Compartmentalized Intracellular Click Chemistry with Biodegradable Polymersomes.

Polymersome nanoreactors that can be employed as artificial organelles have gained much interest over the past decades. Such systems often include biological catalysts (i.e., enzymes) so that they can undertake chemical reactions in cellulo. Examples of nanoreactor artificial organelles that acquire metal catalysts in their structure are limited, and their application in living cells remains fairly restricted. In part, this shortfall is due to difficulties associated with constructing systems that maintain their stability in vitro, let alone the toxicity they impose on cells. This study demonstrates a biodegradable and biocompatible polymersome nanoreactor platform, which can be applied as an artificial organelle in living cells. The ability of the artificial organelles to covalently and non-covalently incorporate tris(triazolylmethyl)amine-Cu(I) complexes in their membrane is shown. Such artificial organelles are capable of effectively catalyzing a copper-catalyzed azide-alkyne cycloaddition intracellularly, without compromising the cells' integrity. The platform represents a step forward in the application of polymersome-based nanoreactors as artificial organelles.

Sodium chloride inhibits effective bubbly drag reduction in turbulent bubbly Taylor-Couette flows.

Using the Taylor-Couette geometry we experimentally investigate the effect of salt on drag reduction caused by bubbles present in the flow. We combine torque measurements with optical high-speed imaging to relate the bubble size to the drag experienced by the flow. Previous studies have shown that a small percentage of air (4%) can lead to dramatic drag reduction (40%). In contrast to previous laboratory experiments, which mainly used fresh water, we will vary the salinity from that of fresh water to the average salinity of ocean water. We find that the drag reduction is increasingly more inhibited for increasing salt concentrations, going from 40% for fresh water to just 15% for sea water. Salts present in the working fluid inhibit coalescence events, resulting in smaller bubbles in the flow and, with that, a decrease in the drag reduction. Above a critical salinity, increasing the salinity has no further effect on the size of bubbles in the flow and thus the drag experienced by the flow. Our new findings demonstrate the importance of sodium chloride on the bubbly drag reduction mechanism, and will further challenge naval architects to implement promising air lubrication systems on marine vessels. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

Recent developments of turbulent emulsions in Taylor-Couette flow.

Emulsions are common in many natural and industrial settings. Recently, much attention has been paid to understanding the dynamics of turbulent emulsions. This paper reviews some recent studies of emulsions in turbulent Taylor-Couette flow, mainly focusing on the statistics of the dispersed phase and the global momentum transport of the system. We first study the size distribution and the breakup mechanism of the dispersed droplets for turbulent emulsions with a low volume-fraction (dilute) of the dispersed phase. For systems with a high volume-fraction (dense) of the dispersed phase, we address the detailed response of the global transport (effective viscosity) of the turbulent emulsion and its connection to the droplet statistics. Finally, we will discuss catastrophic phase inversions, which can happen when the volume-fraction of the dispersed phase exceeds a critical value during dynamic emulsification. We end the manuscript with a summary and an outlook including some open questions for future research. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

Catastrophic Phase Inversion in High-Reynolds-Number Turbulent Taylor-Couette Flow.

Emulsions are omnipresent in the food industry, health care, and chemical synthesis. In this Letter the dynamics of metastable oil-water emulsions in highly turbulent (10^{11}≤Ta≤3×10^{13}) Taylor-Couette flow, far from equilibrium, is investigated. By varying the oil-in-water void fraction, catastrophic phase inversion between oil-in-water and water-in-oil emulsions can be triggered, changing the morphology, including droplet sizes, and rheological properties of the mixture, dramatically. The manifestation of these different states is exemplified by combining global torque measurements and local in situ laser induced fluorescence microscopy imaging. Despite the turbulent state of the flow and the dynamic equilibrium of the oil-water mixture, the global torque response of the system is found to be as if the fluid were Newtonian, and the effective viscosity of the mixture was found to be several times bigger or smaller than either of its constituents.

Supergravitational turbulent thermal convection.

High-Rayleigh number convective turbulence is ubiquitous in many natural phenomena and in industries, such as atmospheric circulations, oceanic flows, flows in the fluid core of planets, and energy generations. In this work, we present a novel approach to boost the Rayleigh number in thermal convection by exploiting centrifugal acceleration and rapidly rotating a cylindrical annulus to reach an effective gravity of 60 times Earth's gravity. We show that in the regime where the Coriolis effect is strong, the scaling exponent of Nusselt number versus Rayleigh number exceeds one-third once the Rayleigh number is large enough. The convective rolls revolve in prograde direction, signifying the emergence of zonal flow. The present findings open a new avenue on the exploration of high-Rayleigh number turbulent thermal convection and will improve the understanding of the flow dynamics and heat transfer processes in geophysical and astrophysical flows and other strongly rotating systems.

Using ray-traversal for 3D particle matching in the context of particle tracking velocimetry in fluid mechanics.

An innovative method based on the traversal of rays, originating from detected particles, through a three-dimensional grid of voxels is presented. The methodology has the main advantage that the outcome of the method is independent of the order of the input; the order of the cameras and the order of the rays presented as input to the algorithm do not influence the outcome. The algorithm finds matches in decreasing value of match quality, ensuring that globally best matches are matched before worse matches. The time complexity of the algorithm is found to scale efficiently with the number of cameras and particles. A variety of show-cases are given to exemplify the algorithm for different geometries and different numbers of cameras. The method is designed for the tracking of tracer or inertial particles in fluid mechanics, for which the particle size generally ranges from O (µm)-O (cm). The method, however, does not impose a size limit on the particles.

Twente mass and heat transfer water tunnel: Temperature controlled turbulent multiphase channel flow with heat and mass transfer.

A new vertical water tunnel with global temperature control and the possibility for bubble and local heat and mass injection has been designed and constructed. The new facility offers the possibility to accurately study heat and mass transfer in turbulent multiphase flow (gas volume fraction up to 8%) with a Reynolds-number range from 1.5 × 104 to 3 × 105 in the case of water at room temperature. The tunnel is made of high-grade stainless steel permitting the use of salt solutions in excess of 15% mass fraction. The tunnel has a volume of 300 l. The tunnel has three interchangeable measurement sections of 1 m height but with different cross sections (0.3 × 0.04 m2, 0.3 × 0.06 m2, and 0.3 × 0.08 m2). The glass vertical measurement sections allow for optical access to the flow, enabling techniques such as laser Doppler anemometry, particle image velocimetry, particle tracking velocimetry, and laser-induced fluorescent imaging. Local sensors can be introduced from the top and can be traversed using a built-in traverse system, allowing, for example, local temperature, hot-wire, or local phase measurements. Combined with simultaneous velocity measurements, the local heat flux in single phase and two phase turbulent flows can thus be studied quantitatively and precisely.

Rough-wall turbulent Taylor-Couette flow: The effect of the rib height.

In this study, we combine experiments and direct numerical simulations to investigate the effects of the height of transverse ribs at the walls on both global and local flow properties in turbulent Taylor-Couette flow. We create rib roughness by attaching up to 6 axial obstacles to the surfaces of the cylinders over an extensive range of rib heights, up to blockages of 25% of the gap width. In the asymptotic ultimate regime, where the transport is independent of viscosity, we emperically find that the prefactor of the [Formula: see text] scaling (corresponding to the drag coefficient [Formula: see text] being constant) scales with the number of ribs [Formula: see text] and by the rib height [Formula: see text]. The physical mechanism behind this is that the dominant contribution to the torque originates from the pressure forces acting on the rib which scale with the rib height. The measured scaling relation of [Formula: see text] is slightly smaller than the expected [Formula: see text] scaling, presumably because the ribs cannot be regarded as completely isolated but interact. In the counter-rotating regime with smooth walls, the momentum transport is increased by turbulent Taylor vortices. We find that also in the presence of transverse ribs these vortices persist. In the counter-rotating regime, even for large roughness heights, the momentum transport is enhanced by these vortices.

Dispersion of Air Bubbles in Isotropic Turbulence.

Bubbles play an important role in the transport of chemicals and nutrients in many natural and industrial flows. Their dispersion is crucial to understanding the mixing processes in these flows. Here we report on the dispersion of millimetric air bubbles in a homogeneous and isotropic turbulent flow with a Taylor Reynolds number from 110 to 310. We find that the mean squared displacement (MSD) of the bubbles far exceeds that of fluid tracers in turbulence. The MSD shows two regimes. At short times, it grows ballistically (∝τ^{2}), while at larger times, it approaches the diffusive regime where the MSD∝τ. Strikingly, for the bubbles, the ballistic-to-diffusive transition occurs one decade earlier than for the fluid. We reveal that both the enhanced dispersion and the early transition to the diffusive regime can be traced back to the unsteady wake-induced motion of the bubbles. Further, the diffusion transition for bubbles is not set by the integral timescale of the turbulence (as it is for fluid tracers and microbubbles), but instead, by a timescale of eddy crossing of the rising bubbles. The present findings provide a Lagrangian perspective towards understanding mixing in turbulent bubbly flows.

The boiling Twente Taylor-Couette (BTTC) facility: Temperature controlled turbulent flow between independently rotating, coaxial cylinders.

A new Taylor-Couette system has been designed and constructed with precise temperature control. Two concentric independently rotating cylinders are able to rotate at maximum rates of f(i) = ± 20 Hz for the inner cylinder and f(o) = ± 10 Hz for the outer cylinder. The inner cylinder has an outside radius of r(i) = 75 mm, and the outer cylinder has an inside radius of r(o) = 105 mm, resulting in a gap of d = 30 mm. The height of the gap is L = 549 mm, giving a volume of V = 9.3 L. The geometric parameters are η = r(i)/r(o) = 0.714 and Γ = L/d = 18.3. With water as working fluid at room temperature, the Reynolds numbers that can be achieved are Re(i) = ω(i)r(i)(r(o) - r(i))/ν = 2.8 × 10(5) and Re(o) = ω(o)r(o)(r(o) - r(i))/ν = 2 × 10(5) or a combined Reynolds number of up to Re = (ω(i)r(i) - ω(o)r(o))(r(o) - r(i))/ν = 4.8 × 10(5). If the working fluid is changed to the fluorinated liquid FC-3284 with kinematic viscosity 0.42 cSt, the combined Reynolds number can reach Re = 1.1 × 10(6). The apparatus features precise temperature control of the outer and inner cylinders separately and is fully optically accessible from the side and top. The new facility offers the possibility to accurately study the process of boiling inside a turbulent flow and its effect on the flow.

Multiple states in highly turbulent Taylor-Couette flow.

The ubiquity of turbulent flows in nature and technology makes it of utmost importance to fundamentally understand turbulence. Kolmogorov's 1941 paradigm suggests that for strongly turbulent flows with many degrees of freedom and large fluctuations, there would only be one turbulent state as the large fluctuations would explore the entire higher dimensional phase space. Here we report the first conclusive evidence of multiple turbulent states for large Reynolds number, Re = O(10(6)) (Taylor number Ta = O(10(12))) Taylor-Couette flow in the regime of ultimate turbulence, by probing the phase space spanned by the rotation rates of the inner and outer cylinder. The manifestation of multiple turbulent states is exemplified by providing combined global torque- and local-velocity measurements. This result verifies the notion that bifurcations can occur in high-dimensional flows (that is, very large Re) and questions Kolmogorov's paradigm.

Logarithmic boundary layers in strong Taylor-Couette turbulence.

We provide direct measurements of the boundary layer properties in highly turbulent Taylor-Couette flow up to Re=2×106) (Ta=6.2×10(12)) using high-resolution particle image velocimetry and particle tracking velocimetry. We find that the mean azimuthal velocity profile at the inner and outer cylinder can be fitted by the von Kármán log law u+=1/κ lny+ +B. The von Kármán constant κ is found to depend on the driving strength Ta and for large Ta asymptotically approaches κ≈0.40. The variance profiles of the local azimuthal velocity have a universal peak around y+≈12 and collapse when rescaled with the driving velocity (and not with the friction velocity), displaying a log dependence of y+ as also found for channel and pipe flows.

Statistics of turbulent fluctuations in counter-rotating Taylor-Couette flows.

The statistics of velocity fluctuations of turbulent Taylor-Couette flow are examined. The rotation rates of the inner and outer cylinders are varied while keeping the Taylor number fixed to 1.49×10(12) [O(Re)=10(6)]. The azimuthal velocity component of the flow is measured using laser Doppler anemometry. For each experiment 5×10(6) data points are acquired and carefully analyzed. Using extended self-similarity [Benzi et al., Phys. Rev. E 48, R29 (1993)] the longitudinal structure function exponents are extracted and are found to weakly depend on the ratio of the rotation rates. For the case where only the inner cylinder rotates the results are in good agreement with results measured by Lewis and Swinney [Phys. Rev. E 59, 5457 (1999)] using hot-film anemometry. The power spectra show clear -5/3 scaling for the intermediate angular velocity ratios -ω(o)/ω(i)∈{0.6,0.8,1.0}, roughly -5/3 scaling for -ω(o)/ω(i)∈{0.2,0.3,0.4,2.0}, and no clear scaling law can be found for -ω(0)/ω(i)=0 (inner cylinder rotation only); the local scaling exponent of the spectra has a strong frequency dependence. We relate these observations to the shape of the probability density function of the azimuthal velocity and the presence of a neutral line.

Interaction and coalescence of large bubbles rising in a thin gap.

We present accurate measurements of the relative motion and deformation of two large bubbles released consecutively in a quiescent liquid confined in a thin-gap cell. Although the second injected bubble was smaller, we observed that, in all cases, it accelerated and caught up with the leading bubble. This acceleration is related to the wake of the leading bubble, which also induces significant changes in the width and curvature of the trailing bubble. On the contrary, the velocity of the leading bubble is unaltered during the whole interaction and coalescence process. Shape adaptation of the two bubbles is observed just prior to coalescence. After pinch-off, the liquid film is drained at a constant velocity.

Ultimate turbulent Taylor-Couette flow.

The flow structure of strongly turbulent Taylor-Couette flow with Reynolds numbers up to Re(i)=2×10(6) of the inner cylinder is experimentally examined with high-speed particle image velocimetry (PIV). The wind Reynolds numbers Re(w) of the turbulent Taylor-vortex flow is found to scale as Re(w)∝Ta(1/2), exactly as predicted by Grossmann and Lohse [Phys. Fluids 23, 045108 (2011).] for the ultimate turbulence regime, in which the boundary layers are turbulent. The dimensionless angular velocity flux has an effective scaling of Nu(ω)∝Ta(0.38), also in correspondence with turbulence in the ultimate regime. The scaling of Nu(ω) is confirmed by local angular velocity flux measurements extracted from high-speed PIV measurements: though the flux shows huge fluctuations, its spatial and temporal average nicely agrees with the result from the global torque measurements.

Torque scaling in turbulent Taylor-Couette flow with co- and counterrotating cylinders.

We analyze the global transport properties of turbulent Taylor-Couette flow in the strongly turbulent regime for independently rotating outer and inner cylinders, reaching Reynolds numbers of the inner and outer cylinders of Re(i) = 2×10(6) and Re(o) = ±1.4×10(6), respectively. For all Re(i), Re(o), the dimensionless torque G scales as a function of the Taylor number Ta (which is proportional to the square of the difference between the angular velocities of the inner and outer cylinders) with a universal effective scaling law G ∝ Ta(0.88), corresponding to Nu(ω) ∝ Ta(0.38) for the Nusselt number characterizing the angular velocity transport between the inner and outer cylinders. The exponent 0.38 corresponds to the ultimate regime scaling for the analogous Rayleigh-Bénard system. The transport is most efficient for the counterrotating case along the diagonal in phase space with ω(o) ≈ -0.4ω(i).