Shear thinning and frequency dependent behaviour of adsorbed polymer layers Part I. Experimental aspects and a first order analysis

Nanorheological measurements were carried out using an oscillatory AFM technique to investigate the viscoelastic properties of adsorbed hydroxypropyl guar (HPG) layers. The oscillations were performed at frequencies between 300 Hz and 1 kHz, with applied oscillation amplitudes of 2 nm. Qualitative data analysis was carried out using complex viscosity and complex modulus transfer functions based on a hydrodynamic lubrication model. The results indicated viscous behaviour at large surface separations and viscoelastic behaviour in the region of polymer layer overlap, as would be expected for adsorbed polymer layers. However, the adsorbed HPG layers also showed an indication of frequency dependent viscoelastic behaviour and shear thinning (reduction of viscosity with frequency). Furthermore, there appeared to be an unanticipated correlation between the apparent thickness of the adsorbed layers and the viscoelastic properties of the system, which may be attributed to the shear thinning behaviour of the layers

Interaction forces between particles stabilized by a hydrophobically modified inulin surfactant.

Submitted versio

Shear thinning and frequency-dependent behaviour of adsorbed polymer layers. Part II. Three-layer flow model.

A novel hydrodynamic model called the three-layer flow (TLF) model has been developed to enable the quantification of viscoelastic properties of adsorbed polymer layers obtained from nanorheological experiments. By comparing the qualitative experimental data to the TLF Model, four specific parameters could be quantified from the best fit curve; these are the apparent thickness of the adsorbed layer, the real and imaginary components of the layer viscosity, and the apparent solvent viscosity. It was found that the comparison between experimental data and the TLF Model showed excellent agreement for separation distances larger than the overlap region. Furthermore, by comparing the TLF Model with experimental data at different frequencies, an apparent link was found between the complex modulus cross-over point and the adsorbed layer thickness. For adsorbed hydroxypropyl guar (HPG) layers, the cross-over point occurred when the surface separation distance was approximately equal to the thickness of a single adsorbed layer, indicating a relatively viscous nature. Finally, a direct comparison between nanorheological data with standard rheometry measurements also provided some indication of the reliability of the model

Non-linear response of colloid monolayers at high-frequency probed by ultrasound-driven microbubble dynamics

Hypothesis: High-frequency interfacial rheology of complex interfaces remains challenging yet it is central to the performance of multiphase soft matter products. We propose to use ultrasound-driven bubble dynamics to probe the high-frequency rheology of a colloid monolayer used as model system with controlled interactions and simultaneous monitoring of the microstructure. We hypothesize that by comparing the response of colloid-coated bubbles with that of a bare bubble under identical experimental conditions, it is possible to detect the non-linear response of the monolayer and use it to extract interfacial rheological properties at 104s−1. Experiments: Using high-speed video-microscopy, the dynamics of colloid-coated bubbles were probed to study the micromechanical response of the monolayer to high-frequency deformation. Protocols analogous to stress-sweep and frequency-sweep were developed to examine the stress–strain relationships. A simple model, motivated by the observed non-linear responses, was developed to estimate the interfacial viscoelastic parameters. Findings: The estimated elastic moduli of colloid monolayers at 104s−1 are about an order of magnitude larger than those measured at 1 s−1. The monolayers exhibit non-linear viscoelasticity for strain amplitudes as small as 1%, and strain-softening behaviour. These findings highlight the applicability of acoustic bubbles as high-frequency interfacial probes.ChemE/Transport Phenomen