Advancing and Receding Contact Angles

How do you measure Advancing and Receding Contact Angles?

Contact angle goniometry is a quick and convenient way to determine material or coating wettability. Measurement involves placing a drop of liquid onto a surface and measuring the angle that a stable drop makes with the surface. More details about the contact angle technique can be found on the Contact Angle Tensiometry technique page. The speed and simplicity of the measurement makes it suitable for both research and quality control applications. However, these simple measurements provide only a small amount of information about the surface, and in some instances, more detail is necessary.

The Young’s angle that is the theoretical basis for contact angle measurements represents the lowest free energy equilibrium state between the solid, liquid, and gas phase of a drop of liquid on a surface [1]. This parameter is an ideal value that has the five following assumptions:

1. Surface is smooth
2. Surface is rigid
3. Surface is chemically homogeneous
4. Surface is insoluble
5. Surface is non-reactive

Because real world samples violate at least one of these assumptions, the measurable contact angle is a range of contact angles. In particular, surface heterogeneities, such as roughness or chemical differences, can allow liquid drops to exist in a number of metastable states [1]. A real-life example of this would be the differences in the shape of water droplets on a car after it rains. Even if the car is clean, the drops of water would form different angles on the surface due to potential differences in the finish, roughness, tilt, etc. Similarly, static drops provide only limited information about the behavior of a surface because these drops are much more likely to exist in these metastable states as opposed to a global lowest-energy state.

 

Dynamic Contact Angles

One way to provide more context into real surfaces is by measuring advancing and receding contact angles. These measurements are referred to as dynamic contact angles, or quasi-static contact angles. By placing the needle close to the surface and steadily injecting liquid onto the surface, the drop will grow and advance onto the sample surface. As the drop volume increases, the baseline (the point where the drop contacts the sample) eventually also moves and advances along the surface. As the drop volume continues to increase, the baseline will also continue to advance, and the advancing contact angle (ACA) is reached.

The receding contact angle (RCA) is made in the opposite way, where liquid is removed from a static drop. Initially, the drop volume and contact angle will decrease. Eventually, the baseline length will also begin to decrease, which represents the receding contact angle. In practice, the receding contact angle tends to be harder to quantify because it may require a large initial drop volume, which may be known a priori. The difference between the advancing and receding contact angle is the contact angle hysteresis. A perfectly homogeneous surface has a theoretical contact angle hysteresis of 0°. Larger hysteresis values are typically associated with larger amounts of surface heterogeneity, and vice versa. Common causes of hysteresis include roughness and chemical heterogeneity. Additionally, a large hysteresis value indicates a low drop mobility [2]. It is possible for a seemingly hydrophobic material to exhibit a large contact angle hysteresis, which suggests high adhesion of a drop to the surface. Korhonen et al. show an example of such a material [3].

There are several advantages for measuring ACA and RCA. One is that the ACA and RCA tend to overcome many of the metastable free energy spots in the contact angle range. This is illustrated in Figure 3. In effect, the advancing contact angle is the largest contact angle in the free energy range, while the receding contact angle is the lowest contact angle in the free energy range. Because of this, these measurements are theoretically the most reproducible. Secondly, the hysteresis value provides information about the surface heterogeneity and how much liquid adheres to the surface [4]. Third, when measuring a static drop, evaporation and/or drop volume may affect the contact angle. When a drop is advanced or receded across a surface, these issues become irrelevant. Finally, superhydrophobic surfaces by definition require ACA and RCA values greater than 150° [4]. Also, a common purpose of these surfaces is to resist adhesion, a parameter directly related to hysteresis.

 

What equipment is necessary for measuring advancing and receding contact angles?

The Attension Theta Flex is a versatile, modular optical tensiometer that can come with a number of different configurations. Automatic drop placement can be achieved with both the single liquid dispenser or the pipette dispenser. For ACA and RCA measurements, the single-liquid dispenser accessory (C201) is recommended. Alternatively, for systems with a pipette dispenser (C311-300), the Needle Adapter Pack (C514DCA) is also appropriate. For either dispensing solution, it is important to use as small of a needle diameter as possible so the needle disrupts the drop shape as littie as possible. 30 gauge needles are typically used for these measurements. Researchers that use a manual syringe can also make ACA and RCA measurements using a syringe with a threaded plunger, which allows for more precise control over drop volume.

 

What is the protocol for measuring advancing and receding contact angles?

An excellent overview of how to make contact angle measurements is given by Huhtamäki et al [2]. The authors provide details how to perform these measurements on many different types of surfaces as well as how to analyze the data. The information provided below is a summary of that overview and some information specific to the OneAttension software.

The procedure for performing an ACA measurement is as follows:

1. Auto-adjust brightness and calibrate instrument using calibration ball
2. Attach the needle to the pump/pipette and fill the needle
3. Clean substrate with an appropriate solvent and dry with clean, inert gas
4. Place substrate on the sample stage
5. Place a 2 μL drop on the substrate
6. Place the needle about halfway into the drop and ensure that the drop is in focus
7. Wait for ~30 seconds to ensure that the drop is stable
8. Dispense liquid at 0.05 μL/s for 5 minutes, start recording
9. Stop liquid dispenser and recording after 5 minutes

The procedure for RCA measurements is as follows:

1. Add liquid to your drop (2 μL/s) until your drop reaches the initial RCA volume (varies depending on hysteresis), see Table 1
2. Remove liquid until the baseline begins to recede approximately ~0.5 mm
3. Re-adjust focus, needle position, and drop position as necessary
4. Wait for 30 seconds for drop to equilibrate
5. Remove liquid at 0.05 μL/s for 300 s and record data

Recommended settings for the recipe are given below:

• Camera recording time – 300 s
• Frame rate – 0.33% (~0.68 FPS)
• 30 gauge needle – checked
• Drop rate – 0.05 μL/s (default – 2 μL/s)
• Drop out size – 40 μL (default – 4 μL)
• Dispense rate – 5 μL/s (default – 20 μL/s)
• Fill rate – 5 μL/s (default – 20 μL/s)
• Volume from Image – uncheck

The initial RCA volume described in step 1 varies depending on the sample. Samples with large hysteresis may require large drop volumes to be placed. If the initial volume is not large enough, the correct RCA may not be reached. A small sample of potential initial volumes is listed in Table 1. For a detailed graph on initial volumes for different combinations of ACA and RCA, please refer to [3]. In both the ACA and RCA, it is important to use a slow drop speed to ensure that dynamic effects, such as a sudden acceleration of liquid into or out of the needle, do not affect the measurement. This is why the drop rate has been decreased drastically from the default value. For some samples, particularly ones with high hysteresis, more than 300 seconds may be required to achieve the RCA. In these cases, the time in step 5 may be increased.

ACA(°)	RCA(°)	Inital RCA Volume (μL)
180	160	20
140	100	50
120	50	70
100	20	150

Table 1 – A sample of initial volumes based on ACA and RCA values.

 

Data Analysis of Advancing and Receding Contact Angles

Place a manual baseline and fit all frames according to the manual baseline. Analyze the first and last frames to see if the baseline is appropriate. If not, shift the baseline and reanalyze frames. Plot the data as mean contact angle (average of the contact angle on each side of the drop) vs. baseline. Ideally, a plateau where the ACA and RCA remains relatively constant can be identified. For this region, average all points of the data and report the contact angle and standard deviation. Figures 5 and 6 illustrate examples of ACA and RCA data that contain data where some data are excluded and the range where the data are averaged.

Automatic Dynamic Contact Angle Feature in OneAttension

Once the conditions for ACA and the appropriate initial volume for RCA are known, the OneAttension software has Automated Dynamic Contact Angle function that allows for ACA and RCA to be performed in a one-touch fashion. The ACA is first measured using a specified time and drop rate, and immediately afterward the RCA is measured. Different recording times, drop rates, and camera frame rates may be chosen for the ACA and RCA. Advancing and receding measurements are separated into different tabs and after analysis, the data are combined into a single graph for easy visualization.

Conclusions

ACA and RCA are useful measurements for optical tensiometry that provide additional information about a surface compared to standard static drop measurements. They can tell the researcher about surface heterogeneity such as roughness, chemical gradients, or contamination. They are also particularly important for measuring superhydrophobic materials because of their ability to characterize liquid adhesion to surfaces. With their wide range of accessories, automation, and user-friendly software, the Theta Flex and other instruments in the Attension line are well-suited for performing these measurements accurately and reproducibly.