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Wettability, contact angle and measurement

Wettability, contact angle and measurement

 

Wettability, contact angle and measurement

When placing a droplet on a solid surface, one of the following conditions may occur:

The drop spreads completely over the solid surface, forming a liquid film on top of it.

The drop spreads over the solid surface and forms a flatter droplet on it.

Droplets do not almost spread out on the solid surface, but "sit" on them to form a highly protruding droplet.

 

Wetting refers to the process of replacing one another immiscible fluid (gas or another liquid) with a liquid on a solid surface. The degree to which a liquid is wetted on a solid surface can be measured by the contact angle, which is the angle between the liquid / air interface and the solid surface (including the liquid phase portion)

 

In the first case, the contact angle is 0 ° and we say that the liquid can completely wet the solid surface; in the second case the contact angle is greater than 0 ° but less than 90 °, we say that the liquid can partially wet The solid surface; in the third case, the contact angle is greater than 90 °, we say that the liquid can not wet the solid surface.

 

For a given three-phase system of liquid / solid surface / atmosphere (which may also be another liquid phase incompatible with the liquid), the contact angle should be a specific value which is determined by the interaction between the three phases , That is, liquid / gas, liquid / solid and solid / gas interface, is determined by the system itself, the result of the pursuit of the minimum total energy. The shape of the droplet on the solid surface is determined by the Y-L equation, while the contact angle functions as a boundary condition. In the ideal case, the relationship between the contact angle and the three-phase interaction force can be described by the following Young's equation (Young's eq.)

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Range and examples of contact angles

4) water droplets on the lotus leaf

Pendant Drop after fittting The range of physically meaningful contact angles is from 0 ° to 180 °. A contact angle of 0 ° means that the liquid completely spreads over the solid surface until a monolayer is formed (if there is no obstruction!). A contact angle of 0 ° to 30 ° indicates that the liquid has a good wettability to the solid surface and will spread well. This range is important for many processes such as paints, coatings, cleaning, sticking Knot and so on. The contact angle between 30 ° and 90 ° means that the liquid has a certain wettability to the solid surface, but it is not very good. When the contact angle is greater than 90 °, the liquid has no wettability to the solid surface. When this angle As the angle increases to about 130 ° -140 °, the liquid begins to exhibit surface repellency on the solid surface. When the contact angle is increased above 150 °, the droplet is in fact just "sitting" on the surface, leaving the surface as soon as it has a chance and exhibiting a high degree of repellency on the surface. This is the case of water droplets on the lotus foliage, with contact angles up to about 170 °, known as the "lotus effect," which are also often referred to as superhydrophobic surfaces, They are self-cleaning and have great potential applications, so it is a hot spot in current research.

 Δθ = θa - θr

The actual non-ideal surface, resulting in Young contact angle characterization of the surface wettability of the traditional practice is not perfect, you must also consider the contact angle hysteresis, so as to fully characterize, such as superhydrophobic surface properties.