Microrheology is a technique used to measure the rheological properties, such as viscosity and viscoelasticity of a material, whether liquid, “soft solids” or solids. This science gathers several techniques aiming to overcome serious limitations encountered with traditional bulk rheology using a rheometer. There are two types of microrheology: passive microrheology and active microrheology.
- Micro rheology: Methods of measurement
Several microrheological methods of measurement already exist. Among all of these methods, the main developed and used nowadays are: light scattering (DLS and DWS), video-particle tracking and two-particle correlation.
On the first hand, dynamic light scattering (DLS), ancestor of today’s microrheology, uses scattered light’s time-correlation function to extract the properties of elastic materials, like viscosity and elastic modulus. This method requires more than 90% of light to be transmitted unscattered in order to work properly.
On the other hand, diffusive wave spectroscopy (DWS) is the evolution of DLS, extended to opaque systems, very high frequencies and good spatial resolution. However, it is still a bulk technique, with the limitations of large (milliliter) sample sizes.
Video tracking allows to measure the compliance and can yield a complete characterization of the linear viscoelasticity of a material, using the motion of tracer particles within this material. Particle tracking is very convenient and allows to gather good statistics. However, cameras and computer technologies limit the use of this method, presenting difficulties for trajectory acquisition and image analysis.
Two-particle correlation consists in one or two particles at a time. With this alternative method, very high sampling frequency is available, and fewer problems with data storage are met. Spatial resolution is also extremely good. However, the difficulty lies in tracking only 2 particles.
Figure 1. Dynamic light scattering (DLS) and Video-particle tracking
- Microfluidic and microrheology
The development and growth of microfluidics over the last few years has increased the need for rheological information, but also presents new opportunities for material property measurement in shear, shear-free and mixed kinematics. A lot of applications in microfluidics involve handling complex fluids, which flow in a non-Newtonian manner. However, microfluidic devices, with their micro-scale geometry, provide a rich platform for rheometric investigations of this type of phenomenon at a small scale.
Among a range of techniques situated at the junction of microfluidics and rheology are the microfluidic devices for the measurement of bulk rheological properties in shear and extensional flow.
Microfluidic capillary viscometry, besides being simple in design and microfabrication, can be an extremely reliable and accurate technique for measuring shear viscosities. Microfluidic capillary viscometers use both flow rate and pressure drop to measure the shear viscosity, following two main approaches: imposing a pressure drop and measuring the flow rate or imposing the flow rate and measuring the resulting pressure drop. For both of these approaches, a variety of methods have been exploited, relying on optical-based techniques for flow measurements, which present some constraints, and pressure sensor for pressure measurements.
Using microfluidic stagnation point flows, rheological information can be analyzed by exploring the optical properties of flows in the region of stagnation points (ex: extensional properties by monitoring birefringence). Two methods for exploring these flows exist: microfluidic implementations of the four-roll mill and the “cross-slot” flow geometry. Some difficulties reside in measuring the global pressure drop or stress field in such devices.
Figure 2 four-roll mill and the “cross-slot” flow
Microfluidic contraction flows are another type of device which allows to study the extensional properties of complex liquids by relating the measured pressure drop across a contraction to the imposed flow rate. Contraction flows allows the coupling between shear and elongational components, leading to significantly different microstructural responses. From a force balance across the contraction, it is possible to evaluate the Trouton ratio of a complex fluid (ratio of extensional viscosity to shear viscosity). Unlike macroscopic studies, microfluidic devices offer the potential to apply these techniques to low viscosity solutions.
Finally, microfluidics brings new opportunities to envisage studying rheological properties of key materials, such as complex liquid (like liquid crystals), single polymers or multiphase liquids (using droplet creation).
This concise review article was written by Wilfried Sire from Elveflow Microfluidics and been posted with permission from Elveflow. Readers can find out more about their work here. Other related information can be found through here
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