What is optical microrheology?
Microrheology and Rheology in general are concerned with understanding how materials flow. A traditional mechanical rotational rheometer measures rheological properties by applying a stress to the sample and detecting the response. It applies this perturbation in various geometries (for example ‘cup and bob’, or ‘cone and plate’). More recently, microfluidics allow access to the ultra-high shear rate regime. This method has the added benefit of low sample volume requirement to still obtain information on the bulk sample properties.
An entirely different approach is microrheology by light scattering. Here, we observe the diffusion of tracer particles. From this, we can deduct the viscoeleastic properties for the rheological characterization of low viscosity and weakly-structured complex fluids. Technically this involves calculation of the mean square displacement (MSD) of the tracers as a function of time. We can convert the MSD into a complex viscosity η*, the storage modulus G’ and the loss modulus G’’.
What is the mean square displacement?
In an ideal Newtonian liquid the mean square displacement grows by simple Brownian diffusion. In other words it grows linearly in time proportional to the Diffusion coefficient D.
Once we determine the mean square displacement from the correlation function, we can then turn it into the complex modulus in Fourier space.
The details of the mathematics are in a white paper and an advanced technical note. For practical tips on how to perform these measurements, see the special chapter in the Zetasizer Accessories Manual. By default the file location of the manual on your computer is at
Start - All Programs - Malvern Instruments - Zetasizer Software - Manuals
- Man0487-1.1 (Zetasizer Nano Accessories Guide).pdf
Performing a microrheology experiment involves measuring the tracer particles in buffer/dispersant, and then in the sample of interest. Optionally, a zeta potential measurement of the tracer particles in buffer and in the sample helps to confirm whether interactions between the sample material and the tracer particles take place (and would thus lead to an incorrect interpretation of the microrheology data). For a variety of samples, Latex Nanosphere Size standards are suitable. But other materials (like biomolecules or proteins) may require alternative surface chemistries. As an example, for bovine serum albumin (BSA) we have found carboxylated melamine particles such as MEL0665 and MEL1180 to be viable tracers. As a potential universal tracer PEGylated polystyrene which is sterically stabilized may be suitable.So long as the PEG coating is pretty dense and sufficiently thick.
Does the tracer particle size matter?
Yes, it is very possible that tracer size will have an influence on the observed behavior. This may especially apply, when testing a gel-like network. The probe size must be larger than the pore size of the underlying microstructure, and results obtained using probe sizes larger than the typical pore dimension should all overlay.
How to try out the microrheology features
If you already have a Zetasizer Nano ZS or ZSP, the feature can be evaluated (30-day free trial) in the current software under
Tools – Options – Feature Keys – Microrheology Features – Install license key
If the “install license key” is greyed out, then there are two possible reasons. Either a functional key is already installed, or the trial license for that computer has expired.
To see and try out the software features, open the “Example Results.dts” data file and select a microrheology record (select the ‘Summary’ workspace, for example record #88), then go to Tools – Utilities – Microrheology Utilities to open a new window displaying taps for mean square displacement, viscoelastic moduli, complex viscosity – and various fitting model tools (context sensitive, generalized Newtonian. Complex viscosity: power law, Cross model, Sisko model; viscoelastic moduli: G” one element Maxwell, G” two element Maxwell, power law; mean square displacement: power law). In the example screen shot to the left, the graph displays the fit of microrheology data to the G” two element Maxwell model. Here, one can adjust the fit range by dragging the upper and lower range indicators (red triangles on the x-axis).
How to check for feature keys
There are several feature keys available in the Zetasizer software. How can you find what you have? Here are two methods to get a list. The summary shows all installed keys on your computer:
- In the Zetasizer software go to Tools – Options – Feature Keys – Feature Keys Summary or
- Double-click on the “Features” symbol in the bottom right corner of the Zetasizer software
This will open a window with a summary of the currently installed feature licenses. It may for example list Microrheology Features: None when the special micro rheology features are not present in the software.
Further Resources
- Technical Note: Selection of Suitable Tracer Particles
- White Paper: An Introduction to Microrheology
- Technical Note: Microrheology: Running measurements on the Zetasizer ZSP/ZS
- Recorded Webinar: Probing low viscosity and weakly-structured complex fluids using DLS microrheology (may require Chrome)
- Youtube video: Microrheology with the ZSP
- Request a Quote for a DLS system for microrheology
- Download the latest Zetasizer family software update
Previously
- FAQ: peak size or z-average – which one to pick in DLS?
- Size Exclusion Chromatography (SEC): when to use MALS
- Which size is right: intensity volume number distributions