Davide A. Hill

The characterization of the total stress of concentrated suspensions of noncolloidal spheres in Newtonian fluids

The total stress of a concentrated suspension of noncolloidal spheres in a Newtonian fluid was characterized by independent measurements in viscometric flows. Using a suspension balance formulation, the normal stress in the vorticity direction (Σ33) for a suspension undergoing simple shear was extracted from Acrivos et al.’s [Int. J. Multiphase Flow 19, 797 (1993)] resuspension data in a Couette device. Employing a new correlation for the relative viscosity μr which obeys the Einstein relation in the dilute limit while diverging at random close packing, it was found that Σ33/τ (where τ is the magnitude of the shear stress) was a strong function of the solid volume fraction φ, scaling as φ3e2.34φ. The relative viscosity, measured in a parallel plate viscometer, was in good agreement with the proposed correlation, while the normal stress differences N1 and N2 for concentrated suspensions (φ=0.30–0.55) were characterized using parallel plate and cone-and-plate geometries, as well as laser profilometry measur...

New high‐contrast developers for poly(methyl methacrylate) resist

New developers for poly(methyl methacrylate) consisting of mixtures of common developing components have been carefully investigated. It has been found that adding a small percentage of methyl ethyl ketone to methyl isobutyl ketone and Cellosolve results in a significant increase in contrast. Results of contrast experiments as well as improvements in electron‐beam lithographic exposures are reported. An explanation of the mechanism of contrast and resolution enhancement is offered.

Wall slip in polymer melts: A pseudo-chemical model

A chemical-type theory for wall slip in polymer melts is developed by modeling the exchange of bridging sites between two opposing polymeric and solid surfaces. Kinetic equations, describing surface coverage by bridging monomers, are formulated and analyzed to evaluate the stability of adhesive contact and slip characteristics of the viscoelastic melt. Order of magnitude estimates of the kinetic coefficients suggest that the polymer–solid interface is always at equilibrium, even under slip. The model displays the following features. The polymer slips at all stresses; the slip velocity, vs, obeys time-free volume superposition and depends on both shear and normal stresses. At small stresses, vs is linear in shear stress and proportional to a function of the work of adhesion; the slip parameter b (the slip extrapolation length scale) takes on the same form as that proposed by de Gennes, but displays an additional dependence on adhesive energy. At constant vs the shear stress is proportional to the adhesive ...

Normal stresses and free surface deformation in concentrated suspensions of noncolloidal spheres in a viscoelastic fluid

Concentrated suspensions of noncolloidal spheres in a constant viscosity elastic fluid were characterized rheologically using rotating plate viscometers and profilometry of the suspension surface deflection near a rotating rod. It was found that the relative viscosity was quantitatively consistent with a previously determined correlation for suspensions based on Newtonian fluids. Moreover, the first normal stress difference N1 was found to be positive and the second normal stress difference N2 negative. Although the magnitude of N1 and N2 increased with the solids volume fraction φ, in general the ratio |N1/N2| decreased as loading increased. Analysis of the normal stress data suggests that the rheological contribution of the solids microstructure was in large part independent of that of the dissolved polymers at high solids loading (φ⩾0.3). The magnitude of N2 at high concentrations approached that measured for similar suspensions in Newtonian fluids, while the magnitude of N1 could be attributed to the ...

Efficient removers for poly(methylmethacrylate)

As nanotechnology approaches molecular scales, issues of surface contamination by unremoved resists will play an important role in device fabrication. Electron beam lithography of polymethylmethacrylate (PMMA) resist is still among the most widely used nanofabrication techniques, so it is relevant to study its residual contamination on both exposed and unexposed surfaces using a variety of resist removers. We systematically characterized the effectiveness of several different strippers. Our experiments show that 1,2-dichloroethane is an efficient PMMA remover, and can produce almost the same surface roughness as the original SiO2 surface (i.e., no contamination), whereas the popular PMMA remover, acetone, cannot. Estimates of the polymer–solvent Flory–Huggins interaction parameters and surface–solvent interfacial energy (from contact angle measurements) satisfactorily predict the effectiveness of the solvents. This method should also be applicable to any PMMA-like films on any substrate to find a good sur...

Zeolite-based electrorheological fluids: Testing, modeling and instrumental artifacts

We examine the rheological response of zeolite-based electrorheological (ER) suspensions in silicone oil, mineral oil, and N-(4-methoxybenzylidine)-4 butylaniline, a low molecular weight, nematic liquid crystal. The carriers were selected to explore the effects of fluid viscosity and dielectric permittivity on the ER properties of the suspensions. A particular form of the White–Metzner constitutive equation correctly reproduces the main, qualitative features of the experiments, with only three parameters. When used in conjunction with an equation describing rheometer dynamics, the rheological model satisfactorily captures artifacts caused by the coupling of inertia and elastic compliance of the instrument with the nonlinear material response.

On the attainment of optimum developer parameters for PMMA resist

Abstract Because of the uniquely high resolution offered by poly(methyl methacrylate) (PMMA) for practically all forms of lithography, its technological significance remains strong. We have found significantly higher contrast developers for PMMA which have shown improved resolution, less susceptibility to proximity effects and better lift-off properties with no loss of sensitivity to exposure. We report data which demonstrate an improvement in contrast by more than 35% with the addition of as little as 1.3% methyl ethyl ketone (mek) to other common developers. We discuss a model which explains the observed contrast enhancement and predicts that for small changes, an increase in either mek concentration, temperature or develop time increases contrast. We include experimental evidence in support of temperature and mek related improvements.

Particle suspensions in liquid crystalline media: Rheology, structure, and dynamic interactions

We present preliminary experimental evidence for the existence of Frank elasticity-induced particle–particle interactions in concentrated suspensions in a nematic carrier. Our model system consists of N-(4-methoxybenzylidine)-4-butylaniline (MBBA; a low molecular weight, thermotropic liquid crystal) compounded with 31.5 vol % zeolite particles. Yield stress and oscillatory measurements on the nematic and isotropic samples (i.e., suspensions with nematic and isotropic carrier), display dramatically different signatures. The yield stress of the nematic system is considerably lower than that of the isotropic material, showing nearly a jump at the nematic to isotropic transition. Under small amplitude oscillations the nematic material exhibits viscoelastic response and memory effects, whereas the isotropic suspension behaves as expected for a Newtonian carrier. Theoretical considerations lend credence to the hypothesis of Frank elasticity-mediated particle–particle interactions.

A molecular dynamics study of macromolecules in good solvents: Comparison with dielectric spectroscopy experiments

Molecular dynamics simulations are employed to study the static and dynamic properties of macromolecules in dilute and semidilute solutions of a good solvent. The results are compared with dielectric spectroscopy experiments. Crossover concentrations, ρ*, that demarcate the dilute and semidilute regimes are identified. The shift from self-avoiding-walk to random-walk behavior is also studied. An investigation is conducted of the normal-mode dielectric relaxation of type-A polar polymers. In dilute systems, a power law molecular weight dependence of the normal mode relaxation times τ∝N2.2 is observed, in accordance with DeGennes’s scaling analysis for self-avoiding-walk chains. This result does not agree with the experimental dependence of the normal mode relaxation time on the size of the polymers. The differences between the simulations and the experimental dynamic results in the dilute regime can be ascribed to the leading assumption of the model, the neglect of hydrodynamic interactions. For higher con...

Molecular dynamics simulations of dielectric relaxation of concentrated polymer solutions

Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectric spectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain sizes up to N=150 beads at reduced densities 0.5⩽ρ⩽0.8 are studied. The dimensions of the chains follow a universal behavior with ρ/ρ*, where ρ* is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E″. As in dielectric spectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of E″ broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more heterogeneous. For denser s...