Tension Superficial

CONTACT ANGLES

Learning Objectives

Study Guidelines

What is a contact angle and what are its causes?

The contact angle is the angle that is formed when a liquid comes into contact with a solid.  The magnitude of the contact angle depends primarily on the ratio of the adhesive forces between liquid and solid and the cohesive forces of the liquid.  When adhesive forces are large in relation to the cohesive forces, the contact angle is less than 90 degrees.  Angles less than 90 degrees will result in a tendency of the liquid to wet the surface.

See a diagram:    

Free body diagram of a contact angle

 

See an illustration:  

Contact angle of a water droplet in contact with water repellent fabric

What is a static contact angle?

A drop of liquid place over a non-absorbent solid may reach equilibrium.  When this condition occurs the contact angle is a static contact angle.  High viscosity liquids (e.g. glycerin) may require a prolonged period to reach a static angle.

What is a dynamic contact angle?

In absorbent surfaces, when the liquid penetrates the substrate, the contact angle changes continuously with time.  For certain materials the process time is critical to workflow.  To measure the dynamic contact angle, and the rates of absorption and expansion, it becomes necessary to capture an image sequence over the process.

Measurement of contact angles

Measure the contact angle for a) water on glass, b) water on soap, and c) water on water repellent fabric.  To do this use a digital camera that has macro capabilites and measure the angle in the photographs. If you do not have access to a digital camera, observe the differences of the contact angles of water on the different materials.

What other parameters should be recorded with the contact angle?

 

RESOURCES