## Recursos de colección

#### University of Twente Publications (58.714 recursos)

University of Twente Repository provides publications of UT scientific staff, in many cases full text and publicly available.

Group = KEUR

1. #### Scaling laws and flow structures of double diffusive convection in the finger regime

Yang, Yantao; Verzicco, Roberto; Lohse, Detlef

2. #### Pattern Formation by Staphylococcus epidermidis via Droplet Evaporation on Micropillars Arrays at a Surface

Susarrey-Arce, A.; Gomez Marin, A.; Massey, A.; Oknianska, A.; Diaz-Fernandez, Y.; Hernandez Sanchez, Jose Federico; Griffiths, E.; Gardeniers, J.G.E.; Snoeijer, J.H.; Lohse, D.; Raval, R.
We evaluate the effect of epoxy surface structuring on the evaporation of water droplets containing Staphylococcus epidermidis (S. epidermidis). During evaporation, droplets with S. epidermidis cells yield to complex wetting patterns such as the zipping-wetting1−3 and the coffee-stain effects. Depending on the height of the microstructure, the wetting fronts propagate circularly or in a stepwise manner, leading to the formation of octagonal or square-shaped deposition patterns.4,5 We observed that the shape of the dried droplets has considerable influence on the local spatial distribution of S. epidermidis deposited between micropillars. These changes are attributed to an unexplored interplay between the zipping-wetting1...

3. #### Volume entrained in the wake of a disk intruding into an oil-water interface

Peters, I.R.; Madonia, M.; Lohse, D.; Meer, R.M. van der
An object moving through a plane interface into a fluid deforms the interface in such a way that fluid from one side of the interface is entrained into the other side, a phenomenon known as Darwin's drift. We investigate this phenomenon experimentally using a disk which is started exactly at the interface of two immiscible fluids, namely, oil and water. First, we observe that due to the density difference between the two fluids the deformation of the interface is influenced by gravity and show that there exists a time window of universal behavior. Second, we show by comparing with boundary...

4. #### Vapour-bubble nucleation and dynamics in turbulent Rayleigh–Bénard convection

Narezo Guzman, Daniela; Frączek, T.; Reetz, C.; Sun, C.; Lohse, D.; Ahlers, G.
Vapour bubbles nucleating at micro-cavities etched into the silicon bottom plate of a cylindrical Rayleigh–Bénard sample (diameter $D=8.8$D=8.8 cm, aspect ratio ${\it\Gamma}\equiv D/L\simeq 1.00$Γ≡D/L≃1.00 where $L$L is the sample height) were visualized from the top and from the side. A triangular array of cylindrical micro-cavities (with a diameter of $30~{\rm\mu}\text{m}$30 μm and a depth of $100~{\rm\mu}\text{m}$100 μm) covered a circular centred area (diameter of 2.5 cm) of the bottom plate. Heat was applied to the sample only over this central area while cooling was over the entire top-plate area. Bubble sizes and frequencies of departure from the bottom plate are...

5. #### Turbulent Taylor–Couette flow with stationary inner cylinder

Ostilla-Monico, R.; Verzicco, R.; Lohse, D.
A series of direct numerical simulations were performed of Taylor–Couette (TC) flow, the flow between two coaxial cylinders, with the outer cylinder rotating and the inner one fixed. Three cases were considered, where the Reynolds number of the outer cylinder was $Re_{o}=5.5\times 10^{4}$Reo=5.5×104, $Re_{o}=1.1\times 10^{5}$Reo=1.1×105 and $Re_{o}=2.2\times 10^{5}$Reo=2.2×105. The ratio of radii ${\it\eta}=r_{i}/r_{o}$η=ri/ro was fixed to ${\it\eta}=0.909$η=0.909 to mitigate the effects of curvature. Axially periodic boundary conditions were used, with the aspect ratio of vertical periodicity ${\it\Gamma}$Γ fixed to ${\it\Gamma}=2.09$Γ=2.09. Being linearly stable, TC flow with outer cylinder rotation is known to have very different behaviour than TC flow with...

6. #### Transition to geostrophic convection: the role of the boundary conditions

Kunnen, R.P.J.; Ostilla-Monico, Rodolfo; Poel, E.P. van der; Verzicco, R.; Lohse, D.
Rotating Rayleigh–Bénard convection, the flow in a rotating fluid layer heated from below and cooled from above, is used to analyse the transition to the geostrophic regime of thermal convection. In the geostrophic regime, which is of direct relevance to most geo- and astrophysical flows, the system is strongly rotating while maintaining a sufficiently large thermal driving to generate turbulence. We directly simulate the Navier–Stokes equations for two values of the thermal forcing, i.e. Ra=1010 and Ra=5×1010, at constant Prandtl number Pr=1, and vary the Ekman number in the range Ek=1.3×10−7 to Ek=2×10−6, which satisfies both requirements of supercriticality and...

7. #### Taylor–Couette turbulence at radius ratio η=0.5: scaling, flow structures and plumes

Veen, R.C.A. van der; Huisman, S.G.; Merbold, S.; Harlander, U.; Egbers, C.; Lohse, D.; Sun, C.
Using high-resolution particle image velocimetry, we measure velocity profiles, the wind Reynolds number and characteristics of turbulent plumes in Taylor–Couette flow for a radius ratio of 0.5 and Taylor number of up to 6:2 109. The extracted angular velocity profiles follow a log law more closely than the azimuthal velocity profiles due to the strong curvature of this D 0:5 set-up. The scaling of the wind Reynolds number with the Taylor number agrees with the theoretically predicted 3=7 scaling for the classical turbulent regime, which is much more pronounced than for the wellexplored D 0:71 case, for which the ultimate regime...

8. #### Role of natural convection in the dissolution of sessile droplets

Dietrich, E.; Wildeman, S.; Visser, C.W.; Hofhuis, K.A.; Kooij, E.S.; Zandvliet, H.J.W.; Lohse, D.
The dissolution process of small (initial (equivalent) radius R0 < 1 mm) long-chain alcohol (of various types) sessile droplets in water is studied, disentangling diffusive and convective contributions. The latter can arise for high solubilities of the alcohol, as the density of the alcohol–water mixture is then considerably less than that of pure water, giving rise to buoyancy-driven convection. The convective flow around the droplets is measured, using micro-particle image velocimetry (mPIV) and the schlieren technique. When non-dimensionalizing the system, we find a universal Sh Ra1=4 scaling relation for all alcohols (of different solubilities) and all droplets in the convective...

9. #### Soil stochastic parameter correlation impact in the piping erosion failure estimation of riverine flood defences, doi:10.1016/j.strusafe.2016.01.004

Aguilar Lopez, J.P.; Warmink, J.J.; Schielen, R.M.J.; Hulscher, S.J.M.H.
Piping erosion has been proved to be one of the failure mechanisms that contributes the most to the total probability of failure on the Dutch flood defence systems. The present study aimed to find the impact of correlation and tail dependence between soil parameters present in the Sellmeijer revised limit state equation for piping safety assessment, particularly between the grain size and hydraulic conductivity parameters. A copula based random sampling method was used as a tool to include this effect in the probabilistic estimation of this type of failure. The method was framed in a real case study for a...

10. #### Response of large-scale coastal basins to wind forcing: influence of topography

Chen, WenLong; Roos, P.C.; Schuttelaars, H.M.; Kumar, M.; Zitman, T.J.; Hulscher, S.J.M.H.
Because wind is one of the main forcings in storm surge, we present an idealised process-based model to study the influence of topographic variations on the frequency response of large-scale coastal basins subject to time-periodic wind forcing. Coastal basins are represented by a semi-enclosed rectangular inner region forced by wind. It is connected to an outer region (represented as an infinitely long channel) without wind forcing, which allows waves to freely propagate outward. The model solves the three-dimensional linearised shallow water equations on the f plane, forced by a spatially uniform wind field that has an arbitrary angle with respect...

11. #### Effect of roll number on the statistics of turbulent Taylor-Couette flow

Ostilla Monico, Rodolfo; Lohse, Detlef; Verzicco, Roberto
A series of direct numerical simulations in large computational domains has been performed in order to probe the spatial feature robustness of the Taylor rolls in turbulent Taylor-Couette flow. The latter is the flow between two coaxial independently rotating cylinders of radius r i and r o , respectively. Large axial aspect ratios Γ=7–8 [with Γ=L/(r o −r i ) , and L the axial length of the domain] and a simulation with Γ=14 were used in order to allow the system to select the most unstable wave number and to possibly develop multiple states. The radius ratio was taken...

12. #### Identifying coherent structures and vortex clusters in Taylor-Couette turbulence

Spandan, Vamsi; Ostilla-Monico, Rodolfo; Lohse, Detlef; Verzicco, Roberto
The nature of the underlying structures in Taylor-Couette (TC) flow, the flow between two co-axial and independently rotating cylinders is investigated by two methods. First, the quadrant analysis technique for identifying structures with intense radial-azimuthal stresses (also referred to as 'Q's) of Lozano-Durán et al., (J. Fluid Mech. 694, 100-130) is used to identify the main structures responsible for the transport of angular velocity. Second, the vortex clusters are identified based on the analysis by del Álamo et al., (J. Fluid. Mech., 561, 329-358). In order to test these criteria, two different radius ratios η = ri/ro are considered, where...

13. #### Controlling the Growth Modes of Femtoliter Sessile Droplets Nucleating on Chemically Patterned Surfaces

Bao, L.; Werbiuk, Z.; Lohse, D.; Zhang, X.
Femtoliter droplet arrays on immersed substrates are essential elements in a broad range of advanced droplet-based technologies, such as light manipulation, sensing, and high throughput diagnosis. Solvent exchange is a bottom-up approach for producing those droplets from a pulse of oil oversaturation when a good solvent of the droplet liquid is displaced by a poor solvent. The position and arrangement of the droplets are regulated by chemical micropatterns on the substrate. Here we show experimentally and theoretically that the growth modes of droplets confined in planar micropatterns on the surface can be manipulated through the laminar flow of the solvent...

14. #### Heat and momentum transport scalings in horizontal convection

Shishkina, Olga; Grossmann, Siegfried; Lohse, Detlef
In a horizontal convection (HC) system heat is supplied and removed exclusively through a single, top, or bottom, surface of a fluid layer. It is commonly agreed that in the studied Rayleigh number (Ra) range, the convective heat transport, measured by the Nusselt number, follows the Rossby (1965) scaling, which is based on the assumptions that the HC flows are laminar and determined by their boundary layers. However, the universality of this scaling is questionable, as these flows are observed to become more turbulent with increasing Ra. Here we propose a theoretical model for heat and momentum transport scalings with...

15. #### An 8-10 GHz upconversion mixer, with a low-frequency calibration loop resulting in better than -73dbc in-band spurs

Withagen, Johan C.J.G.; Annema, A.J.; Nauta, B.; Vliet, F.E. van
Abstract—An 8-10 GHz X-band upconversion quadrature mixer stage implemented in 250 nm SiGe BiCMOS is presented. Orthogonality of the spurious responses caused by clock feed through, I/Q mismatch and baseband harmonics after self-mixing was exploited to realize a baseband calibration scheme reducing all in-band spurs down to below -73dBc, for baseband signals up to a bandwidth of 2MHz and with an IF center frequency up to 100MHz. Utilizing a low-frequency output spectrum analysis of an integrated self-mixer at the upconversion mixer output for calibration, eliminates the need for expensive microwave frequency spectrum analyzers

16. #### Towards microfluidic sperm refinement : impedance-based analysis and sorting of sperm cells

Wagenaar, B. de; Dekker, S.; Boer, H.L. de; Bomer, J.G.; Olthuis, W.; Berg, A. van den; Segerink, L.I.
The use of high quality semen for artificial insemination in the livestock industry is essential for successful outcome. Insemination using semen with a high number of sperm cells containing morphological defects has a negative impact on fertilization outcome. Therefore, semen with a high number of these abnormal cells is discarded in order to maintain high fertilization potential, resulting in the loss of a large number of morphologically normal sperm cells (up to 70-80% of original sample). A commonly occurring morphological sperm anomaly is the cytoplasmic droplet on the sperm flagella. Currently, no techniques are available to extract morphologically normal sperm...

17. #### Photopatterning of hydrogel microarryas in closed microchips

Gumuscu, Burcu; Bomer, Johan G.; Berg, Albert van den; Eijkel, Jan C.T.
To date, optical lithography has been extensively used for in situ patterning of hydrogel structures in a scale range from hundreds of microns to a few millimeters. The two main limitations which prevent smaller feature sizes of hydrogel structures are (1) the upper glass layer of a microchip maintains a large spacing (typically 525 mu m) between the photomask and hydrogel precursor, leading to diffraction of UV light at the edges of mask patterns, (2) diffusion of free radicals and monomers results in irregular polymerization near the illumination interface. In this work, we present a simple approach to enable the...

18. #### Book review: Crime and Everyday Life by Marcus Felson and Mary Eckert, 5th edition

Hartel, P.H.; Junger, M.

19. #### Direct quantification of transendothelial electrical resistance in organs-on-chips

Helm, Marinke W. van der; Odijk, M.; Frimat, Jean-Philippe; Meer, Andries D. van der; Eijkel, Jan C.T.; Berg, Albert van den; Segerink, Loes I.
Measuring transendothelial or transepithelial electrical resistance (TEER) is a widely used method to monitor cellular barrier tightness in organs-on-chips. Unfortunately, integrated electrodes close to the cellular barrier hamper visual inspection of the cells or require specialized cleanroom processes to fabricate see-through electrodes. Out-of-view electrodes inserted into the chip's outlets are influenced by the fluid-filled microchannels with relatively high resistance. In this case, small changes in temperature or medium composition strongly affect the apparent TEER. To solve this, we propose a simple and universally applicable method to directly determine the TEER in microfluidic organs-on-chips without the need for integrated electrodes close...

20. #### Surface- and tip-enhanced raman spectroscopy in catalysis

Hartman, Thomas; Wondergem, Caterina S.; Kumar, Naresh; Berg, Albert van den; Weckhuysen, Bert M.
Surface- and tip-enhanced Raman spectroscopy (SERS and TERS) techniques exhibit highly localized chemical sensitivity, making them ideal for studying chemical reactions, including processes at catalytic surfaces. Catalyst structures, adsorbates, and reaction intermediates can be observed in low quantities at hot spots where electromagnetic fields are the strongest, providing ample opportunities to elucidate reaction mechanisms. Moreover, under ideal measurement conditions, it can even be used to trigger chemical reactions. However, factors such as substrate instability and insufficient signal enhancement still limit the applicability of SERS and TERS in the field of catalysis. By the use of sophisticated colloidal synthesis methods and...

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