Contact Angle Measurements

Static contact angle measurement involves the placement of microliter sized droplets on a sample surface. The angle formed at the boundary of the droplet is the contact angle, which characterizes a surface’s wettability. Dynamic contact angle measurements quantify contact angle hysteresis (the difference between advancing and receding contact angles), roll-off angle and absorption which can be useful for both non-absorbing and sponge-like materials.. The single atomic layer sensitivity makes contact angle measurement ideal for surface cleanliness and contamination detection for medical devices, fabrics, thin films and wafers.

Typical Experimental Results

Various contact angle measurements on different surfaces and zoom levels.

3 state contact angle measurement of air in saline solution.

Extension and contraction method for determining the advancing (left) and receding (right)
angles.

Contact angle as a function of time for advancing angle (left) and receding angle (right) determination.

Water droplet undergoing transition during sliding angle test.

Applications

For more information please read our application notes:
Advancing and Receding Angles of Biomedical Catheters, PDF
Advancing, Receding and Roll-off Angle Measurements through Sliding Angle Method, PDF
Surface Free Energy Analysis of Gelatin Samples, PDF
Density and Surface Tension of Ink, PDF

Instrument: Kyowa DM 701 Contact Angle Meter

Instrument Key Specifications

Advancing and Receding Angles of Biomedical Polymer Catheters

Contact angle measurement by means of extension and contraction methods can be used to determine both advancing and receding angles. The advancing angle is the maximum contact angle possible for the liquid/solid surface system without increasing the interfacial contact area when the drop volume continuously increased. The receding angle is the minimum contact angle possible for the liquid/solid surface system without reducing the interfacial contact area when the drop volume continuously decreased. The difference between the advancing and receding angles is called contact angle hysteresis. The advancing and receding angles are sometimes referred to as dynamic contact angles because they can provide extra useful information in dynamic nature over static contact angle. For example, a small advancing angle is preferred in spin coating process for easy spreading of coating materials while a large receding angle is desired for chemical cleaning solutions for speedy drying up after the cleaning.

In the extension method, a droplet is first deposited on the sample surface. More liquid is then dispensed into the droplet and the contact angle is recorded over time. Eventually, the base of the droplet expands outward to compensate for the increase in droplet volume. The point where the base of the droplet starts to slide outward is where the advancing angle is recorded. The contraction method is the extension method in reverse. Instead of adding liquid to the droplet, liquid is removed from a prepared droplet. Just as in the extension method, there comes a point where the base of the droplet begins to move inward. This point is where the receding angle is recorded.

To perform the extension and contraction methods manually for advancing and receding angle measurement requires skill and steady hands. On the contrary, the DM-701 Contact Angle Meter, manufactured by Kyowa Interface Science Co., Ltd. is capable of performing the dispensing and aspiration of the liquid needed for the extension and contraction methods automatically. This automation minimizes the operator’s error and improves measurement repeatability significantly.

In biomedical industry, catheters are often surface modified or coated to be lubricious and hydrophilic. Advancing and receding angles are measured for catheter surfaces in order to meet product specifications as well as to control interactions between catheter and patient body fluids during operation.

An uncoated biomedical polymer catheter tube was tested using the extension and contraction methods in our lab on the Kyowa DM-701. Some of the measurement results are presented in Figures 1 and 2. After the testing, it was found that the advancing angle was 110°. This is shown as the peak value in contact angle vs. time graph in Figure 2. The receding angle was 72°. It is shown as the minimum value in the receding angle vs. time graph in Figure 2.

Advancing, Receding and Roll-off Angle Measurements through Sliding Angle Method

The DM-701, a fully automatic contact angle meter with a rotating base, manufactured by Kyowa Interface Science Co., Ltd., is capable of measuring the sliding angle of a droplet on a surface via automated rotation of the entire measuring unit. The DM-701 can rotate under software control to any angle from 0° to 90° continuously or intermittently. The stage, light source and capturing camera all rotate simultaneously as one unit. This is advantageous over a stage-rotating setup where only stage is rotated. When only stage undergoes rotation and the camera and light-source are in stationary, the rotated stage may block some of the field of view of the camera, and it may also cause the illumination conditions to change. These disadvantages associated with the stage-only rotation setup could lead to contact angle measurement errors at different rotation angles. On the other hand, the DM-701 has no such concerns at all.

Sliding angle that may also be called roll-off angle has its own practical meaning and usefulness. It is defined as the angle between the sample surface and the horizontal plane at which the liquid drop starts to slide off the sample surface under gravity influence. Its simply conceivable applications are in windshield and building roof designs. The slope and material choices in designs aught to be in favor of rain drops’ rolling off. The angles of the surfaces relative to the ground should be at least larger than the water sliding angles on the surfaces of the windshield and roofing materials.

The advancing and receding angles can also be determined by the sliding angle method. The advancing and receding angles are the liquid drop front and rear end contact angles respectively at the drop slide starting point. The sliding angle method for advancing and receding angle measurements has the advantage of not having the dispenser tip in contact with the droplet over the extension and contraction method. By being out of contact with the droplet, the tip is cleared from any potential influence for the advancing and receding angle measurement. The work of adhesion between liquid and the solid surface can also be determined through the sliding angle method.

The DM-701 can measure the sliding angle by intermittently or continuously rotating the whole contact angle measurement unit. Under the intermittent mode, the unit rotates one degree at a time, holds and rotates again. The boundary and position of the droplet are compared to the boundary and position at starting point to determine if the droplet has moved. Under continuous mode the unit rotates at a fixed speed while the images are taken and compared at a selected frame rate. The DM-701 has a 60 frames per second capture rate that is sufficient to record virtually all transit events during the sliding angle method execution. Due to the direction of unit rotation, the advancing and receding angles are determined at the left (lower) side and right (upper) side of the droplet respectively.

As an example, a series of sliding angle measurements were performed on an extruded plastic tube with the DM-701 using distilled water. A droplet size of 10μL was used. After depositing a droplet on the tube sample surface, the intermittence tilt method was used in this example. The intermittence tilt rotates the measuring unit including the stage with the sample one degree and holds the unit there for a given amount of time. The system takes an image at the start of the hold and another at the end of the hold. Once it finishes this task, the stage rotates one degree again and repeats the process until the rotation reaches the pre-set angle or being aborted for any reason.

The image in Figure 1 shows a droplet had underwent sliding transition. Since the sample on the stage rotated together with the camera and the lamp as a whole unit, the image appears flat in Figure 1. The image was actually taken at a sample surface tilt angle relative to the horizontal plane. The different front and rear end contact angels indicted the rotation and were used for determining the advancing and receding angles. Table 1 summarizes the numerical experimental results that were obtained via the sliding angle method on the extruded plastic tube sample.

Surface Free Energy Analysis of Gelatin Samples

Contact angle measurement can provide useful information about the wetting characteristics of a surface and a liquid. Further, by using different probe liquids with known polar, non-polar, hydrogen-bond energy components, the surface free energy of a solid surface can be determined through contact angle measurement. Surface free energy is the excessive energy existing on the surface of a solid due to imbalanced intermolecular forces among molecules of the solid. The surface free energy provides a more general characterization of a surface chemically and energetically and its analysis is of significance to numerous applications such as wetting, cleaning, contamination, adhesion, friction, lubrication, and wear. For instance, with measured surface free energy values for any pair of solids or solid and liquid the work of adhesion between the two can be analyzed through the Young-Dupré theory.

Table 1 presents the surface free energy analysis performed on two gelatin samples using the Kyowa contact angle meters equipped in our lab. Kyowa Interface Science’s contact angle measurement analysis software, FAMAS, supports five popular surface free energy analysis models. These models include Fowkes’ acid-base, Kitazaki-Hata, Owens-Wendt, Kaelble-Uy, and Wu Model. Each of the five models determines the same or different components that comprise the total surface free energy. As shown in Table 1, the Kaelble-Uy, Wu and OwensWendt models determine the values for each of the components. Because each model has its own assumptions and limitations there is not one that can be universally applicable to all surfaces and probe liquids. Sometimes a particular model will yield useful data and other times it will not based on the combination of solids and probe liquids chosen. In spite of that scientists and engineers may need to work with more than one model, surface energy analysis through contact angle measurement remains a favorite and popular choice for its component level analysis capability and easy of operation.

Density and Surface Tension of Ink

Inkjet printers can produce high quality pictures in a short amount of time. One important aspect to the print quality is the surface tension of the inks. Controlling the surface tension of the inks can help to improve their surface wetting properties to the printing media. One method to determine the surface tension of a liquid is the so-called Pendant Drop method. For surface tension measurements using the Pendant Drop method, a single droplet is suspended in air from a needle tip. The drop shape is then captured by a high speed camera for analysis. A fitting routine is used to analyze the captured image and determine the surface tension of the liquid.

The Pendant Drop method requires the density of the liquid to be known or measured. Other surface tension measurement techniques, such as the Wilhelmy Plate and du Noüy Ring, do not require the liquid density to be known. Nonetheless, the Pendant Drop method requires significantly less of a liquid sample for analysis. Just a few milliliters are sufficient for multiple surface tension measurements with the Pendant Drop method. In addition, the needle tip does not need to be cleaned using burning heat between measurements and is much more resilient to deformation than the Wilhelmy plate or the du Noüy Ring.

Two common printer ink colors are cyan and magenta. To determine the density, the cyan and magenta printer inks were measured with a DDM 2911 Density Meter manufactured by Rudolph Research Analytical (USA). Each ink was carefully injected into the Density Meter at room temperature. The results of the density tests for the cyan and magenta printer inks are shown in Table 1.

With the densities of both the cyan and magenta inks measured, the surface tension of each ink can be determined through the Pendant Drop method. The surface tension measurements were performed with a DM-701 Contact Angle Meter made by Kyowa Interface Science Co. Ltd. (Japan). The DM-701 allows for automatic liquid dispensing and drop size control. Figure 1 shows typical droplets formed by the cyan and magenta printer inks. The drop shapes were analyzed using the Young-Laplace theory.

The surface tension for each ink was approximately 30 mN/m with the cyan ink being slightly greater in value than the magenta ink. Both measured surface tension values fall within typical surface tension values for printer inks. Even though the Pendant Drop method requires more information about the liquid properties to be known than other methods, it still has advantages over other surface tension measurement techniques for certain applications where liquid amount is rather limited.