Dewetting
Encyclopedia
In fluid mechanics
, dewetting is one of the processes that can occur at a solid–liquid or liquid–liquid interface
. Generally, dewetting describes the rupture of a thin liquid film on the substrate (either a liquid itself, or a solid) and the formation of droplets. The opposite process—spreading of a liquid on a substrate—is called spreading. The factor determining the spontaneous spreading and dewetting for a drop of oil placed on a liquid substrate (water here) with ambient gas, is the so-called spreading coefficient
:
where
is the gas-water surface tension
,
is the gas-oil surface tension and 
is the oil water surface tension (measured on the fluids before they are brought in contact with each other)
When
, the spontaneous spreading occurs, and if 
, dewetting occurs.
Spreading and dewetting are important processes for many applications, including adhesion
, lubrication
, painting, printing, and protective coating. For most applications, dewetting is an unwanted process, because it destroys the applied thin film.
In most dewetting studies a thin polymer
film is spun onto a substrate. Even in the case of
the film does not dewet immediately if it is in a metastable state, e.g. if the temperature is below the glass transition temperature of the polymer. Annealing such a metastable film above its glass transition temperature increases the mobility of the polymer-chain molecules and dewetting takes place.
When starting from a continuous film, an irregular pattern of droplets is formed. The droplet size and droplet spacing may vary over several orders of magnitude, since the dewetting starts from randomly formed holes in the film. There is no spatial correlation between the dry patches that develop. These dry patches grow and the material is accumulated in the rim surrounding the growing hole. In the case where the initially homogeneous film is thin (in the range of 100 nm (1E-07 m)), a polygon
network of connected strings of material is formed, like a Voronoi pattern of polygons. These strings then can break up into droplets, a process which is known as the Rayleigh–Taylor instability. At other film thicknesses, other complicated patterns of droplets on the substrate can be observed, which stem from a fingering instability of the growing rim around the dry patch.
Surfactants can have a significant effect on the spreading coefficient. When a surfactant is added, it would decrease the interfacial tension, thus increasing the spreading coefficient (i.e. making S more positive) and allowing wetting to occur.
Fluid mechanics
Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...
, dewetting is one of the processes that can occur at a solid–liquid or liquid–liquid interface
Interface (chemistry)
An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...
. Generally, dewetting describes the rupture of a thin liquid film on the substrate (either a liquid itself, or a solid) and the formation of droplets. The opposite process—spreading of a liquid on a substrate—is called spreading. The factor determining the spontaneous spreading and dewetting for a drop of oil placed on a liquid substrate (water here) with ambient gas, is the so-called spreading coefficient
:where
is the gas-water surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
,
is the gas-oil surface tension and is the oil water surface tension (measured on the fluids before they are brought in contact with each other)When
, the spontaneous spreading occurs, and if , dewetting occurs.Spreading and dewetting are important processes for many applications, including adhesion
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
, lubrication
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...
, painting, printing, and protective coating. For most applications, dewetting is an unwanted process, because it destroys the applied thin film.
In most dewetting studies a thin polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
film is spun onto a substrate. Even in the case of
the film does not dewet immediately if it is in a metastable state, e.g. if the temperature is below the glass transition temperature of the polymer. Annealing such a metastable film above its glass transition temperature increases the mobility of the polymer-chain molecules and dewetting takes place.When starting from a continuous film, an irregular pattern of droplets is formed. The droplet size and droplet spacing may vary over several orders of magnitude, since the dewetting starts from randomly formed holes in the film. There is no spatial correlation between the dry patches that develop. These dry patches grow and the material is accumulated in the rim surrounding the growing hole. In the case where the initially homogeneous film is thin (in the range of 100 nm (1E-07 m)), a polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...
network of connected strings of material is formed, like a Voronoi pattern of polygons. These strings then can break up into droplets, a process which is known as the Rayleigh–Taylor instability. At other film thicknesses, other complicated patterns of droplets on the substrate can be observed, which stem from a fingering instability of the growing rim around the dry patch.
Surfactants can have a significant effect on the spreading coefficient. When a surfactant is added, it would decrease the interfacial tension, thus increasing the spreading coefficient (i.e. making S more positive) and allowing wetting to occur.