Discussion Examples for Contact Angle Measurement | Wettability, Surface Free Energy, and Hydrophilicity

What Is Contact Angle Measurement?

Contact angle measurement is a method in which a liquid droplet is placed on a solid surface and the angle formed at the position where the solid surface and the droplet surface meet is measured.
This angle is called the contact angle.
The contact angle is used to evaluate whether a liquid spreads easily over a solid surface, that is, its wettability.

When a droplet spreads over the surface, the contact angle becomes small, while when the droplet remains rounded, the contact angle becomes large.
When water is used as the measurement liquid, a surface with a small contact angle can be judged to have high hydrophilicity, while a surface with a large contact angle can be judged to have high hydrophobicity.
However, because the contact angle changes depending on the type of measurement liquid, the liquid used must be clearly stated.

Main Items to Include in the Results

In the results of contact angle measurement, organize the sample name, measurement liquid, droplet volume, measurement temperature, measurement time, average contact angle, standard deviation, number of measurements, presence or absence of surface treatment, surface roughness, condition of the measurement image, and other information.
Because contact angle is easily affected by slight surface contamination or roughness, it is important to record the measurement conditions in detail.

Main Items to Include in the Results

• Type of solid sample measured
• Type of measurement liquid
• Droplet volume
• Measurement temperature
• Measurement humidity
• Time after the start of measurement
• Measured contact-angle values
• Average value
• Standard deviation
• Number of measurements
• Presence or absence of surface treatment
• Cleaning method
• Surface roughness
• Difference between the left and right sides of the droplet
• Presence or absence of evaporation or absorption
• Judgment of hydrophilicity or hydrophobicity
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Contact Angle Measurement

Here, the contact-angle measurements of samples subjected to different surface treatments are compared to confirm the process of discussing differences in wettability, hydrophilicity and hydrophobicity, and surface condition.

Contact angle is an indicator of how a liquid droplet spreads on a solid surface.
In general, the smaller the contact angle, the more easily the liquid spreads and the more hydrophilic the surface is considered to be.
Conversely, the larger the contact angle, the more rounded the droplet becomes and the more hydrophobic the surface is considered to be.

Reference Experimental Conditions

Examples of Measurement Samples

Contact-Angle Measurement Results

The following is an example in which a water droplet is placed on each sample under the same conditions and the contact angle is measured five times.
Because contact angle varies depending on slight surface contamination, roughness, droplet volume, and measurement position, multiple measurements are performed and the average value is calculated.

Example Calculation of the Average Contact Angle

The average contact angle is obtained by dividing the sum of the angles measured multiple times by the number of measurements.
For example, the average contact angle of Sample A can be calculated as follows.

Average contact angle = (54.2 + 56.1 + 55.4 + 53.8 + 55.7) ÷ 5

Average contact angle = 275.2 ÷ 5 = 55.04°

When expressed to one decimal place, the average contact angle of Sample A is 55.0°.

How to Think About Standard Deviation

In contact angle measurement, not only the average value but also the variation in the measured values is checked.
The smaller the standard deviation, the more closely the measured values are clustered around the average and the higher the reproducibility is considered to be.

In this reference example, the standard deviations of all samples are approximately 0.6 to 1.1°, indicating that the variation in the measured values is relatively small.
Therefore, the differences in contact angle caused by each surface treatment are considered more likely to reflect differences in surface condition rather than simple measurement variation.

Evaluation of Wettability from Contact Angle

The smaller the contact angle, the more easily the water droplet spreads over the surface and the higher the wettability can be judged to be.
On the other hand, the larger the contact angle, the more rounded the water droplet becomes and the lower the wettability.

Relationship Between Contact Angle and Surface Free Energy

In general, the higher the surface free energy of a solid surface, the more easily liquids such as water spread and the smaller the contact angle becomes.
Conversely, on a surface with low surface free energy, the liquid does not spread easily and the contact angle becomes larger.

In this reference example, Sample D after plasma treatment has the smallest contact angle, while Sample C after hydrophobic treatment has the largest contact angle.
Therefore, Sample D is considered to have high surface free energy, while Sample C is considered to have low surface free energy.

Comparison of Changes Caused by Surface Treatment

Using untreated glass as the reference, the amount by which the contact angle changed as a result of each surface treatment is compared.

Example of How to Write the Results

The average contact angle of untreated glass was 55.0°.
In the sample after cleaning treatment, the average contact angle was 36.2°, which was 18.8° lower than that of the untreated glass.
In addition, the average contact angle of the sample after plasma treatment was 18.6°, which was the smallest value among the samples measured in this experiment.

On the other hand, the average contact angle of the sample after hydrophobic treatment was 102.6°, which was 47.6° larger than that of the untreated glass.
These results show that cleaning treatment and plasma treatment increased surface hydrophilicity, while hydrophobic treatment reduced wettability toward water.

Points for Connecting the Results to the Discussion

In a discussion of contact angle measurement, it is important not only to describe the magnitude of the angle but also to explain how the surface condition changed as a result of surface treatment.

• Can it be said that samples with smaller contact angles allow water to spread more easily?
• Can organic contamination or oil on the surface be considered to have been removed by cleaning treatment?
• Can polar groups be considered to have been introduced onto the surface by plasma treatment, increasing hydrophilicity?
• Can hydrophobic treatment be considered to have lowered the surface free energy and made the water droplet more rounded?
• Is the variation in the measured values small and are the differences among treatment conditions clear?
• Could droplet volume, measurement time, surface roughness, contamination, or other factors have affected the measured values?

Example Discussion

In this experiment, the water contact angle changed greatly depending on the surface treatment.
The average contact angle of untreated glass was 55.0°, whereas after cleaning treatment it was 36.2° and after plasma treatment it was 18.6°, both of which were smaller than that of the untreated sample.
This was considered to result from removal of organic contamination from the surface by cleaning and introduction of hydrophilic functional groups onto the surface by plasma treatment, which strengthened the interaction with water.

On the other hand, the average contact angle of the hydrophobically treated sample was 102.6°, and the water droplet became difficult to spread over the surface.
This was considered to result from the presence of hydrophobic functional groups on the surface, which lowered the surface free energy and reduced affinity for water.
Therefore, the contact angle measurements showed that changes in wettability caused by surface treatment could be quantitatively evaluated.

In addition, the standard deviations of each sample were approximately 0.6 to 1.1°, indicating that the variation in measured values was relatively small.
Therefore, the differences in contact angle among the treatment conditions are difficult to explain solely by measurement error and are considered to reflect differences in surface condition.
However, because contact angle is also affected by surface roughness, droplet volume, measurement position, and time until measurement, it is important to keep the measurement conditions constant during the experiment.

Summary

Contact angle measurement makes it possible to evaluate the wettability of a solid surface from the way a liquid droplet spreads.
In this reference example, the contact angle was smallest for the sample after plasma treatment and largest for the sample after hydrophobic treatment.

The smaller the contact angle, the higher the hydrophilicity is considered to be, while the larger the contact angle, the higher the hydrophobicity is considered to be.
In a report, presenting the average value and variation and then explaining the relationships among surface treatment, surface free energy, and hydrophilicity or hydrophobicity makes it possible to provide a discussion based on the experimental results.

Relationship Between Contact Angle and Wettability

Wettability is a property indicating how easily a liquid spreads over a solid surface.
The smaller the contact angle, the more easily the liquid spreads over the solid surface and the higher the wettability.
The larger the contact angle, the more difficult it is for the liquid to spread over the surface and the lower the wettability.

In contact angle measurements using water, a surface with a small contact angle is judged to have high hydrophilicity because it has a high affinity for water.
Conversely, a surface on which the water droplet remains rounded and the contact angle is large is considered highly hydrophobic because it readily repels water.
Wettability is also related to adhesion, coating, printing, cleaning, surface coating, medical materials, and other applications.

Discussion of Hydrophilic Surfaces

A hydrophilic surface is a surface that has a high affinity for water and allows water droplets to spread easily.
Hydrophilic surfaces often contain polar functional groups such as -OH, -COOH, and -NH2, which readily form hydrogen bonds or dipole interactions with water molecules.
Therefore, water droplets spread over the surface and the contact angle becomes smaller.

When polar functional groups are introduced onto the surface through plasma treatment, UV ozone treatment, oxidation treatment, hydrophilic coating, or other methods, the contact angle may decrease.
If the contact angle decreases, the surface free energy is considered to have increased and the interaction with water to have become stronger.

Discussion of Hydrophobic Surfaces

A hydrophobic surface is a surface with low affinity for water on which water droplets readily remain rounded.
Hydrophobic surfaces may contain many functional groups with low polarity, such as hydrocarbon chains, fluorinated carbon chains, or silicone-based structures.
Because these surfaces interact weakly with water molecules, water droplets do not readily spread.

The larger the contact angle, the more difficult it is for water to wet the surface.
Surfaces subjected to water-repellent treatment or fluorine coating may show larger contact angles.
However, the contact angle is affected not only by chemical composition but also by surface roughness.

What Is Surface Free Energy?

Surface free energy is the energy possessed by a solid surface and can be considered as the energy required to create a surface.
Solids with high surface free energy tend to interact readily with liquids and to be easily wetted.
Solids with low surface free energy tend to interact less readily with liquids and to repel them.

In general, polar surfaces such as glass and metal oxides have high surface free energy and are readily wetted by water.
On the other hand, low-polarity surfaces such as polyethylene and fluororesins have low surface free energy and readily repel water.
Contact angle measurement provides a clue for estimating surface free energy.

Concept of Young’s Equation

The contact angle is determined by the balance of interfacial tensions among the solid, liquid, and gas phases.
On an ideally smooth and homogeneous solid surface, the contact angle can be explained by Young’s equation.
Young’s equation considers the balance among the interfacial free energies of the solid-gas, solid-liquid, and liquid-gas interfaces.

γSV = γSL + γLVcosθ

γSV: solid-gas interfacial free energy, γSL: solid-liquid interfacial free energy, γLV: liquid-gas interfacial tension, θ: contact angle

When the interaction between the solid and liquid is strong, γSL becomes small, the droplet spreads more easily, and the contact angle becomes smaller.
Conversely, when the interaction between the solid and liquid is weak, the contact angle becomes larger.
Young’s equation assumes an ideal surface, but it is important for explaining the basic concept of contact angle.

Discussion of Droplet Shape

Droplet shape is important in contact angle measurement.
Ideally, the droplet should be left-right symmetrical, the substrate surface should be horizontal, and the outline of the droplet should be clear.
If the droplet is asymmetrical, possible causes include tilting of the substrate, surface nonuniformity, excessive droplet volume, or excessive momentum during droplet deposition.

If the droplet outline is unclear, the error in reading the contact angle becomes larger.
In addition, when the droplet evaporates or is absorbed into the surface, the contact angle changes over time.
It is important to read the value at a fixed time after droplet deposition.

Effect of Surface Roughness

Surface roughness greatly affects contact angle.
When the surface is rough, the actual surface area becomes larger, and the apparent contact angle changes depending on whether the liquid enters the surface irregularities.
On a hydrophilic surface, roughness may further promote wetting and reduce the contact angle.

On a hydrophobic surface, surface roughness may trap air in the irregularities, making the water droplet more rounded and increasing the apparent contact angle.
This is also related to the design of water-repellent and superhydrophobic surfaces.
When surface roughness is present, the measured contact angle includes not only the influence of chemical composition but also that of the surface topography.

Wenzel Model and Cassie-Baxter Model

When considering contact angles on rough surfaces, the Wenzel model and Cassie-Baxter model may be used.
In the Wenzel model, the liquid enters the surface irregularities and completely wets the solid surface.
In this case, surface roughness may enhance the original hydrophilicity or hydrophobicity.

In the Cassie-Baxter model, the droplet rests on top of the irregularities and air remains trapped within them.
In this state, the contact area between the water droplet and the solid becomes smaller and the apparent contact angle may become larger.
The presence of this type of air layer is important in superhydrophobic surfaces.

Effect of Surface Contamination

Contact angle is very sensitive to surface contamination.
If oil, dust, fingerprints, organic matter, detergent residue, or other substances adhere to the surface, the chemical properties of the solid surface change and the contact angle may change greatly.
In particular, when oil adheres to a hydrophilic surface, the water contact angle may increase.

Conversely, removing surface contamination through cleaning may reduce the contact angle.
When comparing the effects of surface treatment, it is necessary to distinguish whether the change is caused by the treatment itself or by removal of contamination through cleaning.
Handling of the sample before measurement is important.

Effect of Surface Treatment

Contact angle measurement is often used to evaluate the effects of surface treatment.
Plasma treatment, UV ozone treatment, acid treatment, alkali treatment, silane coupling treatment, fluorine coating, and other treatments change the chemical composition or roughness of the surface.
As a result, the contact angle also changes.

In hydrophilic treatment, polar functional groups may increase on the surface and the water contact angle may decrease.
In hydrophobic treatment, low-surface-free-energy functional groups may be introduced and the water contact angle may increase.
Changes in contact angle provide a clue for determining whether the surface treatment was successful.

Effect of Droplet Volume

Droplet volume is also important in contact angle measurement.
If the droplet is too small, its outline becomes difficult to read and it is also more easily affected by evaporation.
If the droplet is too large, gravity may flatten the droplet and make the contact angle appear smaller.

Therefore, the droplet volume must be kept constant in comparative experiments.
Even for the same sample, the measured value may change if the droplet volume differs.
It is desirable to follow the droplet volume specified by the measurement apparatus or laboratory manual and perform measurements at multiple points to calculate the average value.

Effect of Measurement Time

Contact angle may change over time immediately after a droplet is placed.
The contact angle may gradually decrease because the droplet spreads over the surface, is absorbed into the surface, evaporates, or because the surface structure changes.
In porous or hydrophilic materials, absorption of water may cause the contact angle to decrease over time.

Therefore, when comparing static contact angles, the time after droplet deposition at which the measurement is taken must be standardized.
When investigating time dependence, the contact angle is recorded as a function of time.
If the measurement time is not standardized, it becomes difficult to determine whether the difference is caused by surface wettability or simply by elapsed time.

Discussion of Dynamic Contact Angles

In addition to static contact angle, contact angle includes advancing and receding contact angles.
The advancing contact angle is the contact angle when a droplet spreads, while the receding contact angle is the contact angle when a droplet shrinks.
The difference between these is called contact-angle hysteresis.

If contact-angle hysteresis is large, surface roughness, chemical nonuniformity, contamination, droplet pinning, or other factors may be involved.
Static contact angle alone may not sufficiently explain the actual wetting behavior of a surface.
Measuring dynamic contact angles also makes it possible to evaluate how easily a droplet moves over the surface.

Effect of the Type of Measurement Liquid

Contact angle changes depending on the type of liquid used for measurement.
This is because liquids such as water, diiodomethane, ethylene glycol, and glycerin have different surface tensions and different polar and dispersive components.
Water contact angle alone cannot accurately evaluate all aspects of surface free energy.

To determine surface free energy in detail, contact angles may be measured using multiple liquids with different polarities and analyzed by methods such as the Owens-Wendt method.
In experiments using only water, evaluation mainly focuses on hydrophilicity and hydrophobicity toward water.
It is important to clearly state the type of measurement liquid in the discussion.

Estimation of Surface Free Energy

There are methods for estimating surface free energy from contact angle measurements.
A representative approach is to measure the contact angles of multiple liquids and consider surface free energy as being divided into dispersive and polar components.
This makes it possible to evaluate what types of interactions are likely to occur at the surface.

For example, a surface with a large polar component interacts readily with water and other polar liquids and tends to exhibit hydrophilicity.
A surface dominated by the dispersive component and having a small polar component may tend to exhibit hydrophobicity.
However, because estimation of surface free energy depends on the model used, the analytical conditions must be clearly stated.

Effects of Temperature and Humidity

Temperature affects the surface tension and viscosity of liquids as well as evaporation rate.
In general, the surface tension of water decreases as temperature increases.
In addition, at higher temperatures, droplets evaporate more rapidly and their shape is more likely to change over time.

Evaporation also proceeds more readily at low humidity.
If the droplet volume changes because of evaporation, the contact angle may change over time.
In comparative experiments, it is important to keep temperature and humidity as constant as possible and to standardize the measurement time.

Causes of Error in Contact Angle Measurement

Causes of error in contact angle measurement include surface contamination, surface roughness, sample tilt, variation in droplet volume, impact during droplet deposition, droplet evaporation, absorption, errors in contour recognition, lighting conditions, differences in measurement time, and differences in readings among operators.
Because contact angle is sensitive to surface condition, even slight contamination or irregularities may change the value.

Even on the same sample, the contact angle may differ depending on the measurement position.
This may be because the surface is nonuniform, surface treatment is uneven, or microscopic contamination or scratches are present.
Therefore, it is important to perform measurements at multiple points and report the average value and standard deviation.

When the Results Can Be Considered Good

Contact angle measurement can be considered to have produced good results when repeated measurements on the same sample show little variation and a consistent trend corresponding to differences in surface treatment or material is obtained.
For example, if the contact angle decreases after hydrophilic treatment and increases after hydrophobic treatment, the result can be considered reasonable and consistent with changes in surface chemistry.

A good discussion can also be made if changes in contact angle can be related to changes in surface free energy or surface functional groups.
It is important to explain variation in the measured values and differences from theoretical values while also considering the effects of surface roughness and contamination.

Example Discussions When the Experiment Did Not Go Well

When contact angle measurement does not go well, possible causes should be considered from results such as large variation in values, large differences between left and right contact angles, results opposite to expectations, immediate absorption of the droplet, difficulty in observing the droplet outline, or large changes over time.
Organizing the causes according to surface condition, droplet volume, measurement time, apparatus settings, and environmental conditions makes the discussion easier.

How to Write Points for Improvement

In a discussion of contact angle measurement, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to sample preparation, droplet formation, measurement conditions, and data analysis.

Improvements to Sample Preparation

• Thoroughly clean the surface before measurement
• Avoid attaching fingerprints or oil
• Fix the sample horizontally
• Standardize the surface-treatment conditions
• Keep the storage time after treatment consistent
• Select multiple measurement positions

Improvements to Droplet Formation

• Keep the droplet volume constant
• Minimize impact during droplet deposition
• Place the droplet gently
• Use droplets without bubbles
• Use the same measurement liquid
• Avoid contamination of the measurement liquid

Improvements to Measurement and Analysis

• Standardize the measurement time after droplet deposition
• Keep temperature and humidity constant
• Measure contact angles on both the left and right sides
• Perform multiple measurements and calculate the average value
• Report the standard deviation
• Capture a clear image of the droplet outline
• Consider the effects of surface roughness and contamination

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of contact angle measurement, simply writing that “the angle was small” or “the surface was hydrophilic” results in a superficial discussion.
A good discussion relates droplet spreading, interfacial free energy, surface functional groups, surface roughness, and measurement conditions.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of contact angle measurements.
Adjust the necessary parts according to your own experimental results.

• Contact angle is an indicator used to evaluate the wettability of a liquid droplet on a solid surface.
• The smaller the contact angle, the more easily the liquid spreads over the solid surface.
• A surface with a small water contact angle is considered to have high hydrophilicity.
• A surface with a large water contact angle is considered to have high hydrophobicity.
• A surface with high surface free energy tends to be readily wetted by water.
• A surface with low surface free energy tends to repel water.
• In Young’s equation, the contact angle is determined by the balance of interfacial free energies.
• Surface roughness may greatly change the apparent contact angle.
• Surface contamination and fingerprints are major causes of error in contact angle measurement.
• To compare contact angles, the droplet volume, measurement time, temperature, and measurement position must be standardized.

Points to Check When Discussing Contact Angle Measurement

Checking the following points before writing the report makes the discussion easier to write.

• Is the definition of contact angle explained?
• Is the relationship with wettability described?
• Are hydrophilicity and hydrophobicity judged from the contact angle?
• Are surface free energy and contact angle related?
• Is the concept of Young’s equation explained?
• Are the effects of surface functional groups considered?
• Is the effect of surface roughness considered?
• Are surface contamination and uneven treatment considered as causes of error?
• Are droplet volume and measurement time standardized?
• Are multiple measurements performed and the average value reported?
• Are standard deviation and variation discussed?
• Do the points for improvement correspond to the causes of error?

Summary

Contact angle measurement is an experiment that evaluates the wettability of a surface from the angle of a liquid droplet placed on a solid surface.
The smaller the contact angle, the more easily the droplet spreads over the surface and the higher the hydrophilicity is considered to be.
The larger the contact angle, the more rounded the droplet becomes and the higher the hydrophobicity is considered to be.
This difference can be explained by the interaction between the solid surface and the liquid and by surface free energy.

In Young’s equation, the contact angle is determined by the balance among the interfacial free energies of the solid-gas, solid-liquid, and liquid-gas interfaces.
When polar functional groups increase on the surface, the interaction with water becomes stronger and the contact angle tends to decrease.
Conversely, on a surface containing many hydrophobic functional groups with low surface free energy, the water contact angle becomes larger.

In a report, rather than simply writing that “the surface was hydrophilic” or “the surface was hydrophobic,” organize and discuss wettability, surface free energy, Young’s equation, surface functional groups, surface roughness, droplet volume, measurement time, surface contamination, causes of error, and points for improvement.
Contact angle measurement is an important experiment that allows the chemical and physical properties of material surfaces to be evaluated simply.