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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 referred to as a the contact angle. The most common method for measurement involves looking at the profile of the drop and measuring two-dimensionally the angle formed between the solid and the drop profile with the vertex at the three-phase line as shown in the graphic below. Young's equation 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 contact angle over 90° 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.
Types of Contact Angle Measurements Static Contact Angle This is perhaps the most common type of measurement. A single reading on a static sessile drop shortly after its creation. A static contact angle is captured when a thermodynamic equilibrium is reached between the three phases: solid, liquid, and gas. Static contact angle provides valuable information about the properties of the surface. Any ramé-hart instrument can be used to capture the static contact angle. Contact angle is often used to measure cleanliness. Organic contaminants will prevent wetting and result in higher contact angles on hydrophilic surfaces. 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.
Surface roughness can also affect the static contact angle. For more on roughness, see our Sept 2010 Newsletter. Any contact angle taken on a drop which is motion is considered a dynamic contact angle measurement. These include, but are not limited to, tilting plate contact angle measurements, adding and removing volume, and time-dependent studies. Tilting Plate Method The tilting plate method captures the contact angles measurements on both the left and right sides of a sessile drop while the solid surface is being inclined typically from 0° to 90°. As the surface is inclined, gravity causes the contact angle on the downhill side to increase while the contact angle on the uphill side decreases. Respectively, these contact angles are referred to advancing and receding angles. The difference between them is the contact angle hysteresis. In some cases, the drop will will roll off the solid as wetting occurs at the roll-off angle. The last valid readings are captured and normally represent the advancing and receding contact angles. In some cases, the solid can tilt all the way to 90° without the drop releasing. The final left and right contact angles are used. The graphic below shows a sessile drop as the solid is inclined.
Add and Remove Volume Method There are other ways to capture advancing and receding contact angles. One study requires adding volume to the drop dynamically to the maximum volume permitted without increasing the three-phase line. The resulting maximum possible 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 three-phase line 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. This method produces good results for determining the advancing angle. However, it's difficult to capture the receding contact angle using this method as the needle which must be imbedded in the drop to remove the volume is also disturbing the geometry of the drop profile. An alternate method for capturing the receding contact angle is the evaporating method. The drop is measured repeatedly as the volume evaporates. Just prior to dewetting and subsequent reduction in the three-phase line, the receding contact angle is measured.
Time-dependant Dynamic Studies Researchers often will watch the contact angle over time to study the effects of absorption, evaporation, and more curious phenomena such as the Cassie to Wenzel transitional states. Other time-dependant studies track the change in contact angle over time as environmental factors (such as temperature and humidity) change. In some cases, the drop is modified by adding and agent that will increase or decrease surface tension.
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.
Wilhelmy Plate Method An alternate method for measuring the contact angle involves lowering a plate into a test liquid and then removing it; and while doing so, measuring the force on the plate. This method is more complicated than the sessile drop method, requires large volumes of liquid, does not determine heterogeneity, and requires that the solid samples be fabricated to exacting dimensions and have two identical surfaces. It also requires a precision force scale. Due to these disadvantages, ramé-hart no longer offers an instrument based on the Wilhelmy plate method. 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. |
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