Cassie's law
Encyclopedia
Cassie's law describes the effective contact angle
θ
c for a liquid on a composite surface
. The law explains how simply roughing up a surface increases the apparent surface angle. The law is stated as:
where by θ
1 is the contact angle for component 1 with areal fraction f1 and θ
2 is the contact angle for component 2 with areal fraction f2 present in the composite material. This equation takes on special meaning when in a 2-component system one component is air with a contact angle
of 180°
. With cosine(180) = –1, the equation reduces to:
which implies that with a small f1 and a large θ
1 it is possible to create surfaces with a very large contact angle. Cassie's research pointed out that the water repelling quality of duck
s is due to the very nature of the composite formed between air and feather and not by other causes such as the presence of exceptional proofing agents like oils. Water strider
s also exploit this phenomenon. Artificial superhydrophobic materials such as nanopin film
exist in the laboratory that also make use of this law.
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...
θ
Theta
Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth...
c for a liquid on a composite surface
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
. The law explains how simply roughing up a surface increases the apparent surface angle. The law is stated as:
where by θ
Theta
Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth...
1 is the contact angle for component 1 with areal fraction f1 and θ
Theta
Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth...
2 is the contact angle for component 2 with areal fraction f2 present in the composite material. This equation takes on special meaning when in a 2-component system one component is air with a contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...
of 180°
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
. With cosine(180) = –1, the equation reduces to:
which implies that with a small f1 and a large θ
Theta
Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth...
1 it is possible to create surfaces with a very large contact angle. Cassie's research pointed out that the water repelling quality of duck
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...
s is due to the very nature of the composite formed between air and feather and not by other causes such as the presence of exceptional proofing agents like oils. Water strider
Water strider
Gerridae is a family of true bugs in the order Hemiptera, commonly known as water striders, water bugs, magic bugs, pond skaters, skaters, skimmers, water scooters, water skaters, water skeeters, water skimmers, water skippers, water spiders, or Jesus bugs...
s also exploit this phenomenon. Artificial superhydrophobic materials such as nanopin film
Nanopin film
Nanopin film is an experimental material in nanotechnology developed in 2005 with unusual superhydrophobic properties . A droplet of water makes contact with the surface of this film and forms an almost perfect sphere with a contact angle of 178°. The film is able to do this because it is covered...
exist in the laboratory that also make use of this law.