A filament stretching device for measuring the extensional viscosity of low-viscosity liquids is presented. The fluid sample is held between two disks which move apart at an increasing velocity so that the extension rate, based on the filament midpoint diameter, is constant. The device was used to measure the extensional stress growth coefficients of three ideal elastic solutions, including the model fluid M1 and a shear-thinning model fluid A1. The results indicate that all solutions containing high molecular weight polymer exhibit significant strain hardening as the fluid is extended. For the ideal elastic fluids, steady state in extensional stress was observed at strain above 4.5 and the steady Trouton ratio obtained for the fluids range from 2 to 5×103. For the fluid M1 the extensional viscosities obtained are higher than the apparent extensional viscosity obtained by other methods. This is the first time that the steady extensional viscosity has been measured for polymer solutions. The results obtained enable one to evaluate the numerous constitutive equations that have been proposed for polymer solutions.

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The scaling behaviour of the zero shear rate viscosity of semidilute unentangled DNA solutions, in the double crossover regime driven by temperature and concentration, is mapped out by systematic experiments. The viscosity is shown to have a power law dependence on the scaled concentration c/c * , with an effective exponent that depends on the solvent quality parameter z. The determination of the form of this universal crossover scaling function requires the estimation of the θ temperature of dilute DNA solutions in the presence of excess salt, and the determination of the solvent quality parameter at any given molecular weight and temperature. The θ temperature is determined to be T θ ≈ 15 • C using static light scattering, and the solvent quality parameter has been determined by dynamic light scattering. Typeset by REVT E Xand AIP 15 • C 20 • C 25 • C 30 • C 35 • C z 0 0.11 0.22 0.32 0.43 2.9 kbp c * 0.371 0.313 0.278 0.253 0.234 z 0 0.16 0.31 0.46 0.60 5.9 kbp c * 0.251 0.201 0.173 0.155 0.142 z 0 0.19 0.37 0.54 0.71 8.3 kbp c * 0.214 0.165 0.141 0.125 0.114 z 0 0.22 0.43 0.63 0.83 11.1 kbp c * 0.184 0.139 0.117 0.103 0.093 z 0 0.33 0.64 0.95 1.24 25 kbp c * 0.123 0.084 0.068 0.059 0.052 z 0 0.44 0.86 1.27 1.66 45 kbp c * 0.092 0.058 0.045 0.039 0.034 z 0 0.69 1.37 2.03 2.66 114.8 kbp c * 0.057 0.031 0.023 0.019 0.017 z 0 1.11 2.18 3.22 4.22 289 kbp c * 0.036 0.016 0.012 0.010 0.008 minutes at their maximum concentrations. This was done to prevent aggregation of long DNA chains [Heo and Larson, 2005]. The shear rate range of the instrument, under the applied geometry, is from 0.01 to 100 s −1 . At each shear rate, a delay of 30 seconds was employed so that the DNA chains have sufficient time to relax to their equilibrium state. Some typical relaxation times observed in dilute and semidilute solutions are given in Table I. At each temperature, a 30 minutes incubation time was employed for sample equilibration. III. SOLVENT QUALITY CROSSOVER OF THE ZERO SHEAR RATE VISCOSITY A. Zero shear rate viscosity of semidilute solutionsThe scaling behaviour of the zero shear rate viscosity of semidilute polymer solutions can be determined by measuring the viscosity as a function of concentration and temperature for a range of molecular weights, and then representing this behaviour in terms of the crossover variables z and c/c * . In order to do so, however, as discussed earlier in section I, it is first necessary to

Electrocrystallization of hydroxyapatite (HAp) on titanium was achieved by cathodic polarization in solution containing calcium nitrate and ammonium dihydrogen phosphate. The composition and pH of the bath were found to significantly affect the nature and surface morphology of the deposit. The effect of bath temperature was also studied. X-ray diffraction tests and microscopic inspections confirmed the formation of well-crystallized HAp at pH 0 ) 6.0 at any temperature between 70 and 95 °C, whereas, at pH 0 ) 4.2, less-crystallized, thicker, and more porous coatings that contained traces of octacalcium phosphate were observed. The influence of potassium chloride and sodium nitrite on the composition and surface morphology of the deposit was also evaluated. A speciation-precipitation model was applied to better understand the effect of bath conditions. The standard enthalpy of activation was ∼40 kJ mol -1 , indicating that the reaction kinetics is controlled by the interfacial area. The corrosion resistance of the coatings was determined by open-circuit potential and cyclic potentiodynamic polarization measurements in a simulated body fluid. The samples coated at pH 0 ) 6.0 exhibited nobler behavior. The ability to modify the chemistry and surface morphology of the coating by fine control of bath composition, pH, and temperature makes electrochemical deposition a versatile process for deposition of coatings on implants, with a tailored body response.

The swelling of the viscosity radius, α η , and the universal viscosity ratio, U ηR , have been determined experimentally for linear DNA molecules in dilute solutions with excess salt, and numerically by Brownian dynamics simulations, as a function of the solvent quality. In the latter instance, asymptotic parameter free predictions have been obtained by extrapolating simulation data for finite chains to the long chain limit. Experiments and simulations show a universal crossover for α η and U ηR from θ to good solvents in line with earlier observations on synthetic polymer-solvent systems. The significant difference between the swelling of the dynamic viscosity radius from the observed swelling of the static radius of gyration, is shown to arise from the presence of hydrodynamic interactions in the non-draining limit. Simulated values of α η and U ηR are in good agreement with experimental measurements in synthetic polymer solutions reported previously, and with the measurements in linear DNA solutions reported here.