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Scientific Highlights – Online Annual Report 2017

Optimized mass transfer in minichannel reactors

Optimierte Strömung in Minikanal-Reaktoren ©Copyright: Dr. Schubert, Markus, HZDR, Experimentelle Fluiddynamik

Vibrations can help significantly
to improve the transition of
gases intor the liquid phase by
triggering helical capillary waves.

Source: HZDR, Dept.
Experimental Thermal Fluid

Today, millions of minichannel reactors with a ceramic honeycomb catalyst are used to clean exhaust gases in motor vehicles. Due to their excellent flow-mechanical and reactive properties, they are also increasingly being used for gas-liquid reactions in the chemical industry. One problem of gas-liquid reactors, however, is the poor solubility of gaseous reactants such as hydrogen or oxygen. Vibrations can help significantly to improve the transition of the gases into the liquid phase.

As part of DFG Priority Program 1506 'Transport Processes at Fluidic Interfaces', HZDR scientists studied Taylor bubble flow in minichannels. The researchers were able to show that electro-mechanical stimulation of the channels can increase mass-transfer rates by up to 186 percent.

For their investigations, the researchers first used high-resolution x-ray microtomography to determine the precise, three-dimensional Taylor bubble shapes and the film-thickness profiles in round and square minichannels. The measured data served as a basis for a review of various numerical simulation methods. In further experiments, the scientists studied mass transfer using hydraulically immobilized individual bubbles of carbon dioxide and oxygen, with and without the addition of surfactants.

Using the data from these experiments, the researchers succeeded in developing more comprehensive models on mass transfer and to prove their validity. In a key experiment, the HZDR researchers studied the mass transfer of individual Taylor bubbles in a round, individual channel, which they set in motion mechanically. The deformation of the Taylor bubble was recorded by high-speed microscopy in addition to the x-ray measurement method. This enabled the researchers to also explain the basic mechanism improving mass transfer: helical capillary waves are triggered by the vibrations at the interface between gas and liquid.

How do plants take up nanoparticles from the environment?

Aufnahme von Cerdioxid-Nanopartikeln in Pflanzen (1) ©Copyright: Dr. Schymura, Stefan

Allocation of radio-
labeled nano particles along the
leaf-veins before and ...

Source: S. Schymura

Aufnahme von Cerdioxid-Nanopartikeln in Pflanzen ©Copyright: Dr. Schymura, Stefan

... after dissolution.

Source: S. Schymura

Nanoparticles are used in many products and industrial processes. However, the potential risks for humans and the environment are still largely unclear. HZDR researchers used radiolabeled cerium-dioxide nanoparticles to examine how such particles are taken up by plants. Nanoscale cerium dioxide is used, for example, as an additive for diesel fuels, in exhaust catalysts, and as an UV-absorbing paint additive. The high sensitivity of the method used by the researchers enables the study of the very low exposure levels that are to be expected in the environment.

In order to differentiate between the uptake and translocation of intact particles and dissolved cerium, the researchers created two different types of chemically and physically identical radiolabeled nanoparticles. In the first case, the radioactive nuclide Ce-139 was evenly distributed in the particles. Cerium-dioxide nanoparticles were directly activated for this purpose using proton irradiation. In the second case, the nuclide Ce-139 was concentrated near the surface. For this purpose, the scientists let the radioactive nuclide diffuse into the cerium-dioxide particles at elevated temperatures.

Using the example of ryegrass and sunflowers, the scientists were able to prove that the plants primarily take up intact particles. Furthermore, autoradiographic images of the sunflower leaves also showed that the absorbed nanoparticles are initially transported as intact particles within the plant and, over time, they slowly dissolve and become evenly distributed throughout the leaf tissue.

Although the results cannot be readily generalized to apply to all other particles, the work clearly shows that for a realistic assessment of the environmental risks of nanotechnologies there is no alternative to comprehensive and systematic examinations of the specific uptake routes, and that radiolabeling is a strong tool in gaining this process understanding.

Magnetic frustration makes crystals superfluid and supersolid

Darstellung Spinell-Verbindung ©Copyright: V. Tsurkan

According to its structure,
Mangan-Chrom-Sulfide is a
typical spinel. A strong magnetic
field is able to decouple the
substructures of the lattice.

Foto: V. Tsurkan

Can a material be solid and liquid at the same time? Theoretical physics has been predicting such 'supersolid' phases for over 50 years. Although final experimental proof has yet to be furnished, scientists from the HZDR and the University of Augsburg have now found significant indications that such conditions really do exist. In a manganese-chromium-sulfide crystal they found extremely unusual spin orders, which form as a result of what is known as magnetic frustration. Frustrated magnets represent a promising way of generating exotic quantum states - e.g. spin liquids, spin ice, or complex spin molecules. In an external magnetic field, defined spin patterns can form which are stabilized by field-induced disturbances.

In the manganese-chromium sulfide (MnCr2S4), the chromium spins align themselves parallel to the external magnetic field. Due to magnetic frustration, the manganese spins exhibit different types of transverse and longitudinal order, which, by analogy with other known (bosonic) systems, can be described as superliquid and supersolid phases.

Two surprising properties became apparent during the magnetization and ultrasonic experiments in the examined material up to 60 tesla. First, a very robust magnetization plateau with an unusual spin structure was revealed. Second, two transitional phases were observed, indicating the possible occurrence of supersolid phases.

The magnetization plateau corresponds to the full polarization of all chromium moments without the participation of manganese. At 40 tesla – in the middle of the plateau – sound waves spread almost without loss: the external magnetic field here exactly compensates the chromium-manganese exchange field, so that the chromium and manganese sublattices are decoupled. By analogy with predictions of the quantum lattice-gas models, the changes in the spin order of the manganese ions on the edges of the magnetization plateau can be interpreted as transitions to supersolid phases.

Diamonds form in the interior of gas planets

Grafik Diamantregen Neptun ©Copyright: Greg Stewart / SLAC National Accelerator Laboratory

Inside Neptune, high pressure
and temperatures can form
diamonds from complex 

Foto: Greg Stewart / SLAC
National Accelerator Laboratory

The great ice planets of our solar system – Neptune and Uranus – contain traces of hydrocarbons such as methane in their atmospheres, and far larger quantities are suspected inside these planets. For decades, astrophysicists have been speculating about the processes that take place inside these planets. As pressure increases and temperatures rise from the surface towards the core, complex hydrocarbons such as polymers can be expected to form first. When the hydrogen separates out completely, pure carbon in the form of diamonds can develop, which, due to their density, should sink further into the interior of the planetary core.

Scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have now succeeded for the first time in experimentally mimicking these processes. Together with colleagues from Germany and the USA, they were able to show that 'diamond rain' really does develop under the conditions that exist deep inside our solar system's ice giants.

At the Stanford Linear Accelerator Center (SLAC) in California, they exposed polystyrene – a solid hydrocarbon with the molecular formula (C8H8)n – to dynamic pressures of up to about 150 gigapascals and temperatures of approximately 5,000 kelvins. Comparable conditions are to be expected approximately 10,000 kilometers below the planetary surfaces of Neptune and Uranus. With the help of the SLAC's ultra-powerful x-ray laser, the researchers were able to observe by x-ray diffraction the process of phase separation with dehydrogenation and the formation of diamonds in situ. They were able to show that very high pressures are required to initiate the separation of carbon and hydrogen.

This suggests that diamond formation requires about ten times higher pressures than previously assumed on the basis of static tests. The results will help to determine the mass/radius ratios of carbonaceous exoplanets more precisely; they provide boundary conditions for their inner layer structure, and help to improve the planet-formation models for Neptune and Uranus, because, in these models, carbon-hydrogen separation can influence the convective transport of heat.

Squamous cell carcinomas of the head and neck: detecting therapy resistance at an earlier time-point during treatment

FMISO-PET-Bildgebungsmethode ©Copyright: NCT Dresden/ Philip Benjamin, Anna Bandurska-Luque

The FMISO-PET imaging is an
appropriate method of detecting
clinical effectiveness of combined
radiation and chemotherapy for 
head-and-neck tumors at an 
earlier time-point. 

Foto: NCT Dresden / Philip
Benjamin, Anna Bandurska-

Lack of oxygen in tumor cells correlates with poor prognosis in radiation therapy and a lower effectiveness of many anti-cancer drugs, potentially including immune therapy. This relationship has been known for a long time. However, it is difficult to identify patients with hypoxic tumors – preferably prior to the start of treatment or at an early stage during radio(chemo)therapy. In a study, scientists from the HZDR and the University Hospital Carl Gustav Carus Dresden were able to show that persisting hypoxia in the tumor cells in the second week of radiochemotherapy is a predictor of poor prognosis. A total of 50 patients with squamous cell carcinoma were included in the prospective clinical study.

By analyzing the study data, the scientists have now succeeded in proving that positron emission tomography with [18F]-fluoromisonidazole (FMISO-PET) two weeks after the beginning of therapy is suitable as a biomarker for the early detection of persisting hypoxia and thus therapy-resistant tumors. FMISO-PET and computer-tomography images were acquired at four different time-points before and during combined radiation and chemotherapy. The most significant changes in the FMISO-PET signal over the six- to seven-week therapy were seen during the first one to two weeks. The scientists were able to show that patients with a continued lack of oxygen in the tumor as depicted by the FMISO-PET two weeks after the beginning of therapy responded worse to the therapy overall. They had a higher risk of developing recurrences at the site of the primary tumor.

This FMISO-PET-based biomarker for head and neck squamous cell carcinoma, which was developed at the Dresden OncoRay Center, will make it possible in future to identify high-risk patients at an early stage, in order to include them in therapy-intensification studies, for example. These studies are currently being prepared at the University Hospital Carl Gustav Carus.