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Information on Contact Angle

A contact angle can be measured by producing a drop of pure liquid on a solid. The angle formed between the solid/liquid interface and the liquid/vapor interface and which has a vertex where the three interfaces meet is referred to as a the contact angle.

Young's equation as shown below is used to describe the interactions between the forces of cohesion and adhesion and measure what is referred to as surface energy.

A drop with a large contact angle is hydrophobic. This condition is exemplified by poor wetting, poor adhesiveness and the solid surface free energy is low. A drop with a small contact angle is hydrophilic. This condition reflects better wetting, better adhesiveness, and higher surface energy.

Contact angle is a primary tool used to measure cleanliness. Organic contaminants will prevent wetting and result in higher contact angles. As a surface is cleaned and treated to remove contaminants the contact angle typically will decrease as wetting improves and surface energy increases.

In the fabrication of semiconductors, for example, contact angle is frequently used to characterize the wettability of the silicon wafer in an effort to characterize the efficacy of production processes and surface modifications such as etching, passivation, ultrasonic agitation, and other surface treatments and cleaning processes -- as well as to quantify the effects of resins, primers, oxidation, bonding, annealing, and polishing.

Various types of dynamic studies are conducted to determine more advanced wetting properties. One study requires adding volume to the drop dynamically to the maximum volume permitted without increasing the interfacial area between the liquid and solid phases. The resulting contact angle is referred to as the advancing angle. Volume is then removed from the drop. When the maximum volume that can be removed without reducing the solid/liquid interface is reached, the resulting contact angle is measured. This angle is the receding angle. When the receding angle is subtracted from the advancing angle, the result is called the contact angle hysteresis. The hysteresis characterizes surface topology and can help quantify contamination, surface chemical heterogeneity, and the effect of surface treatments, surfactants and other solutes. The advancing and receding angle can also be measured using the ramé-hart Tilting Base option. 

Contact angle is an invaluable metric for understanding material surface properties -- adhesion, wettability, and solid surface free energy. Contact angle is used to measure cleanliness, the effects of surface treatments, adhesiveness, repellency. To learn more about surface energy and contact angle, view this PDF file.

In recent years, many researchers have been exploring the Cassie and Wenzel states in an attempt to better understand superhydrophobicity. In short, in a Cassie state, a drop sits on top of asperities and there are air voids below the drop as shown in the picture below. The percentage of surface area that touches the drop is used to define the apparent (or measured) contact angle. In a Wenzel state, the drop fills all of the area under the drop and apparent contact angle is calculated based on roughness. For a detailed overview of Cassie and Wenzel, see our Sept 2008 Newsletter here.

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