Dramatic Increase In Viscosity Research Articles

Brownian Dynamics Simulations of Electro-Rheological Fluids, II

Abstract Electro-rheological fluids are technologically relevant colloidal dispersions that on application of an applied electric field manifest a yield stress and dramatic increase in viscosity. To assist optimisation of these fluids we have performed molecular simulations to understand the basic mechanisms operating in these fluids. In this work a dispersion of field-aligned dipoles is simulated in shear flow by non-equilibrium Brownian dynamics. In agreement with experiment, a plot of the simultated relative, η r , against a dimensionless characteristic ER parameter, the Mason Number, Mn, exhibits a plateau region (at low electric fields and/or high shear rates) prior to a steep increase in viscoisty for Mn smaller. Although, the simulations exhibit only an approximate data collapse according to the Mn. Following on from the first paper in this series we relate the origins of the ER effect in more detail to the temporal structural changes that take place in the fluid. We find for example, that the flui...

Shear thickening (dilatancy) in concentrated dispersions

AbstractA new criterion is introduced for the onset of shear thickening in a concentrated dispersion. The criterion follows from the assumption that shear thickening occurs when the shear forces overrule the interparticle forces. A force balance at small interparticle distances is used to predict the dependence of this critical shear rate on the volume fraction. It is deduced that the critical shear rate is proportional to the interparticle distance, has a linear dependence on the magnitude of the stabilizing force, an inverse linear dependence on the dispersion medium viscosity, and an inverse linear dependence on the particle radius. The model is confirmed experimentally with viscosity measurements on various electrostatically stabilized dispersions. The validity of the model is also checked with data obtained from the literature. Experiments indicate that polydisperse dispersions exhibit pronounced shear thickening but with a less dramatic increase in viscosity than monodisperse dispersions.

On the interaction between convection and crystallization in cooling magma chambers

We investigate the interaction of thermal convection and crystallization in large aspect-ratio magma chambers. Because nucleation requires a finite amount of undercooling, crystallization is not instantaneous. For typical values of the rates of nucleation and crystal growth, the characteristic time-scale of crystallization is about 10 3–10 4 s. Roof convection is characterized by the quasi-periodic formation and instability of a cold boundary layer. Its characteristic time-scale depends on viscosity and ranges from about 10 2 s for basaltic magmas to about 10 7 s for granitic magmas. Hence, depending on magma viscosity, convective instability occurs at different stages of crystallization. A single non-dimensional number is defined to characterize the different modes of interaction between convection and crystallization. Using realistic functions for the rates of nucleation and crystal growth, we integrate numerically the heat equation until the onset of convective instability. We determine both temperature and crystal content in the thermal boundary layer. Crystallization leads to a dramatic increase of viscosity which acts to stabilize part of the boundary layer against instability. We compute the effective temperature contrast driving thermal convection and show that it varies as a function of magma viscosity and hence composition. In magmas with viscosities higher than 10 5 poise, the temperature contrast driving convection is very small, hence thermal convection is weak. In low-viscosity magmas, convective breakdown occurs before the completion of crystallization, and involves partially crystallized magma. The convective regime is thus characterized by descending crystal-bearing plumes, and bottom crystallization proceeds both by in-situ nucleation and deposition from the plumes. We suggest that this is the origin of intermittent layering, a form of rhythmic layering described in the Skaergaard and other complexes. We show that this regime occurs in basic magmas only at temperatures close to the liquidus and never occurs in viscous magmas. This may explain why intermittent layering is observed only in a few specific cases.

Rheokinetics for reactive polymer processing

AbstractAn important aspect of all reactive polymer processing methods, including reactive injection molding (RIM) and reactive extrusion (REX) is the dramatic increase in viscosity of the material being processed. This paper describes an instrument designed to study rheokinetics of polymerizations relevant to reactive processing. The instrument is capable of measuring simultaneously both viscosity (dynamic or “steady”) and reaction rate during isothermal polymerizations in a closed system. Design aspects of the instrument and some preliminary results are presented.

Chromosomale Strukturen von Pseudomonas testosteroni. I. Isolierung und Charakterisierung der chromosomalen Komplexe / Chromosomal Structures of Pseudomonas testosteroni. I. Isolation and Characterization of the Chromosomal Complexes

After lysis of Pseydomonas testosteroni with lysozyme and non-ionic detergents different DNA-protein complexes can be separated in 5-25% (w/v) neutral sucrose gradient. The protein to DNA ratio of these complexes varies between 0.5-4.5 to 1, whereby the faster sedimenting forms contain more protein than the slower sedimenting ones. Different initial rates of DNase digestion may indicate various degrees of DNA packing in these complexes. The chromosomal complexes of Pseudomonas testosteroni are relatively stable towards pronase. Treatment with RNase or sodium dodecylsulphate is accompanied by a dramatic increase in viscosity and decrease in relative density. It suggests that DNA in these complexes is maintained in its supercoiled form by RNA molecule(s) in a similar way as in isolated chromosome of E. coli.