Interfacial rheology is a challenging field of research because of the magnitude of forces in the interface is really small. Rotating ring and bicone methods have been developed, but they have been demonstrated to work only with macromolecular compounds at the interfaces. The KSV NIMA ISR utilizes a small magnetic probe which is moved with an oscillated magnetic field. The method reduces the inertia and enhances the sensitivity of the probe compared to the rotating ring method and makes it possible to measure low molecular weight surface active compounds.
Working principle
Magnetized probe is moved at air-water or oil-water interface using magnetic field created by Helmholtz coil. The probe movement is recorded optically from above. The complex surface modulus is calculated from the strain and signal phase shift and can be divided into elastic and viscous properties of the thin film.
Helmholtz coil
The ISR instrument uses Helmholtz coil to create a uniform magnetic field in the measuring area. Helmholtz coil refers to two identical circular magnetic coils that are apart from each other the same distance as the coil radius. This allows moving the magnetized probe in a measuring area by controlling the magnetic field. Since no mechanical connection is needed the instrument sensitivity increases dramatically allowing measurements of very weak viscoelastic forces.
Instrumental setup
The measurement probes are design to float easily at the liquid interface due to the light weight and water repellent coating. The metal part inside the probe is already magnetized and the probe is ready to be used instantly. Since thin film viscoelasticity can vary between different samples we offer different sizes of probes to match the measurement demands.
The probe moves inside a glass channel that creates a small meniscus on both sides of the surface. This channel guides the probe to move in a straight line and ensures uniform flow geometry. To ensure the glass channel is centred every time a channel holder is used to guide it in the centre of the trough. This way magnetic probe is easy to set in the middle of the magnetic field every time. The camera and lens are mounted on a xyz- stage providing easy optical setup.
Dynamic measurement
Allows defining:
- Elastic (storage) modulus, G’
- Viscous (loss) modulus, G’’
- Dynamic interfacial viscosity, μs*
In dynamic rheological experiments three variables are measured: applied stress (σ), strain (γ) and phase angle (φ) between the stress and strain oscillation.
Dynamic viscoelastic modulus G* is obtained as a function of oscillation frequency (ω) from the measurement, and it can be separated into two components, elastic (or storage) modulus G’ and viscous (or loss) modulus G’’. In the measurement either stress
The dynamic testing includes three different measurement types:
Frequency sweep allows measuring the viscoelastic properties at different frequencies. Since rheology is highly time dependent this method is extremely useful. It allows defining thin film rheological behaviour in different time scales and provides information whether viscosity or elasticity is the dominating part.
Single frequency measurement can be used to define the time dependency of viscoelastic properties. When combined with KSV NIMA Langmuir trough the viscoelastic changes can be defined as a function of surface pressure.
Amplitude sweep measurement allows defining the linear region of the viscoelastic film and allows defining suitable oscillation amplitude for different thin films. Shear thinning and thickening can also be detected with this method.
Creep test
Allows studying:
- Surface / interfacial viscosity, η
- Elastic moduli, G
- Relaxation times, τ
In creep compliance test mode, the instrument provides information to obtain whether the system behaves more like an ideal Newtonian liquid (dashpot model) or ideal elastic (spring model). Viscoelastic systems are more complex as they combine both elements. These can be modelled with Maxwell and Kelvin-Voigt models. In the creep compliance measurements constant stress (σ) is applied and related strain (γ) is measured.
With Maxwell model the Elastic modulus, G is calculated from immediate response after stress is applied and the viscosity derives from the time dependent deformation (slope).
In Kelvin-Voigt model both spring and dashpot are parallel and the strain response to appllied stress is time dependent and nonlinear. The response can be separated to elastic, G, and viscous, η, components. From these components the relaxation time, τ, can be determined
In most cases the viscoelastic behaviour needs to be modelled using different combination of both Maxwell and Kelvin-Voigt models such as Standard Linear Solid model and Generalized Maxwell model.
Instead of strain compliance (J) is often used in literature since it takes into account used force.
Addition to the interfacial shear rheology also the dilational rheology can be measured as complementary technique using Langmuir trough. It is notable that these are not directly comparable since the surface packing density changes and absorption/desorption may occur in the dilational measurements.
