On line diffusing wave spectroscopy during rheological measurements: A new instrumental setup to measure colloidal instability and structure formation in situ

- A novel Rheo-DWS set-up was used to monitor the destabilization of food colloids.
- The presence and oscillation of the bob tool in the rheometer do not affect the scattering of light.
- To obtain accurate data, the turbidity parameter l* needs to be ≪ L.
- Gelation measured by Rheo-DWS was in agreement with rheology and transmission DWS.

A novel setup was employed to investigate the destabilization of colloids, using a rheometer in line with diffusing wave spectroscopy (DWS) in backscattering mode. The newly designed system consists of a concentric cylinder couette geometry fitted with a window to perform simultaneous light scattering measurements. To test the ability of this Rheo-DWS setup to follow the dynamics of destabilization of model systems such as skim milk and emulsions, parallel experiments were also carried out using a separate transmission DWS equipment. For skim milk, containing 10% of volume of protein particles (casein micelles) of about 200 nm in diameter, the physical constraints imposed by the rheological setup did not allow calculation of the correct value of the diffusion coefficient, as the diffusion approximation could not be fulfilled. On the other hand, for concentrated milk (4 × the original volume fraction) and for oil in water emulsions, the characteristic decay constant, τ, yielded correct size values calculated using known DWS theory. The ability to follow the dynamics of early gelation using Rheo-DWS was tested by measuring the changes occurring during gelation of skim milk, concentrated milk and emulsions. There were no statistically significant differences between the gelation times measured in situ using backscattering DWS and transmission DWS. The data obtained by light scattering were in full agreement with rheology experiments. The results demonstrate the potential of such setup to follow the dynamics of colloidal destabilization and structure formation in situ while subjecting them to simultaneous rheological measurements.