July 2008

Advancing and Receding Contact Angles are typically measured using a tilting base option. As the solid is tilted from 0° to 90°, the uphill angle (or receding angle) decreases while the downhill angle (or advancing angle) increases. As some point the drop may release at the Roll-off Angle. The difference between the advancing and receding angle is called the Contact Angle Hysteresis. An alternate method of measuring the advancing and receding angles requires volume of liquid to be added or and removed while dynamically measuring the contact angle as shown below.

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.

Contact Angle Hysteresis is the difference between the Advancing and Receding Contact Angles and is used to characterize surface roughness. It is affected by cleanliness, the presence of Surfactants, and other Surface Treatments.

A Dynamic Contact Angle is measured on a drop that is in motion -- for example, while volume is being added to or removed from the drop -- or while tilt is occurring.

Goniometer or Contact Angle Goniometer is instrument that is used to precisely measure static and dynamic contact angles of liquids on solids. The modern contact angle goniometer was invented by Dr. William Albert Zisman at the Naval Research Laboratory in Washington DC and built by ramé-hart in New Jersey. The original Zisman design, aka the NRL Goniometer Model 100-00, was in production for nearly four decades. The current generation ramé-hart goniometer replaces the microscope with a digital camera and imaging software to collect and measure contact angle. Additionally, the new generation of instruments can calculate surface energy, surface tension as well as perform advancing and receding measurements and other more advanced tasks.

Hydrophilicity occurs when a drop forms with a small contact angle, say under 10°, and wetting is complete and surface energy is very high. If the contact angle approaches 0°, the material is said to be superhydrophilic.

Hydrophobicity occurs when a drop forms with a large contact angle, say over 100°. In this condition wetting is considered poor and surface energy is low. If the contact angle exceeds 150°, such as water on a lotus leaf, then the material is considered superhydrophobic.

Interfacial Tension is similar to Surface Tension except that the external phase is a liquid rather than a gas.

A Pendant Drop hangs, typically from a needle, and can be used to measure Surface Tension.

Roll-off Angle is the maximum tilt angle before a drop releases and rolls off a solid and can be measured using a tilting base option. See Advancing and Receding Contact Angles. Drops on hydrophobic solids tend to have lower roll-off angles than drops on hydrophilic surfaces which often don't even roll off.

A Sessile Drop sits on a solid surface and can be used to measure Contact Angle and Surface Tension.

A Static Contact Angle is measured on a sessile liquid drop on a solid in a gas phase or a captive bubble gas drop in a liquid phase on (the bottom of) a solid when the drop is not changing.

Surface Energy or Surface Free Energy is the excess energy at the surface of a material compared with the material as a whole. There are various methods for measuring surface energy. Young's Equation is one method which can be used to quantify surface energy.

Surface Tension is described as a measurement of energy on the surface of a liquid surrounded by a gas which allows it to behave like an elastic sheet and can be measured using the Pendant Drop or Sessile Drop methods using a ramé-hart Tensiometer.

Surface Treatments consist of Surfactants, which minimize surface tension of liquids, surface coatings and surface altering treatments (such as hardening, diffusion, and glazing) which are designed to alter the Surface Energy of solids.

Surfactant, derived from "surface acting agent", is a wetting agent used to lower the surface tension of a liquid allowing improved wetting. They can also reduce the interfacial tension between two liquids (such as oil and water).

Tensiometer is any instrument that can measure Surface Tension. The ramé-hart Model 250 and 500 instruments with DROPimage Advanced, for example, can be used as tensiometers as well as goniometers.

Wettability defines the degree to which a solid will wet. If a drop spreads out indefinitely and the Contact Angle approaches 0°, then total wetting is occurring. In most cases, however, the drop will bead up and only partial wetting (or non-wetting) will occur.  The extent to which a solid will wet can be quantified by measuring the Contact Angle.

Young's Equation is used to explain the balance of forces of a liquid drop on a solid surface.

If you have any corrections, amplifications, or suggestions for additional terms, please contact us.