Da-Ming Zhu

Viscoelastic signature of physisorbed macromolecules at the solid-liquid interface

Viscoelastic behavior of a solution boundary layer at a solid-liquid interface could differ from that of bulk solution due to molecular adsorption at the interface. Such a property can be used as a characteristic signature to indicate the molecular adsorption at the interface. In this work, we systematically measured the viscoelastic properties of polyethylene glycol (PEG) solution boundary layers in contact with a gold surface using a quartz crystal resonator technique. The results show that viscosity and shear modulus of the PEG boundary layer increase with the PEG concentration in the solution; the increasing rate depends on the molecular weight. For relatively small PEG molecules, the viscoelastic property of the PEG solution boundary layer is almost indistinguishable from that of the bulk solution of the same concentration, indicating no adsorption at the interface. For larger PEG polymers (with molecular weights above a few thousands grams per mole), the viscoelastic property of the solution boundary layer differs distinctively from that of the corresponding bulk solution. The difference can be attributed to physisorption of PEG molecules on the Au surface, which alters the viscoelastic behaviors of the boundary layer. The results suggest that adsorption behaviors of macromolecules at a solid-liquid interface might be inferred from the changes of the viscoelastic properties of a solution boundary layer.

Synthesis and atomic force microscopy characterization of GeFe nanophase materials

Nanocrystalline GeFe samples with Fe concentration varying from a few percent to about 86% were synthesized using an inert gas condensation method. The samples were compressed into thin disks in vacuum before exposure to air. Magnetization measurements found that the magnetization of the samples cannot be described by a simple Langevin function. Atomic force microscopy has been used to characterize the average size of the nanocrystals in the samples. For samples with a low concentration of Fe, atomic force microscopy images show distinct individual clusters with an average size of about 500 A in diameter. For samples with a higher concentration of Fe, individual clusters cannot be distinguished, and atomic force microscope images show a mountain‐like, rugged morphology on the sample surfaces.