Surface rheology is a description of the
rheological properties of a
free surface
In physics, a free surface is the surface of a fluid that is subject to zero parallel shear stress,
such as the interface between two homogeneous fluids.
An example of two such homogeneous fluids would be a body of water (liquid) and the air in ...
. When perfectly pure, the
interface between fluids usually displays only
surface tension
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...
. The stress within a fluid interface can be affected by the adsorption of
surfactants in several ways:
* Change in the surface concentration of surfactants when the in-plane flow tends to alter the surface area of the interface (Gibbs' elasticity).
*
Adsorption/
desorption
Desorption is the physical process where Adsorption, adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding e ...
of the surfactants to/from the interface.
Importance of surface rheology
The mechanical properties (
rheology
Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applie ...
) of
dispersed media such as
liquid foams and
emulsions
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...
is strongly affected by surface rheology. Indeed, when they consist of two (or more) fluid phases, deforming the material implies deforming the constitutive phases (
bubbles,
drops) and thus their
interfaces.
The measurement of surface rheological properties is described by
storage and loss moduli. In the case of a linear response to a sinusoidal deformation, the loss modulus is the product of the viscosity by the frequency. One of the difficulties of surface rheology measurements come from the fact that the adsorbed layers are usually rather compressible (at the difference of bulk fluids which are essentially incompressible), and both compression and shear parameters should be determined. This determination requires different type of rheometric instruments, for instance oscillating
drops for the
compression properties and oscillating
bicones for the
shear properties.
These two methods allow investigating the variation of the parameters upon the amplitude of the deformation. The responses of adsorbed layers to deformations are frequently non-linear, making this variation measurement relevant to rheological studies.
References
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Rheology