a new domain of Rheology
The best solution for viscoelastic analysis at rest (elasticity, viscosity, gel point...)
PARTICLES MEAN SQUARE DISPLACEMENT (MSD)
Brownian motion of the particles is reported as the particles Mean Square Displacement (MSD) versus correlation time.
This MSD curve enables the characterisation of the viscoelastic properties of the sample. The particles MSD in a purely viscous fluid grows linearly with time (cf. Fick's Law), while in a viscoelastic fluid the particles are limited in their displacement as they are "trapped" in the 3 dimensional microstructure network, leading to a plateau in the MSD curve.
The MSD curve is the viscoelastic signature of the samples, and contains all the information one might need to characterise a sample rheology.
This non-contact and non-destructive measurement enables the monitoring on the very same sample of the viscoelastic evolution versus ageing time, by repeating the acquisition of particles MSD curves.
MEASUREMENT AT REST
Measurement is performed without any mechanical stress. It allows the analysis of fragile materials (weak gels, creams, etc...) without sample modification nor destruction.
VISCOELASTIC PROPERTIES VS. TIME
- Gelation process ;
- Structure recovery ;
- Long-term stability ;
ON THE VERY SAME SAMPLE
EASY SAMPLE HANDLING
- No evaporation or drying (Rheolaser) ;
- Open environment (Horus) ;
- No geometry configuration ;
- Disposable measurement cell (Rheolaser) ;
Microrheology : A new way of investigating soft materials
“Microrheology looks at the thermal motion of small particles embedded in a material in order to extract its bulk rheological properties.
This experimental technique has opened to investigation material properties that are difficult to access or inaccessible by conventional rheology such as the viscoelastic response of fragile materials. It is a non intrusive technique and thus particularly well suited to study fragile materials such as weak gels (emulsions, yahourt, cosmetics). No macroscopic stress is applied to the sample which avoids its destruction or its modification. This technique increases our microscopic understanding of these complicated materials. Microrheology directly probes the microstructure of the material. The analysis of the mean square displacement is related to the meshsize of a semi dilute solution of polymer, or to the meshsize of a gel. From these data, structural data may be extracted.
This technique detects microscopic changes of the structure and is thus very relevant to this study of the syneresis of a gel, of an emulsion and thus to predict its stability.”
University of Bordeaux I
Institut Universitaire de France
THE MICRORHEOMETER IS THE PARTICLE
Passive Microrheology consists in using micron sized particles* to measure the local deformation of a sample due to thermal energy (Brownian motion).
*i.e.: particles contained in the liquid dispersion (emulsions, suspensions,foams).
The measurement is performed at rest, as no mechanical stress is applied to the sample. This technique allows monitoring of samples evolution, such as gelation, rheology ageing, stability, but also drying of coatings.
THE MICRON SCALE
Microrheology is a new domain of Rheology, which studies viscoelastic materials at the micrometer length scale. This complementary technique is powerful to analyse the viscoelastic structure of soft materials like colloids, polymers, gels, emulsions.
Rheology is the study of flow and deformation of materials in response to an applied stress. For complex viscoelastic systems, small deformation measurements reveal both the solid-like and fluid-like responses. This is usually performed by oscillatory mechanical rheometers which apply a small amplitude shear strain or stress, thereby ensuring linear response. These instruments require a strong expertise of the operator.
Our microrheology analysers use MS-DWS (Mutli-Speckle Diffusing Wave Spectroscopy) principle of measurement. It corresponds to Dynamic Light Scattering (DLS) extended to concentrated dispersions. It measures the particles brownian motion, which depends on the viscoelastic structure of the sample.
This technique consists in sending a coherent laser beam into the sample, leading to interfering waves which create a speckle pattern captured with a video camera. The variations of this speckle image are directly linked to the particles brownian motion, their speed, and the distance they explore.