Dewetting
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In
fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids ( liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and bio ...
, dewetting is one of the processes that can occur at a solid–liquid, solid–solid or liquid–liquid
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Inte ...
. Generally, dewetting describes the process of retraction of a
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
from a non-wettable surface it was forced to cover. The opposite process—spreading of a liquid on a substrate—is called ''
wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
''. The factor determining the spontaneous spreading and dewetting for a drop of liquid placed on a solid substrate with ambient gas, is the so-called spreading coefficient : :S\ = \gamma_\text - \gamma_\text - \gamma_\text where is the solid-gas
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...
, is the solid-liquid surface tension and is the liquid-gas surface tension (measured for the mediums before they are brought in contact with each other). When , the spontaneous spreading occurs, and if , partial wetting is observed, meaning the liquid will only cover the substrate to some extent. The equilibrium
contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
\theta_\textis determined from the Young-Laplace equation. Spreading and dewetting are important processes for many applications, including
adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be ...
,
lubrication Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology. Lubrication mechanisms such as fluid-lubric ...
, painting, printing, and protective coating. For most applications, dewetting is an unwanted process, because it destroys the applied liquid film. Dewetting can be inhibited or prevented by photocrosslinking the thin film prior to annealing, or by incorporating nanoparticle additives into the film. Surfactants can have a significant effect on the spreading coefficient. When a surfactant is added, its amphiphilic properties cause it to be more energetically favorable to migrate to the surface, decreasing the interfacial tension and thus increasing the spreading coefficient (i.e. making S more positive). As more surfactant molecules are absorbed into the interface, the free energy of the system decreases in tandem to the surface tension decreasing, eventually causing the system to become completely
wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
. In biology, by analogy with the physics of liquid dewetting, the process of tunnel formation through endothelial cells has been referred to as
cellular dewetting Cellular dewetting refers to the process of nucleation and enlargement of transendothelial cell macroaperture (TEM) tunnels in endothelial cells (Figure 1). This phenomenon is analogous to the nucleation and growth of dry patches in viscous liquids ...
.


Dewetting of polymer thin films

In most dewetting studies a thin
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
film is spin-cast onto a substrate. Even in the case of S<0 the film does not dewet immediately if it is in a metastable state, e.g. if the temperature is below the
glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
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. The process of dewetting occurs by the nucleation and growth of randomly formed holes, which coalesce to form a network of filaments, before breaking into droplets. 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 ), a
polygon In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two toge ...
network of connected strings of material is formed, like a
Voronoi pattern In mathematics, a Voronoi diagram is a Partition of a set, partition of a plane (geometry), plane into regions close to each of a given set of objects. In the simplest case, these objects are just finitely many points in the plane (called seeds, s ...
of polygons. These strings then can break up into droplets, a process which is known as the Plateau-Rayleigh 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. File:100nm hole (2).jpg, Circular hole formed in a 100 nm thick film of polystyrene. The blue color of the film is due to
Structural coloration Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wit ...
and depends on the film's thickness. File:AFM height profile for 100nm hole.jpg, AFM height profile of a dewetting hole's rim. Notice that the material that is removed by dewetting is accumulated at the rim around the hole; initial film thickness (height): 100 nm. File:100nm tessellation.jpg, Voronoi tessellation pattern of polygons achieved by the coalescence of dewetting holes. File:100nm droplets.jpg, If given enough time, this network of polygons decays into separate droplets. File:200nm unstable.jpg, Rim instability in the case of a thicker (200 nm) polystyrene film.


Dewetting of metal thin films

Solid-state dewetting of the metal thin films describe the transformation of a thin film into an energetically favoured set of droplets or particles at temperatures well below the melting point. The driving force for dewetting is the minimization of the total energy of the free surfaces of the film and substrate as well as of the film-substrate interface. The dedicated heating stage in SEM has been widely used to accurately control sample temperature through a thermocouple to observe the in-situ behaviour of the material, and can be recorded as a video format. Meanwhile, the two-dimensional morphology can be directly observed and characterised. ie. the partially dewetted Ni film is itself a workable fuel electrode for SOCs as it provides long TPB lines if the structure is fine enough, the connectivity of the nickel and pore phases as well as the TPB lines can be used for SOFC characterisation.


References


External links

{{Sister project links, wikt=no , commons=Dewetting , b=no , n=no , q=no , s=no , v=no , voy=no , species=no , d=no Fluid mechanics Surface science