Mechanical vibrations of pendant liquid droplets.

A simple optical deflection technique was used to monitor the vibrations of microlitre pendant droplets of deuterium oxide, formamide, and 1,1,2,2-tetrabromoethane. Droplets of different volumes of each liquid were suspended from the end of a microlitre pipette and vibrated using a small puff of nitrogen gas. A laser was passed through the droplets and the scattered light was collected using a photodiode. Vibration of the droplets resulted in the motion of the scattered beam and time-dependent intensity variations were recorded using the photodiode. These time-dependent variations were Fourier transformed and the frequencies and widths of the mechanical droplet resonances were extracted. A simple model of vibrations in pendant/sessile drops was used to relate these parameters to the surface tension, density and viscosity of the liquid droplets. The surface tension values obtained from this method were found to be in good agreement with results obtained using the standard pendant drop technique. Damping of capillary waves on pendant drops was shown to be similar to that observed for deep liquid baths and the kinematic viscosities obtained were in agreement with literature values for all three liquids studied.

Shear banding in drying films of colloidal nanoparticles.

Drying suspensions of colloidal nanoparticles exhibit a variety of interesting strain release mechanisms during film formation. These result in the selection of characteristic length scales during failure processes such as cracking and subsequent delamination. A wide range of materials (e.g., bulk metallic glasses) release strain through plastic deformations which occur in a narrow band of material known as a shear band. Here we show that drying colloidal films also exhibit shear banding. Bands are observed to form a small distance behind the drying front and then to propagate rapidly at ∼45° to the direction of drying. It is shown that the spacing of the bands depends on salt concentration and the evaporation rate of the colloidal suspension. These combined observations suggest that there is a critical shear rate (related to the film yield stress) which controls the ratio of bandwidth to band spacing. Local deformations were measured in the early stages of drying using fluorescent tracer particles. The measurements were used to show that the existence of shear bands is linked to the compaction of particles perpendicular to the drying front. The spacing of shear bands was also found to be strongly correlated with the characteristic length scale of the compaction process. These combined studies elucidate the role of plastic deformation during pattern formation in drying films of colloidal nanoparticles.

Factors affecting the formation of insulin amyloid spherulites.

Thermally induced amyloid aggregation of bovine insulin can produce a number of distinct aggregate morphologies. In this work amyloid spherulites were analysed using cross polarized optical microscopy and light scattering. A new semi-quantitative methodology to estimate the balance of spherulites and free fibrils is reported and, from this analysis, the effects of pH, temperature, salt, and protein concentration on spherulite formation were quantitatively determined for the first time. The number and size of spherulites measured with polarized light microscopy were related to changes in the colloidal stability of the solution and fibril nucleation times (measured by static light scattering). Importantly, changes in pH between 1.75 and 2 were found to result in a dramatic decrease in the spherulite radii, which were related to differences in the conformational stability of the protein. Moreover, estimates of the final spherulite volume fraction clearly indicate that amyloid spherulite formation is the dominant pathway for insulin aggregation in HCl solutions at low pH and protein concentrations below ~5 mg ml(-1), with the balance shifting towards fibrils as the concentration increases.