153 research outputs found
Aging of natural rubber studied via Fourier-transform rheology and double quantum NMR to correlate local chain dynamics with macroscopic mechanical response
Bottom-Up Assembly of DNAâSilica Nanocomposites into Micrometer-Sized Hollow Spheres
Although DNA nanotechnology has developed into a highly innovative and lively field of research at the interface between chemistry, materials science, and biotechnology, there is still a great need for methodological approaches for bridging the size regime of DNA nanostructures with that of micrometerâ and millimeterâsized units for practical applications. We report on novel hierarchically structured composite materials from silica nanoparticles and DNA polymers that can be obtained by selfâassembly through the clamped hybridization chain reaction. The nanocomposite materials can be assembled into thin layers within microfluidically generated waterâinâoil droplets to produce mechanically stabilized hollow spheres with uniform size distributions at high throughput rates. The fact that cells can be encapsulated in these microcontainers suggests that our concept not only contributes to the further development of supramolecular bottomâup manufacturing, but can also be exploited for applications in the life sciences
Impact of thixotropy on flow patterns induced in a stirred tank : numerical and experimental studies
Agitation of a thixotropic shear-thinning ïŹuid exhibiting a yield stress is investigated both experimentally and via simulations. Steady-state experiments are conducted at three impeller rotation rates (1, 2 and 8 sâ1) for a tank stirred with an axial-impeller and ïŹow-ïŹeld measurements are made using particle image velocimetry (PIV) measurements. Threedimensional numerical simulations are also performed using the commercial CFD code ANSYS CFX10.0. The viscosity of the suspension is determined experimentally and is modelled using two shear-dependant laws, one of which takes into account the ïŹow instabilities of such ïŹuids at low shear rates. At the highest impeller speed, the ïŹow exhibits the familiar outward pumping action associated with axial-ïŹow impellers. However, as the impeller speed decreases, a cavern is formed around the impeller, the ïŹow generated in the vicinity of the agitator reorganizes and its pumping capacity vanishes. An unusual ïŹow pattern, where the radial velocity dominates, is observed experimentally at the lowest stirring speed. It is found to result from wall slip effects. Using blades with rough surfaces prevents this peculiar behaviour and mainly resolves the discrepancies between the experimental and computational results
Collective effects in spin-crossover chains with exchange interaction
The collective properties of spin-crossover chains are studied.
Spin-crossover compounds contain ions with a low-spin ground state and low
lying high-spin excited states and are of interest for molecular memory
applications. Some of them naturally form one-dimensional chains. Elastic
interaction and Ising exchange interaction are taken into account. The
transfer-matrix approach is used to calculate the partition function, the
fraction of ions in the high-spin state, the magnetization, susceptibility,
etc., exactly. The high-spin-low-spin degree of freedom leads to collective
effects not present in simple spin chains. The ground-state phase diagram is
mapped out and compared to the case with Heisenberg exchange interaction. The
various phases give rise to characteristic behavior at nonzero temperatures,
including sharp crossovers between low- and high-temperature regimes. A
Curie-Weiss law for the susceptibility is derived and the paramagnetic Curie
temperature is calculated. Possible experiments to determine the exchange
coupling are discussed.Comment: 9 pages, 13 color figures, published versio
Macro- and microscale structure formation and aging in different arrested states of Laponite dispersions
Valency engineering of monomeric enzymes for self-assembling biocatalytic hydrogels
All-enzyme hydrogels are efficient reagents for continuous flow biocatalysis. These materials can be obtained by self-assembly of two oligomeric enzymes, modified with the complementary SpyTag and SpyCatcher units. To facilitate access to the large proportion of biocatalytically relevant monomeric enzymes, we demonstrate that the tagging valency of the monomeric (S)-stereoselective ketoreductase Gre2p from Saccharomyces cerevisiae can be designed to assemble stable, active hydrogels with the cofactor-regenerating glucose 1-dehydrogenase GDH from Bacillus subtilis. Mounted in microfluidic reactors, these gels revealed high conversion rates and stereoselectivity in the reduction of prochiral methylketones under continuous flow for more than 8 days. The sequential use as well as parallelization by ânumbering upâ of the flow reactor modules demonstrate that this approach is suitable for syntheses on the semipreparative scale
Effect of weak attractive interactions on flow behavior of highly concentrated crystalline suspensions
Microstructure, local dynamics, and flow behavior of colloidal suspensions with weak attractive interactions
We present a comprehensive micro- and macrorheological study of the effect of weak depletion attraction (Κdepâââ1â10âkBT) on dense colloidal suspensions stabilized by short-range repulsive interactions. We used aqueous polymer dispersions as model system and demonstrated the unique capabilities of multiple particle tracking (MPT) to disclose structural changes in such technically important systems exhibiting many characteristic features of hard sphere systems. Below the hard sphere freezing point Ïc, viscosity increases monotonically with increasing Κdep due to the transition from a fluid to a fluid/crystalline and finally to a gel state. Above Ïc, increasing attraction strength first results in a viscosity reduction corresponding to the formation of large, permeable crystals and then in a viscosity increase when a network of dense, small crystals forms. The fraction of the fluid and crystal phase, particle concentration in each phase as well as the modulus of the micro-crystals are obtained, the latter decreases with Κdep. Above the colloidal glass transition strong heterogeneities and different local particle mobility in the repulsive and attractive arrested states are found. Particles are trapped in the cage of neighboring particles rather than in an attractive potential well. The intermediate ergodic state exhibits uniform tracer diffusivity
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