Learning
MESUREMENT OF CONTACT ANGLE
Contact Angle and Young Equasion
Consider a liquid droplet at rest on a flat, solid surface. A cross-sectional view of the droplet is given below. Its shape is characteristically lenticular. The angle formed by the solid surface and the tangent line to the upper surface at the end point is called the contact angle.
The contact angle is a result of the interface/surface tensions (surface free energies) between liquid and solid surrounded by vapor, and is measured according to the Young equation given below:
Evaluations
From the contact angle, physical properties of interaction between solid and liquid like wettability, affinity, adhesiveness and repellency can be studied. Typical evaluations are as shown below:
3: Evaluating Cleanness of Solid Surface
Measurement of the contact angle of a water droplet is a quick and simple way to evaluate cleanliness of a solid surface as below:
Advantages of Contact Angle Data
Solid materials like metals and glasses generally have large surface free energy (the Υs should be large) and water droplets on those materials should form a small contact angle. On the other hand, when the solid surface is contaminated with organic materials, water droplet will form a larger contact angle. The contact angle is sensitive enough to show different results even with a small amount of contamination such as monolayer orders. That’s why contact angle is used for evaluating cleanness of solid surface.
The followings are other advantages for which Contact Angle data is widely used:
- You can measure it without special knowledge about its complex theories like the Young equation.
- Skilled operations or procedures are not required for measurement. These days, contact angle meters are often coupled with computers to make operation easier.
Since contact angles are very sensitive to contamination, external factors will affect the results. Factors like temperature, humidity, solid surface roughness, and static electricity, etc. all affect results, so measuring in a controlled environment is important.
The θ/2 method (a half angle method) is generally used to determine the contact angle. The method calculates the contact angle from the angle (θ1) between the droplet base line and the line passing beyond the apex of the droplet as shown in the figure below. Based on the assumption that the droplet profile forms a segment of an arc, 2θ1=θ is formulated by geometric theorem.
The θ/2 method essentially supposes that the droplet forms a partial sphere. Therefore, if the droplet becomes non-spherical due to gravity, an inaccurate measurement will result.. (Generally, small droplets within a few micro liters are used for contact angle measurement to reduce the effect of gravity to an acceptably negligible level.) For further study, obtaining a direct tangent line or applying curve fitting are also possible.
MESUREMENT OF DYNAMIC CONTACT ANGLES
What is Dynamic Contact Angle?
So far, we've described the contact angle on the assumption that the droplet rests on a solid surface. That contact angle data is useful if the shape of the droplet rests immediately after attaching to a solid surface and the goal is simple to study variations of wettability of different solid and liquid combinations.
But in the case of many process fields like coating or cleaning, liquids and solids once attached, do not maintain a constant state. The liquid moves, and acts to expose its fresh surface and wet fresh surface of the solid in turn. The phenomena of wetting is different from the static state, making measurement of contact angles in a dynamic state a significant procedure.
There are several techniques to measure dynamic contact angles:
Variations over time under Sessile Drop Method
If a droplet changes its surface tension due to an additive agent, or the solid surface has the property of liquid absorption, the droplet will spread over the solid with time. The speed of spreading depends on a combination of several factors, and can be understood from variations in the contact angle over time.
Contact Angle changes with time function.
Extension/Contraction Method
This method measures the contact angle in the state in which the droplet is attached to both the needle tip and solid surface, and the droplet amount is increased or decreased. The ends of the droplet show the hysteresis of contact angle variations by increasing (or decreasing) volume. The angle formed while increasing volume is called the Advancing Angle, likewise the angle while decreasing volume is called the Receding Angle.
Contact Angle changes when increasing droplet volume (called the Advancing Angle) and contracting it (called the Receding Angle). The Advancing Angle should be larger than the Receding Angle.
Sliding Method
This method measures the contact angle of a droplet attached on a tilted solid surface.
When a droplet is attached to a solid surface and the solid surface is tilted little by little, the droplet will lunge forward and finally slide downward. The angles formed in the fore and the rear of the droplet lunging forward are respectively called the Advancing Angle (θa) and the Receding Angle (θr). The tilting angle of a solid surface when the droplet starts sliding downward is called the Sliding Angle (α).
This method presents various data, not only the advancing and receding contact angles, but also the sliding angle and adhesive work of the liquid against the solid.
