Interfacial Rheology
Just as rheology is the study of flow in bulk fluids, interfacial rheology is the study of the
flow properties of interfaces. These flow properties are important in
determining, for example, the behaviour of emulsions and foams. Important rheological properties are viscosity, viscous (loss) modulus, elastic (storage) modulus and creep. There
are two main types of interfacial flow:
- Dilational / dilatational flow - This type of flow occurs due to expansion or compression of the
interface. Interfacial rheological parameters can be determined by
changing the area of the interface, and measuring the resulting change
in interfacial tension. An oscillatory sine wave area change can be
applied, so that phase
change, and hence dynamic storage (elastic) and loss (viscous) moduli
can be determined. Applying a step change in interfacial area allows
determination of creep. The measurement can be performed in two ways:
- Drop shape analysis-
Soluble surfactants can be examined using a goniometer, where the
volume of a drop can be varied by applying pressure to the liquid
tubing system, and measuring the interfacial tension via drop shape
analysis. The most reproducible method of changing the drop volume is
with a piezoelectric transducer such as the KSV PD 100.
- Langmuir trough - oscillating the barriers of a Langmuir trough and measuring the
resulting change in surface pressure provides the desired information
for insoluble materials.
- Shear flow -
Occurs due to longitudinal deformation of the interface, i.e. in
1-dimension. Behaviour under shear flow can deviate significantly from
dilational flow. The most sensitive method of measuring shear flow is
using the KSV ISR 400, which applies a magnetic field to a teflon-coated
needle floated at a fluid/fluid interface, the position of which is
determined microscopically. Analagous to dilational flow, the effect of
sine-wave formed deformation provides phase change, and hence dynamic
storage (elastic) and loss (viscous) shear moduli and a step change in
drop volume allows determination of creep.
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