Rheology is a branch of science involving the study of material flow. Interfacial rheology concerns the unique two-dimensional systems formed between two immiscible phases, such as liquid/gas, and liquid/liquid.

The stability of interface greatly depends on the viscoelastic properties of the interface, which makes it a crucial aspect for many industries where foams, emulsions and dispersions are used such as pharmaceuticals, foods, beverages, cosmetics and coatings.

Interfacial rheology is a challenging field of research because of the magnitude of forces in the interface is exceedingly small. Rotating ring and bicone methods have been developed, but they only work with macromolecular compounds at the interfaces. The KSV NIMA ISR utilizes a small magnetic probe that is moved with an oscillating magnetic field. The method reduces the inertia and enhances the sensitivity of the probe compared to the rotating ring method to enable the measurement of 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

Interfacial Shear Rheology instruments use a Helmholtz coil to create a uniform magnetic field in the area of measurement. A Helmholtz coil uses two identical circular magnetic coils that are separated from each other with the same distance as the coil radius. This allows the magnetized probe to move in the measuring area by controlling the magnetic field. Since no mechanical connection is used the instrument sensitivity is dramatically increased to allow the measurement of very weak viscoelastic forces. 

Instrumental setup

The measurement probes are designed to float easily on the liquid interface due to their lightweight and water repellent coating. The metal inside the probe is magnetized and the probe is ready to use. Since thin film viscoelasticity can vary between different samples there are different sizes of probes to match the experimental conditions.

The probe moves inside a glass channel that creates a small meniscus on both sides of the surface. This channel guides the probe in a straight line and ensures uniform flow geometry. To ensure the glass channel is centered every time a channel holder is used to guide it in the center of the trough. In this way, the magnetic probe easily set in the middle of the magnetic field. The camera and lens are mounted on a xyz-stage to simplify setup. 

Dynamic measurement

Dynamic measurement allows the definition of:

  • 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.

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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

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The dynamic testing includes three different measurement types:

  • Frequency sweep allows measurement of the viscoelastic properties at different frequencies. This method is extremely useful since rheology is time dependent. It allows defining thin film rheological behavior in different time scales and provides information about the dominance of viscosity or elasticity.
  • Single frequency measurement can be used to define the time dependency of viscoelastic properties. When combined with a 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

Creep testing allows the study of:

  • 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 modeled with Maxwell and Kelvin-Voigt models. In the creep compliance measurements constant stress (σ) is applied and related strain (γ) is measured.

In the 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).

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In the Kelvin-Voigt model both the spring and the dashpot are parallel and the strain response to applied 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

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in most cases the viscoelastic behavior needs to be modeled using different combinations of the Maxwell and the Kelvin-Voigt models, such as Standard Linear Solid model and Generalized Maxwell model.

Instead of strain compliance (J), which is often used in literature, it takes into account the used force.

In addition to the interfacial shear rheology, dilational rheology can be measured as complementary technique using a Langmuir trough. Please note that these techniques are not directly comparable since the surface packing density changes and absorption/desorption may occur in the dilational measurements.

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