Maziyar Jalaal

Slip of Spreading Viscoplastic Droplets

The spreading of axisymmetric viscoplastic droplets extruded slowly on glass surfaces is studied experimentally using shadowgraphy and swept-field confocal microscopy. The microscopy furnishes vertical profiles of the radial velocity using particle image velocimetry (PIV) with neutrally buoyant tracers seeded in the fluid. Experiments were conducted for two complex fluids: aqueous solutions of Carbopol and xanthan gum. On untreated glass surfaces, PIV demonstrates that both fluids experience a significant amount of effective slip. The experiments were repeated on glass that had been treated to feature positive surface charges, thereby promoting adhesion between the negatively charged polymeric constituents of the fluids and the glass surface. The Carbopol and xanthan gum droplets spread more slowly on the treated surface and to a smaller radial distance. PIV demonstrated that this reduced spreading was associated with a substantial reduction in slip. For Carbopol, the effective slip could be eliminated entirely to within the precision of the PIV measurements; the reduction in slip was less effective for xanthan gum, with a weak slip velocity remaining noticeable.

On the rheology of Pluronic F127 aqueous solutions

The rheology of aqueous solutions of Pluronic F127 is studied as a function of concentration, temperature, and shear rate. At sufficiently low temperatures, the solutions behave like Newtonian fluids; a simple empirical model is proposed for the viscosity as a function of temperature and concentration. The solutions undergo a transition to a gel at higher temperature, above which a complex rheological behavior is observed. In this regime, the solutions are viscoplastic with a yield stress that can be as large as hundreds of Pascal. We provide Herschel–Bulkley fits to the rheology for a range of temperatures above the gel point. At much higher temperatures, the rheology of the solutions becomes unsteady and difficult to characterize in terms of a steady-state flow curve.

The Effect of Temperature Dependency of Surface Emissivity in 1-D and 2-D Nonlinear Heat Radiation Equations

Knowledge of the temperature dependency of the physical properties such as surface emissivity, which controls the radiative problem, is fundamental for determining the thermal balance of many scientific and industrial processes. The surface emissivity generally depends on surface temperature, wavelength, surface material geometry (curvature, roughness), and direction of observation, and often changes with oxidation, melting, coating, and even surface pollution. Current work studies the influences of temperature dependency of surface emissivity on heat transfer in a lumped system. The problem was investigated for both one-dimensional (1-D) and two-dimensional (2-D) systems. For 1-D equations, two recent analytical methods called the homotopy perturbation method (HPM) and parameterized perturbation method (PPM) are presented. Unlike classic perturbation methods, these techniques do not need small parameters for nonlinear heat transfer equations. Thus, we can apply them for large values of surface emissivity. For the 2-D domain, a finite-element code is used to obtain the unsteady distribution of temperature. Three different functions were chosen to describe the thermal behavior of surface emissivity. The solutions of 1-D nonlinear equations are compared with the accurate numerical fourth-order Runge–Kutta method, and excellent agreement (maximum error of 0.0021%) was observed. The capabilities of employed analytical methods are discussed and it is shown that HPM needs fewer series terms in comparison with PPM. For both 1-D and 2-D equations, it is illustrated that the temperature gradient increased by adding to the order of emissivity variation versus temperature.