2 results
Modelling steady shear flows of Newtonian liquids with non-Newtonian interfaces
- Patrick T. Underhill, Amir H. Hirsa, Juan M. Lopez
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- Journal:
- Journal of Fluid Mechanics / Volume 814 / 10 March 2017
- Published online by Cambridge University Press:
- 31 January 2017, pp. 5-23
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In countless biological and technological processes, the flow of Newtonian liquids with a non-Newtonian interface is a common occurrence, such as in monomolecular films in ‘solid’ phases atop of aqueous bulk fluid. There is a lack of models that can predict the flow under conditions different from those used to measure the rheological response of the interface. Here, we present a model which describes interfacial hydrodynamics, including two-way coupling to a bulk Newtonian fluid described by the Navier–Stokes equations, that allows for shear-thinning response of the interface. The model includes a constitutive equation for the interface under steady shear that takes the Newtonian functional form but where the surface shear viscosity is generalized to be a function of the local shear rate. In the limit of a highly viscous interface, the interfacial hydrodynamics is decoupled from the bulk flow and the model can be solved analytically. This provides not only insight into the flow but also a means to validate the numerical technique for solving the two-way coupled problem. The numerical results of the coupled problem shed new light on existing experimental results on steadily sheared monolayers of dipalmitoylphosphatidylcholine (DPPC), the primary constituent of lung surfactant and the bilayers of mammalian cell walls. For low packing density DPPC monolayers, a Newtonian shear-independent surface shear viscosity model can reproduce the interfacial flows, but at high packing density, the shear-thinning properties of the new model presented here are needed.
Directed Mineralization on Polyelectrolyte Multilayer Films
- Maria C. Advincula, Pritesh A. Patel, Patrick T. Mather, Daniel Underhill, Bryan D. Huey, A. Jon Goldberg
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- Journal:
- MRS Online Proceedings Library Archive / Volume 975 / 2006
- Published online by Cambridge University Press:
- 26 February 2011, 0975-DD06-03
- Print publication:
- 2006
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Silica formation aided by polypeptides is being actively investigated for a wide range of applications including biomaterials synthesis, ceramics and controlled release systems. We envision that biocatalyzed mineralization could have application as a dental material where in situ formation of mineral layers could provide needed wear-resistance or sealing capability. The approach would be more clinically relevant, if a polymer host could be used to carry and specifically position the biocatalyst on a surface and additionally maintain the catalyst activity.
Accordingly, we studied the influence of simple catalytic polypeptides on silica formation from prehydrolyzed alkoxide precursor solutions onto a surface. The polypeptides were localized on to the surface as multilayered thin films using the layer-by-layer (LbL) assembly of polyelectrolytes. Polylysine (PLL) or another biocatalytic polycation, poly(ethyleneimine) (PEI), was adsorbed layer-by-layer up to 10 bilayers on silicon wafers in combination with a negatively charged polyelectrolyte polymer host, poly(sodium-4-styrene sulfonate) (PSS) to prepare PEI-(PLL/PSS)10, PEI-(PEI/PSS)10 and PEI-(PEI/PSS/PLL/PSS)10 multilayer films. Pre-hydrolyzed alkoxysilane solutions were placed dropwise on the catalytic films for silicification. Additionally, the effects of precursor concentration, solvent and drying were evaluated. The morphology, roughness and contact mechanical stiffness of the formed silica were investigated using optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM).
The resulting silica morphology was plate-like or spherical, and porous with average particle size depending on the catalyst and its positions on the surface. Without a catalyst the silica formed over longer times with a fine, gel-like appearance. The morphology of silica produced on the substrate was different from that of particles catalyzed in solution with the same polypeptide catalyst. Additionally, it was found that the homogeneity of PEI-(PLL/PSS)10 films increased with drying temperature, silica precursor concentration and the presence of ethanol. The contact mechanical stiffness of the silica particles (40 N/m) catalyzed from PEI-(PLL/PSS)10 films was lower than the non-silicified areas (48 N/m) suggesting that regions of the silica were amorphous and hydrated. These results show that a polypeptide applied to a surface as a multiple layer with an oppositely charged polymer host (PSS) maintains its activity for silicification. The generally coherent nature of the mineral coating suggests its potential for enhancing critical restorative dental interfaces; however properties like porosity, hydration and their effect on hardness and permeability will need further study.