The viscous properties of a liquid or amorphous solid are primarily determined by inter-particle forces within the solution, including friction and attraction between molecules in the macrostructure. These Van der Waals forces are critical facets of a sample’s resistance to deformation, or flow, which defines the material’s viscosity.
Shear viscosity is expressed under two distinct forms:
This blog post will explore five of the fundamental measuring techniques for obtaining the dynamic viscosity and the kinematic viscosity of fluidic samples.
Measuring viscosity via a capillary tube is one of the oldest methods of determining the kinematic viscosity of a sample, requiring prior knowledge of the density and volume of the sample of interest. This fluid is passed through a vertical U-tube of known dimensions and a very small diameter. The time taken for the sample to travel through the capillary correlates to its kinematic viscosity.
A rotational viscometer applies relatively weak levels of torque to a liquid sample to encourage mechanical deformation. The amount of torque required to cause rotation across a horizontal plane in the sample is measured and is relative to sample viscosity. Using a rotational rheometer allows analysts to plot a full flow curve of the material’s flow characteristics in response to varying levels of shear force and determine more advanced material parameters. Alternative viscometers only allow for single-point measurement and provide only shear viscosity measures.
Viscosity can also be measured by applying oscillating vibrations to the sample and monitoring the damping effects of the fluid. These can be assessed by monitoring power input, the decay time of oscillations, or changes in the resonated frequency.
Microfluidic rheometry is an innovative method for determining the dynamic viscosity of fluids in small sample volumes by forcing a liquid sample through a microfluidic channel in a laminar flow. At Formulaction, Fluidicam Rheo uses this principle to flow the fluid side by side with reference material. Dynamic viscosity is measured by comparing the differential flow rates, the viscosity of the reference material, and the position of the interface between the two fluids within the microfluidic channel.
Passive micro-rheology is a more complex measurement of a sample’s rheological characteristics. It measures similar properties to rotational rheometry but is adapted to more complicated and fragile structures such as gels, weak pastes, and viscoelastic materials that could break under extremely low shear. Unlike traditional rotational rheometers, non-contact rheology enables a quantitative assessment of the rheological properties of a sample at rest without mechanical stress. This is achieved using Rheolaser MASTER which is based on multi-speckle diffusing wave spectroscopy (MS-DWS). This optical method acquires the mean square displacement (nm2) of particles within the media. Mean square displacement can be used to assess the elasticity and viscosity of a sample over time.
Viscosity Measurements with Formulaction
Formulaction has presented a range of innovative viscosity measurement technologies for industrial and commercial fields handling complex formulations and dispersions. We supply a portfolio of novel instruments for analyzing the bulk rheological properties and dynamic viscosity of liquid, solid, and amorphous products in response to shear forces or destabilization over time.