Rheology refers to the study of flow and deformation of materials under imposed stress. The limitations of material behavior are the ideal (Hookean) solid, which is distinguished by an elastic modulus and does not flow, and the ideal (Newtonian) fluid, which is distinguished by a viscosity and has slight elasticity.
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A wide range of soft materials present between these extremes display a mix of elastic and viscous responses or viscoelasticity. Based on the timescale of the applied deformation, these materials can store energy i.e. an elastic response and dissipate energy, which is viscous response.
Complex Fluids
Let us consider the class of materials dubbed 'complex fluids' that comprise surfactant systems, synthetic and biopolymer or protein solutions, and colloidal and non-colloidal emulsions and suspensions (dispersions). Although these complex fluids include a liquid-base, they cover supra-molecular structures produced by dispersed-phase particulates or constituent polymer molecules.
These microstructures transmit viscoelastic properties. Such fluids are present in many everyday products including personal care, foodstuffs, and industrial and household chemicals. Their rheological properties determine processing behavior and end-use performance of the product.
Rotational Rheometer
A traditional rotational rheometer is capable of measuring the shear flow or deformation properties of materials through a mechanical system to apply a controlled force onto a sample, which is of milliliter-scale volume. The response of the sample to the imposed deformation is recorded, allowing frequency-dependent linear viscoelastic moduli and shear rate-dependent viscosity i.e. G´ to be measured modulus and G" the viscous modulus.
A modern rotational rheometer is an advanced instrument that can measure material properties over various decades; however there are innate limitations to mechanical methods that prevent some types of material characterization.
In low viscosity materials, macromolecular relaxation times are of the order of milliseconds, and hence high frequencies are needed to differentiate their viscoelastic response. In such materials, accessing the dynamics is not possible with a rotational rheometer, where measurements of oscillation are controlled by instrument inertia from about 100rads-1, which is orders of magnitude of less than the frequencies needed to probe timescales related to molecular relaxation times.
The weak modulus and highly strain-sensitive structures that tend to develop in complex fluids need extremely low stresses for linear viscoelastic characterization, which is usually a small fraction of the lowest torque range available from a rotational rheometer, which is designed for testing over various decades of torque.
Microrheology
Microrheology is a term used to describe a wide range of methods that extract bulk and local rheological properties of soft materials by calculating and assessing the motion of colloidal probe particles in the sample. Microrheology can even be utilized with respect to microfluidic-based measurements, where complex fluids are exposed to controlled flow and deformation treatments in sub-millimeter scale channels to obtain rheological properties. In this article, embedded probe particle-type Microrheology techniques are discussed in detail.
Active and Passive Probe-based Microrheology
In active Microrheology, rheological properties are obtained from the forced motion of colloidal probe particles in the system, which can expand measurements from the linear into the non-linear regime. However, it should be noted that shifting into the non-linear regime present a number of challenges for theory and interpretation, because the non-equilibrium state of the sample microstructure has to be comprehended to relate probe motion to the underlying rheological properties. Also, active Microrheology methods can extend into the use of different particles. This is called a two-particle correlation.
In passive Microrheology, linear rheological properties are obtained from the motion of colloidal probe particles undergoing thermal changes in a system at thermodynamic equilibrium that is no external forces are applied on the probe particles.
Relationships have been obtained that allow quantitative rheological data across a broad range of frequencies, and which have been demonstrated to hold across a range of complex fluid types.
In general, active techniques allow measurements on materials, which have considerable elastic properties, while passive techniques can be used for measuring weakly-structured materials which have low values of mostly viscous modulus.
Measurement of Probe Particle Behavior
A further classification for Microrheology measurements includes light scattering Microrheology and particle tracking Microrheology. The former illustrates the case where the average motion of an ensemble of probe particles is measured by a scattering technique.
Dynamic Light Scattering is applicable for samples having optical characteristics that range from transparent to slightly turbid. The latter alter illustrates the case where the motion of individual probe particles is followed utilizing a video-microscope, and the probe particle tracks are successively investigated through image processing software.
Conclusion
Microrheology is used to describe a range of methods that extract bulk and local rheological properties of soft materials by determining and studying the motion of embedded colloidal probe particles in the sample.
In an active and developing field of research, a number of review papers have been published in the scientific literature that offer elaborate references to all Microrheology techniques and the applications that have been targeted.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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