Search results for "Shear flow"
showing 10 items of 59 documents
Shear Induced Demixing and Rheological Behavior of Aqueous Solutions of Poly(N-isopropylacrylamide)
2003
The interrelation between the phase separation behavior and the rheological performance of aqueous solutions of high molecular weight (M w = 1 600 kg/mol) poly(N-isopropylacrylamide) was investigated. The system demixes upon heating and the cloud point temperature, T cp decreases steadily with rising polymer concentration up to 10 wt.-%. The application of shear supports phase separation and reduces T cp markedly. This observation is interpreted in terms of destruction of intersegmental clusters formed in the quiescent state owing to favorable interactions. Intrinsic viscosities and Huggins coefficients as well as the viscosities, η at higher polymer concentrations are closely connected wit…
Calculation of shear influences on the phase separation of polymer blends exhibiting upper critical solution temperatures
1994
Calculations were performed on the basis of a generalized Gibbs energy of mixing G γ, which is the sum of the Gibbs energy of mixing of the stagnant system and E s, the energy stored in the system during stationary flow. With increasing shear rate γ, the demixing temperatures shift to lower values (shear-induced mixing; diminution of the heterogeneous area), then to higher values (shear-induced demixing), and finally to lower values again before the effects fade out. The details of the rather complex phase diagrams resulting for a given shear rate are primarily determined by a band in the T/χ plane (χ = mole fraction) within which (∂2 E s/∂χ2) T UCST where they submerge. Bound to a suitable…
Shear-Induced Crystallization and Shear-Induced Dissolution of Poly(ethylene oxide) in Mixtures with Tetrahydronaphthalene and Oligo(dimethyl siloxan…
2003
Cloud point temperatures (T cp ) and crystallization temperatures (T 1/s ) were measured at different constant shear rates for the ternary system tetrahydronaphthalene/poly(ethylene oxide)/oligo(dimethyl siloxane-b-ethylene oxide) using a rheo-optical device and in the case of T 1/s additionnaly a viscometer. This system enables for the first time a joint investigation of both transitions with a given mixture. Shear favors the homogeneous liquid state and the formation of crystals. T cp (liquid/liquid demixing, UCST) shifts to lower and T 1/s (liquid/solid, segregation of PEO) to higher temperatures by several degrees as the shear rate, γ, is increased up to 500 s -1 . The normalized shift …
Complex miscibility behaviour for polymer blends in flow
1995
Abstract Experimental observations of the effect of shear flow on the miscibility of binary polymer blends are compared to calculations based on a generalized Gibbs energy of mixing Gγ˙. This mixing free energy characterizes the steady state established at shear rateγ˙, as the sum of G z , the equilibrium Gibbs energy and E s , the energy the system stores while flowing.
Shear influence on the phase separation of oligomer blends
1994
Shear influences on the phase separation behaviour of four different blends of ethylene glycol/propylene glycol oligomers, exhibiting upper critical solution temperatures, were investigated. Cloud point curves at rest (turbidity measurements) are reported for all systems, spinodal conditions (light scattering) and tie-lines (analysis of the coexisting phases) are given for some examples. Phase diagrams under shear were obtained from rheological data. They demonstrate that the demixing temperatures of systems where both glycols bear OH end-groups are lowered up to ca. 1°C by shear rates of 1000s−1; the critical composition, generalized to the non-equilibrium conditions of flow, is markedly s…
Bimodal drop size distributions during the early stages of shear induced coalescence
2005
Drop sizes and drop size distributions were determined by means of an optical shear cell in combination with an optical microscope for the systems polyisobutylene/poly(dimethylsiloxane) (I) and poly(dimethyl-co-methylphenylsiloxane)/poly(dimethylsiloxane) (II) at low concentrations of the suspended phases and at different constant shear rates ranging from 10 to 0.5 s-1 . After pre-shearing the two-phase mixtures (I: 50 s-1; II: 100 s-1) for the purpose of producing small drop radii, the shear rate was abruptly reduced to the preselected value and coalescence was studied as a function of time. In all cases one approaches dead end drop radii, i.e. breakup is absent. The drop size distribution…
Morphology of PEO/PDMS blends during shear: Coexistence of two droplet/matrix structures and additive effects
2005
Abstract The morphologies of blends of polyethyleneoxide (PEO 37) and poly(dimethylsiloxane)s (PDSM), with viscosity ratios, λ , of approximately one (PDMS 230) or 2.8 (PDMS 314, being the component of higher viscosity) and interfacial tensions on the order of 10 mN/m, were investigated at 70 °C as a function of shear rate (up to 10 s −1 ) and of time. For the system PEO 37/PDMS 230 we have also studied the influence of the compatibilizer dimethyl–ethyleneoxide–copolymer (PDMS- co -PEO), which is only reasonably soluble in PEO. To investigate the morphologies we have used an optical shear cell in combination with a light microscope. The most important observation consists in the formation o…
On the Role of Drop Break Up for Coalescence Processes and Morphology Development in Polymer Blends under Shear
2005
Drop sizes and drop-size distributions were determined as a function of time at constant shear rates, γ, by means of an optical shear cell in combination with an optical light microscope after preshearing the samples at high γ. The systems under investigation were PIB 3/PDMS 152 (PIB: polyisobutylene, PDMS: poly(dimethylsiloxane), numbers: average molar masses in kg/mol) and COP 26*/PDMS 48 (COP: poly(dimethyl-co-methylphenylsiloxane), asterisk: apparent molar mass); all measurements refer to 25 °C. Systems and conditions were chosen such that shear rates in the vicinity of the intersection of the coalescence and the break up curves become experimentally accessible. Under these conditi…
Molecular dynamics simulations in hybrid particle-continuum schemes: Pitfalls and caveats
2017
Heterogeneous multiscale methods (HMM) combine molecular accuracy of particle-based simulations with the computational efficiency of continuum descriptions to model flow in soft matter liquids. In these schemes, molecular simulations typically pose a computational bottleneck, which we investigate in detail in this study. We find that it is preferable to simulate many small systems as opposed to a few large systems, and that a choice of a simple isokinetic thermostat is typically sufficient while thermostats such as Lowe-Andersen allow for simulations at elevated viscosity. We discuss suitable choices for time steps and finite-size effects which arise in the limit of very small simulation bo…
Colloidal layers in magnetic fields and under shear flow
2005
The behaviour of colloidal mono- and bilayers in external magnetic fields and under shear is discussed and recent progress is summarized. Superparamagnetic colloidal particles form monolayers when they are confined to a air–water interface in a hanging water droplet. An external magnetic field allows us to tune the strength of the mutual dipole–dipole interaction between the colloids and the anisotropy of the interaction can be controlled by the tilt angle of the magnetic field relative to the surface normal of the air–water interface. For sufficiently large magnetic field strength crystalline monolayers are found. The role of fluctuations in these two-dimensional crystals is discussed. Fur…