{{more references, date=November 2011 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 In the physical sciences, an interface is the boundary between two spatial regions occupied by different matter, or by matter in different physical states. The interface between matter and air, or matter and vacuum, is called a surface, and studied ...

usually displays only surface tension. But when surfactants are adsorbed on the interface, because they lower the surface tension, the stress within the interface is affected by the flow for several reasons. * Change in the surface concentration of surfactants when the in-plane flow tends to alter the surface area of the interface (Gibbs' elasticity). *

Adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

desorption Desorption is the physical process where a previously adsorbed substance is released from a surface. This happens when a molecule gains enough energy to overcome the activation barrier of the bounding energy that keeps it in the surface. There ...

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 Plasticity (physics), plastic flow rather than deforming Elasticity (phy ...

) of

dispersed media A dispersed medium consists of two media that do not mix. More specifically, it contains discrete elements of one medium which are dispersed in a continuous second medium. The two media can be of very different nature. In particular, they can be a ...

such as liquid foams and emulsions 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 instruments, for instance oscillating drops for the

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

properties and oscillating bicones for the

shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...

properties. These two methods allow investigating the variation of the parameters upon the amplitude of the deformation. This is very useful as the responses of adsorbed layers to deformations are frequently non-linear. Rheology