Periodically driven Taylor-Couette turbulence

We study periodically driven Taylor-Couette turbulence, i.e. the flow confined between two concentric, independently rotating cylinders. Here, the inner cylinder is driven sinusoidally while the outer cylinder is kept at rest (time-averaged Reynolds number is $Re_i = 5 \times 10^5$). Using particle image velocimetry (PIV), we measure the velocity over a wide range of modulation periods, corresponding to a change in Womersley number in the range $15 \leq Wo \leq 114$. To understand how the flow responds to a given modulation, we calculate the phase delay and amplitude response of the azimuthal velocity. In agreement with earlier theoretical and numerical work, we find that for large modulation periods the system follows the given modulation of the driving, i.e. the system behaves quasi-stationary. For smaller modulation periods, the flow cannot follow the modulation, and the flow velocity responds with a phase delay and a smaller amplitude response to the given modulation. If we compare our results with numerical and theoretical results for the laminar case, we find that the scalings of the phase delay and the amplitude response are similar. However, the local response in the bulk of the flow is independent of the distance to the modulated boundary. Apparently, the turbulent mixing is strong enough to prevent the flow from having radius-dependent responses to the given modulation.Comment: 12 pages, 6 figure

A fluorescence approach to investigate repartitioning of coalescing agents in acrylic polymer emulsions

Repartitioning of co-solvents between particles of latex emulsions was investigated by means of a fluorescence method based on the detection of the amount of co-solvent via the solvatochromic shift of the emission maximum of a fluorescent probe, copolymerized at a low concentration. Complete repartitioning of co-solvents between particles of latex materials with a low Tg (ca. 25 °C) occurred within minutes. For a hydrophilic latex with a Tg of 68 °C, equilibration was achieved within an hour. Repartitioning was faster for more hydrophobic co-solvents. For a hydrophobic latex of similar Tg, co-solvent repartitioning took place on the same time scale, but complete equilibration was not reached. Possibly, there is an additional slow component in the repartitioning, or the prolonged presence of co-solvent causes a structural change in the latex particles that affects the outcome of the experiment

On the nature of crosslinks in thermoreversible gels

Applied Science

Dynamic mechanical studies on thermo-reversible ageing processes in gels of polyvinyl chloride and of gelatin

Applied Science

Manufacture of syntactic foams: pre-mold processing

Manufacturing processes for syntactic foams made of hollow microspheres and starch were studied. Various manufacturing parameters in relation to the “buoyancy method” were identified and inter-related. An equation based on unit-cell models with the minimum inter-microsphere distance concept for a relation between volume expansion rate of bulk microspheres in aqueous starch and microsphere size was derived and successfully used to predict experimental data. It was demonstrated that the inter-microsphere distance can be calculated numerically for microspheres with known statistical data. The equation relating between volume expansion rate and microsphere size was further extended to accommodate a relation between inter-microsphere distance and microsphere size but with limited accuracy for binders of low starch content. An alternative empirical linear equation for the relation between inter-microsphere distance and microsphere size is proposed for wider applications. A simple method for estimation of syntactic foam density prior to completion of manufacture is suggested. Shrinkage after molding of syntactic foam is discussed in relation with different stages such as slurry, dough and solid. A two-step manufacturing process involving molding and then forming is suggested for syntactic foam dimensional control

Novel syntactic foams made of ceramic hollow micro-spheres and starch: theory, structure and properties

Novel syntactic foams for potential building material applications were developed using starch as binder and ceramic hollow micro-spheres available as waste from coal-fire power stations. Foams of four different microsphere size groups were manufactured with either pre- or post-mould gelatinization process. They were of ternary system including voids with a foam density range of approximately 0.33–0.44 g/cc. Compressive failure behaviour and mechanical properties of the manufactured foams were evaluated. Not much difference in failure behaviour or in mechanical properties between the two different processes (pre- and post-mould gels) was found for a given binder content. Compressive failure of all syntactic foams was of shear on plane inclined 45 to compressive loading direction. Failure surfaces of most syntactic foams were characterized by debonded microspheres. Compressive strength and modulus of syntactic foams were found to be dependant mainly on binder content but mostly independent of micro-sphere size. Some conditions of relativity arising from properties of constituents leading to the rule of mixtures relationships for compressive strength and to understanding of compressive/transitional failure behaviour were developed. The developed relationships based on the rule of mixtures were partially verified. Some formation of starch webs on failure surfaces was discussed

3-D molecular architectures in thin film liquid crystalline networks

In-situ photopolymerization of polyfunctional liquid-crystalline monomers leads to anisotropic networks with a well-controlled and stable molecular organization. By polymerizing chiral monomers cholesteric networks are formed. The pitch of the molecular helix in these networks can be controlled between 180 nm and infinity. By a special process making use of photo-induced diffusion during the polymerization process, cholesteric networks are made with a pitch that varies gradually over the thickness of the film. These films may be optimized on reflection of circularly polarized light over the entire visible spectrum. The films are highly useful as non-absorbing polarizers for power-efficient liquid crystal displays

Unidirectional dimerization and stacking of ureidopyrimidinone end groups in polycaprolactone supramolecular polymers

The effect of stacking of end groups on the rheol. behavior of supramol. polymer melts is reported. Oscillatory shear expts. in the transition zone from the pseudo rubber plateau to the flow region of telechelic polycaprolactones (PCLs) with ureidopyrimidinone (UPy) end groups directly attached to PCL can be fitted with a single Maxwell element. This demonstrates that dimerization of the UPy groups is unidirectional and that reversible chain scission is faster than reptation. If the UPy groups are connected to the polymer via a urethane linker, a low-frequency plateau in G' is obsd. This is ascribed to the formation of a network of stacked UPy dimers, aided by urethane hydrogen bonding. Below their m.p., these stacks form long fibers in the urethane linked supramol. poly(Me caprolactone), which were obsd. with at. force microscopy (AFM). Steric hindrance interferes with stacking, since the plateau in G' is lower in a urethane linked polymer with bulky adamantyl-UPy end groups