Understanding Contact Angle

A contact angle can be measured by producing a drop of liquid on a solid. The angle formed between the solid/liquid interface and the liquid/vapor interface is the contact angle. In practice, the most common method involves observing the drop profile and measuring the angle formed at the three-phase line.

Young’s equation is used to describe the balance between cohesive and adhesive forces at the interface and is often used as part of surface energy analysis.

A drop with a contact angle above 90° is generally considered hydrophobic. This condition is associated with poor wetting, poor adhesiveness, and lower surface free energy. A small contact angle is generally considered hydrophilic, reflecting better wetting, better adhesiveness, and higher surface energy.

Static contact angle is the most common type of contact angle measurement. It is typically a single reading captured from a sessile drop shortly after its creation, once the system has reached an equilibrium among the solid, liquid, and gas phases.

Static contact angle provides valuable information about the properties of a surface. It is often used to evaluate cleanliness and the effects of surface treatments. Organic contamination can inhibit wetting and produce higher contact angles on otherwise hydrophilic surfaces. As a surface is cleaned or treated, the contact angle often decreases as wetting improves and surface energy increases.

In semiconductor fabrication, for example, contact angle is frequently used to characterize the wettability of silicon wafers and to evaluate the effects of etching, passivation, agitation, cleaning, oxidation, bonding, annealing, polishing, primers, and resins.

Surface roughness can also influence measured contact angle, which is one reason contact angle is often interpreted together with knowledge of the sample history and surface condition.

Any contact angle measured while the drop or surface condition is changing can be considered a dynamic contact angle measurement. Dynamic studies include tilting plate measurements, adding or removing volume from a drop, and time-dependent experiments that track contact angle as the system evolves.

Dynamic contact angle methods are especially useful for studying advancing and receding contact angles, contact angle hysteresis, absorption, evaporation, and changes caused by environmental or process conditions.

In the tilting plate method, contact angles are measured on both the left and right sides of a sessile drop while the solid surface is inclined, often from 0° up to 90°. As the sample tilts, gravity causes the downhill contact angle to increase and the uphill contact angle to decrease.

These two measurements are referred to as the advancing and receding contact angles. The difference between them is the contact angle hysteresis. In some cases, the drop may roll off the surface; in others, the surface may tilt all the way to 90° without release.

Another way to study advancing and receding contact angles is to dynamically add and remove liquid from a sessile drop. As volume is added, the maximum contact angle that can be reached before the three-phase line advances is taken as the advancing angle.

Volume can then be removed to study receding behavior. When the maximum volume is removed without pulling back the three-phase line, the corresponding angle is used as the receding angle. The difference between advancing and receding angles is the contact angle hysteresis.

This method is often useful for characterizing contamination, surface chemical heterogeneity, topology, and the effects of treatments, surfactants, and solutes. In practice, the receding angle can be more difficult to capture cleanly because the needle used to withdraw liquid disturbs the drop geometry. An evaporation-based receding-angle method is often used as an alternative.

Researchers often monitor contact angle over time to study absorption, evaporation, and changes in wetting behavior. Time-dependent experiments can also track how contact angle responds to environmental changes such as temperature and humidity or to changes in the test liquid itself.

These studies are also useful for observing transitions such as the Cassie and Wenzel states in rough or textured surfaces. In a Cassie state, the drop sits on asperities with air pockets beneath it. In a Wenzel state, the liquid fills the roughness. These states can produce very different apparent contact angles and are central to many superhydrophobicity studies.

An alternate method for measuring contact angle is the Wilhelmy plate method, in which a plate is lowered into and withdrawn from a liquid while the force on the plate is measured.

This method is more complicated than the sessile-drop approach, requires larger liquid volumes, depends on precisely fabricated samples with two matching surfaces, and does not directly characterize heterogeneity in the same way as drop-profile methods. It also requires a precision force measurement system.

Because of these disadvantages for many common surface science workflows, ramé-hart focuses on optical contact angle instrumentation rather than Wilhelmy plate systems.

Contact angle is an invaluable metric for understanding material surface properties such as adhesion, wettability, repellency, cleanliness, and solid surface energy. If you would like help choosing an instrument or measurement method for your application, our team can help.