wave simulations - Kurnia, R.; Groesen, E. van
This contribution concerns a specific simulation method for coastal wave engineering applications. As is common to reduce computational costs the flow is assumed to be irrotational so that a Boussinesq-type of model in horizontal variables only can be used. Here we advocate the use of such a model that respects the Hamiltonian structure of the wave equations. To avoid approximations of the dispersion relation by an algebraic relation that is needed for finite element/difference methods, we propose a spatial-spectral implementation which can model dispersion exactly for all wave lengths. Results with a relatively simple spatial-spectral implementation of the advanced theoretical...

6. Enhancing acoustic cavitation using artificial crevice bubbles - Zijlstra, A.G.; Fernandez Rivas, D.; Gardeniers, J.G.E.; Versluis, M.; Lohse, D.
We study the response of pre-defined cavitation nuclei driven continuously in the kHz regime (80, 100 and 200 kHz). The nuclei consist of stabilized gaspockets in cylindrical pits of 30 μμm diameter etched in silicon or glass substrates. It is found that above an acoustic pressure threshold the dynamics of the liquid–gas meniscus switches from a stable drum-like vibration to expansion and deformation, frequently resulting in detachment of microbubbles. Just above this threshold small bubbles are continuously and intermittently ejected. At elevated input powers bubble detachment becomes more frequent and cavitation bubble clouds are formed and remain in the vicinity...

7. Wettability control on carbon nanofiber surfaces: from hydrophilicity to superhydrophobicity - Nair, H.; Tiggelaar, R.M.; Houselt, A. van; Lefferts, L.

8. Control of wettability on carbon nanofiber surfaces: from hydrophilicity to superhydrophobicity - Nair, H.; Tiggelaar, R.M.; Houselt, A. van; Lefferts, L.

9. Modelling non-equilibrium bed load in a parameterized dune evolution model - Duin, O.J.M. van; Ribberink, J.S.; Dohmen-Janssen, C.M.; Hulscher, S.J.M.H.

10. Water-Induced Blister Formation in a Thin Film Polymer - Berkelaar, R.P.; Bampoulis, P.; Dietrich, E.; Jansen, H.P.; Zhang, X.; Kooij, E.S.; Lohse, D.; Zandvliet, Harold J.W.
A failure mechanism of thin film polymers immersed in water is presented: the formation of blisters. The growth of blisters is counterintuitive as the substrates were noncorroding and the polymer does not swell in water. We identify osmosis as the driving force behind the blister formation. The dynamics of the blister formation is studied experimentally as well as theoretically, and a quantitative model describing the blister growth is developed, which accurately describes the temporal evolution of the blisters.

11. Laser impact on a drop - Klein, A.L.; Visser, C.W.; Bouwhuis, W.; Lhuissier, H.E.; Snoeijer, J.H.; Villermaux, E.; Lohse, D.; Gelderblom, H.
The energy deposition in a liquid drop on a nanosecond time scale by impact of a laser pulse can induce various reactions, such as vaporization or plasma generation. The response of the drop can be extremely violent: The drop gets strongly deformed and propelled forward at several m/s, and subsequently breaks up or even explodes. These effects are used in a controlled manner during the generation of extreme ultraviolet (EUV) light in nanolithography machines for the fabrication of leading-edge semiconductor microchips. Detailed understanding of the fundamentals of this process is of key importance in order to advance the latest lithography...

12. Self-assembly triggered by self-assembly: optically active, paramagnetic micelles encapsulated in protein cage nanoparticles - Galindo Millan, J.J.; Brasch, M.; Anaya-Plaza, E.; Escosura, A de la; Velders, A.H.; Reinhoudt, D.N.; Torres, T.; Koay, M.S.T.; Cornelissen, J.J.L.M.

13. A methodology for highly accurate results of direct numerical simulations: Drag force in dense gas–solid flows at intermediate Reynolds number - Yang, Y.; Kriebitzsch, S.H.L.; Hoef, M.A. van der; Peters, E.A.J.F.; Kuipers, J.A.M.
Simulations with an iterative immersed boundary method (IBM) are performed to predict the drag force for gas–solid flows at intermediate Reynolds number (Re). A methodology is developed to obtain highly accurate IBM results at relatively low computational cost. First of all, “resolution-free” gas–solid forces are estimated for a face-centered-cubic (FCC) array of monodisperse spheres in terms of the resolution convergence. This data is subsequently used to compute the relocation of the marker points, so as to correct for the resolution dependence of the simulated force on coarser grids. We then assume that the relocation derived from FCC arrays is also...

14. Integrated DEM–CFD modeling of the contact charging of pneumatically conveyed powders - Korevaar, M.W.; Padding, J.T.; Hoef, M.A. van der; Kuipers, J.A.M.
A model is proposed that incorporates contact charging (also known as triboelectric charging) of pneumatically conveyed powders in a DEM–CFD framework, which accounts for the electrostatic interactions, both between particles and between the particles and conducting walls. The simulation results reveal that the influence of the electrostatic interaction between particle and wall is significant and should not be neglected, since it is found to influence both the spatial distribution of the powder in the duct, and the acquired charge of the particles. We find that there is a critical mean charge: only when the mean charge of the particles exceeds...

15. Simulation of Particle Mixing and Segregation in Bidisperse Gas Fluidized Beds - Olaofe, O.O.; Patil, A.V.; Deen, N.G.; Hoef, M.A. van der; Kuipers, J.A.M.
The mixing and segregation of particles of various types in gas–solid fluidized beds is a common phenomenon that is observed in experimental investigations. Generally, it is necessary to understand the phenomenon of mixing and segregation in gas-fluidized beds for the optimal design operation and scale-up of many industrial processes. To gain more insight into these, bed dynamics have been studied using a fully coupled Computational Fluid Dynamics/Discrete Element Method model (CFD/DEM), in which the particles are tracked individually using Newton's law of motion, and a newly developed continuum-based Multi-Fluid Model [MFM, van Sint Annaland et al. (2009a). Chem. Eng. Sci....

16. Nanobubble formation on a warmer substrate - Xu, C.; Peng, S.; Ciao, G.G.; Gutowski, V.; Lohse, D.; Zhang, X.
The solvent exchange procedure is an often-used protocol to produce surface nanobubbles. In this procedure, the substrate is exposed to a good solvent for gas which is then mixed and rinsed with a poor solvent for gas and the nanobubbles form on the solid–liquid interface. Here we study the effects of temperatures of the substrate and the first solvent on nanobubble formation. Atomic force microscopy with temperature control was used to examine the formation of nanobubbles at temperatures between 37 C and 54 C. It was found that the probability of nanobubble formation was larger on substrates at higher temperatures. Moreover,...

17. Forces encountered by a sphere during impact into sand - Joubaud, S.; Homan, T.A.M.; Gasteuil, Y.; Lohse, D.; Meer, R.M. van der
We describe direct measurements of the acceleration of an object impacting on a loosely packed granular bed under various pressures, using an instrumented sphere. The sphere acts as a noninvasive probe that measures and continuously transmits the acceleration as it penetrates into the sand, using a radio signal. The time-resolved acceleration of the sphere reveals the detailed dynamics during the impact that cannot be resolved from the position information alone. Because of the unobstructed penetration, we see a downward acceleration of the sphere at the moment the air cavity collapses. The compressibility of the sand bed is observed through the...

18. Nonlinear instability and convection in a vertically vibrated granular bed - Shukla, P.; Ansari, I.H.; Meer, R.M. van der; Lohse, D.; Alam, M.
The nonlinear instability of the density-inverted granular Leidenfrost state and the resulting convective motion in strongly shaken granular matter are analysed via a weakly nonlinear analysis of the hydrodynamic equations. The base state is assumed to be quasi-steady and the effect of harmonic shaking is incorporated by specifying a constant granular temperature at the vibrating plate. Under these mean-field assumptions, the base-state temperature decreases with increasing height away from the vibrating plate, but the density profile consists of three distinct regions: (i) a collisional dilute layer at the bottom, (ii) a levitated dense layer at some intermediate height and (iii)...

19. Exploring the phase diagram of fully turbulent Taylor–Couette flow - Ostilla Monico, R.; Poel, E.P. van der; Verzicco, R.; Grossmann, S.; Lohse, D.
Direct numerical simulations of Taylor–Couette flow, i.e. the flow between two coaxial and independently rotating cylinders, were performed. Shear Reynolds numbers of up to 3×10 5 , corresponding to Taylor numbers of Ta=4.6×10 10 , were reached. Effective scaling laws for the torque are presented. The transition to the ultimate regime, in which asymptotic scaling laws (with logarithmic corrections) for the torque are expected to hold up to arbitrarily high driving, is analysed for different radius ratios, different aspect ratios and different rotation ratios. It is shown that the transition is approximately independent of the aspect and rotation ratios, but...

20. Exposing nanobubble-like objects to a degassed environment - Berkelaar, R.P.; Dietrich, E.; Kip, G.A.M.; Kooij, E.S.; Zandvliet, H.J.W.; Lohse, D.

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