Interface (matter)

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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 in surface science. In thermal equilibrium, the regions in contact are called phases, and the interface is called a phase boundary. An example for an interface out of equilibrium is the grain boundary in polycrystalline matter.

The importance of the interface depends on the type of system: the bigger the quotient area/volume, the greater the effect the interface will have. Consequently, interfaces are very important in systems with large interface area-to-volume ratios, such as colloids.

Interfaces can be flat or curved. For example, oil droplets in a salad dressing are spherical but the interface between water and air in a glass of water is mostly flat.

Surface tension is the physical property which rules interface processes involving liquids. For a liquid film on flat surfaces, the liquid-vapor interface keeps flat to minimize interfacial area and system free energy. For a liquid film on rough surfaces, the surface tension tends to keep the meniscus flat, while the disjoining pressure makes the film conformal to the substrate. The equilibrium meniscus shape is a result of the competition between the capillary pressure and disjoining pressure. [1]

Interfaces may cause various optical phenomena, such as refraction. Optical lenses serve as an example of a practical application of the interface between glass and air.

One topical interface system is the gas-liquid interface between aerosols and other atmospheric molecules.

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Phase (matter) Region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform; region of material that is chemically uniform, physically distinct, (often) mechanically separable

In the physical sciences, a phase is a region of space, throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetization and chemical composition. A simple description is that a phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable. In a system consisting of ice and water in a glass jar, the ice cubes are one phase, the water is a second phase, and the humid air is a third phase over the ice and water. The glass of the jar is another separate phase.

Surface tension Tendency of a liquid surface to shrink to reduce surface area

Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension allows insects, to float and slide on a water surface without becoming even partly submerged.

Sintering Process of forming and bonding material by heat or pressure

Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.

Meniscus (liquid)

A concave meniscus occurs when the particles of the liquid are more strongly attracted to the container (adhesion) than to each other (cohesion), causing the liquid to climb the walls of the container. This occurs between water and glass. Water-based fluids like sap, honey, and milk also have a concave meniscus in glass or other wettable containers.

Foam Form of matter

Foam is an object formed by trapping pockets of gas in a liquid or solid.

In fluid dynamics, the capillary number (Ca) is a dimensionless quantity representing the relative effect of viscous drag forces versus surface tension forces acting across an interface between a liquid and a gas, or between two immiscible liquids. For example, an air bubble in a liquid flow tends to be deformed by the friction of the liquid flow due to viscosity effects, but the surface tension forces tend to minimize the surface area. The capillary number is defined as:

Wetting Ability of a liquid to maintain contact with a solid surface

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Wetting deals with three phases of matter: gas, liquid, and solid. It is now a center of attention in nanotechnology and nanoscience studies due to the advent of many nanomaterials in the past two decades.

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, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle. However, in practice a dynamic phenomenon of contact angle hysteresis is often observed, ranging from the advancing (maximal) contact angle to the receding (minimal) contact angle. The equilibrium contact is within those values, and can be calculated from them. The equilibrium contact angle reflects the relative strength of the liquid, solid, and vapour molecular interaction.

In fluid statics, capillary pressure is the pressure between two immiscible fluids in a thin tube, resulting from the interactions of forces between the fluids and solid walls of the tube. Capillary pressure can serve as both an opposing or driving force for fluid transport and is a significant property for research and industrial purposes. It is also observed in natural phenomena.

A dispersion is a system in which distributed particles of one material are dispersed in a continuous phase of another material. The two phases may be in the same or different states of matter.

Cheerios effect Phenomenon that occurs when floating objects attract one another

In fluid mechanics, the Cheerios effect is the phenomenon that occurs when floating objects that do not normally float attract one another. Wetting, an example of the "Cheerios effect," is when breakfast cereal clumps together or clings to the sides of a bowl of milk. It is named after the common breakfast cereal Cheerios and is due to surface tension. The same effect governs the behavior of bubbles on the surface of soft drinks.

Young–Laplace equation

In physics, the Young–Laplace equation is a nonlinear partial differential equation that describes the capillary pressure difference sustained across the interface between two static fluids, such as water and air, due to the phenomenon of surface tension or wall tension, although use of the latter is only applicable if assuming that the wall is very thin. The Young–Laplace equation relates the pressure difference to the shape of the surface or wall and it is fundamentally important in the study of static capillary surfaces. It is a statement of normal stress balance for static fluids meeting at an interface, where the interface is treated as a surface :

Wilhelmy plate Device used to measure surface tension

A Wilhelmy plate is a thin plate that is used to measure equilibrium surface or interfacial tension at an air–liquid or liquid–liquid interface. In this method, the plate is oriented perpendicular to the interface, and the force exerted on it is measured. Based on the work of Ludwig Wilhelmy, this method finds wide use in the preparation and monitoring of Langmuir films.

Capillary length

The capillary length or capillary constant, is a length scaling factor that relates gravity and surface tension. It is a fundamental physical property that governs the behavior of menisci, and is found when body forces (gravity) and surface forces are in equilibrium.

Capillary condensation

Capillary condensation is the "process by which multilayer adsorption from the vapor [phase] into a porous medium proceeds to the point at which pore spaces become filled with condensed liquid from the vapor [phase]." The unique aspect of capillary condensation is that vapor condensation occurs below the saturation vapor pressure, Psat, of the pure liquid. This result is due to an increased number of van der Waals interactions between vapor phase molecules inside the confined space of a capillary. Once condensation has occurred, a meniscus immediately forms at the liquid-vapor interface which allows for equilibrium below the saturation vapor pressure. Meniscus formation is dependent on the surface tension of the liquid and the shape of the capillary, as shown by the Young-Laplace equation. As with any liquid-vapor interface involving a meniscus, the Kelvin equation provides a relation for the difference between the equilibrium vapor pressure and the saturation vapor pressure. A capillary does not necessarily have to be a tubular, closed shape, but can be any confined space with respect to its surroundings.

In physics, the maximum bubble pressure method, or in short bubble pressure method, is a technique to measure the surface tension of a liquid, with surfactants.

Disjoining pressure, in surface chemistry, according to an IUPAC definition, arises from an attractive interaction between two surfaces. For two flat and parallel surfaces, the value of the disjoining pressure can be calculated as the derivative of the Gibbs energy of interaction per unit area in respect to distance. There is also a related concept of disjoining force, which can be viewed as disjoining pressure times the surface area of the interacting surfaces.

The Gibbs–Thomson effect, in common physics usage, refers to variations in vapor pressure or chemical potential across a curved surface or interface. The existence of a positive interfacial energy will increase the energy required to form small particles with high curvature, and these particles will exhibit an increased vapor pressure. See Ostwald–Freundlich equation. More specifically, the Gibbs–Thomson effect refers to the observation that small crystals are in equilibrium with their liquid melt at a lower temperature than large crystals. In cases of confined geometry, such as liquids contained within porous media, this leads to a depression in the freezing point / melting point that is inversely proportional to the pore size, as given by the Gibbs–Thomson equation.

Capillary bridges

Usually, we understand the term capillary bridge as a minimized surface of liquid or membrane, created between two rigid bodies with an arbitrary shape. Capillary bridges also may form between two liquids. Plateau defined a sequence of capillary shapes known as (1) nodoid with 'neck', (2) catenoid, (3) unduloid with 'neck', (4) cylinder, (5) unduloid with 'haunch' (6) sphere and (7) nodoid with 'haunch'. The presence of capillary bridge, depending on their shapes, can lead to attraction or repulsion between the solid bodies. The simplest cases of them are the axisymmetric ones. We distinguished three important classes of bridging, depending on connected bodies surface shapes:

In physics and engineering, the thin-film equation is a partial differential equation that approximately predicts the time evolution of the thickness h of a liquid film that lies on a surface. The equation is derived via lubrication theory which is based on the assumption that the length-scales in the surface directions are significantly larger than in the direction normal to the surface. In the non-dimensional form of the Navier-Stokes equation the requirement is that terms of order and are negligible, where is the aspect ratio and is the Reynold's number. This significantly simplifies the governing equations. However lubrication theory, as the name suggests, is typically derived for flow between two solid surfaces hence the liquid forms a lubricating layer. The thin-film equation holds when there is a single free surface. With two free surfaces the flow must be treated as a viscous sheet.


  1. Hu, Han; Weinberger, Christopher R.; Sun, Ying (2014-11-17). "Effect of Nanostructures on the Meniscus Shape and Disjoining Pressure of Ultrathin Liquid Film". Nano Letters . American Chemical Society (ACS). 14 (12): 7131–7137. doi:10.1021/nl5037066. ISSN   1530-6984.